//===--- ImportDecl.cpp - Import Clang Declarations -----------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This file implements support for importing Clang declarations into Swift.
//
//===----------------------------------------------------------------------===//

#include "CFTypeInfo.h"
#include "ImporterImpl.h"
#include "swift/Strings.h"
#include "swift/AST/ASTContext.h"
#include "swift/AST/GenericSignatureBuilder.h"
#include "swift/AST/Attr.h"
#include "swift/AST/Builtins.h"
#include "swift/AST/Decl.h"
#include "swift/AST/DiagnosticsClangImporter.h"
#include "swift/AST/ExistentialLayout.h"
#include "swift/AST/Expr.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/GenericSignature.h"
#include "swift/AST/Module.h"
#include "swift/AST/NameLookup.h"
#include "swift/AST/ParameterList.h"
#include "swift/AST/Pattern.h"
#include "swift/AST/PrettyStackTrace.h"
#include "swift/AST/ProtocolConformance.h"
#include "swift/AST/Stmt.h"
#include "swift/AST/Types.h"
#include "swift/Basic/Defer.h"
#include "swift/Basic/PrettyStackTrace.h"
#include "swift/ClangImporter/ClangModule.h"
#include "swift/Parse/Lexer.h"
#include "swift/Config.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Attr.h"
#include "clang/Basic/CharInfo.h"
#include "swift/Basic/Statistic.h"
#include "clang/Lex/Preprocessor.h"
#include "clang/Sema/Lookup.h"

#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/StringSwitch.h"
#include "llvm/Support/Path.h"

#include <algorithm>

#define DEBUG_TYPE "Clang module importer"

STATISTIC(NumTotalImportedEntities, "# of imported clang entities");
STATISTIC(NumFactoryMethodsAsInitializers,
          "# of factory methods mapped to initializers");

using namespace swift;
using namespace importer;

namespace swift {
namespace inferred_attributes {
  enum {
    requires_stored_property_inits = 0x01
  };
} // end namespace inferred_attributes
} // end namespace swift

namespace {

struct AccessorInfo {
  AbstractStorageDecl *Storage;
  AccessorKind Kind;
};

enum class MakeStructRawValuedFlags {
  /// whether to also create an unlabeled init
  MakeUnlabeledValueInit = 0x01,

  /// whether the raw value should be a let
  IsLet = 0x02,

  /// whether to mark the rawValue as implicit
  IsImplicit = 0x04,
};
using MakeStructRawValuedOptions = OptionSet<MakeStructRawValuedFlags>;
} // end anonymous namespace

static MakeStructRawValuedOptions
getDefaultMakeStructRawValuedOptions() {
  MakeStructRawValuedOptions opts;
  opts -= MakeStructRawValuedFlags::MakeUnlabeledValueInit; // default off
  opts |= MakeStructRawValuedFlags::IsLet;                  // default on
  opts |= MakeStructRawValuedFlags::IsImplicit;             // default on
  return opts;
}

static bool isInSystemModule(DeclContext *D) {
  return cast<ClangModuleUnit>(D->getModuleScopeContext())->isSystemModule();
}

static AccessLevel getOverridableAccessLevel(const DeclContext *dc) {
  return (dc->getAsClassOrClassExtensionContext()
            ? AccessLevel::Open : AccessLevel::Public);
}

/// Create a typedpattern(namedpattern(decl))
static Pattern *createTypedNamedPattern(VarDecl *decl) {
  ASTContext &Ctx = decl->getASTContext();
  Type ty = decl->getType();

  Pattern *P = new (Ctx) NamedPattern(decl);
  P->setType(ty);
  P->setImplicit();
  P = new (Ctx) TypedPattern(P, TypeLoc::withoutLoc(ty));
  P->setType(ty);
  P->setImplicit();
  return P;
}

/// Create a var member for this struct, along with its pattern binding, and add
/// it as a member
static std::pair<VarDecl *, PatternBindingDecl *>
createVarWithPattern(ASTContext &ctx, DeclContext *dc, Identifier name, Type ty,
                     VarDecl::Specifier specifier, bool isImplicit,
                     AccessLevel access,
                     AccessLevel setterAccess) {
  // Create a variable to store the underlying value.
  auto var = new (ctx) VarDecl(
      /*IsStatic*/false,
      specifier,
      /*IsCaptureList*/false,
      SourceLoc(), name, dc->mapTypeIntoContext(ty), dc);
  if (isImplicit)
    var->setImplicit();
  var->setInterfaceType(ty);
  var->setValidationToChecked();
  var->setAccess(access);
  var->setSetterAccess(setterAccess);

  // Create a pattern binding to describe the variable.
  Pattern *varPattern = createTypedNamedPattern(var);
  auto *patternBinding = PatternBindingDecl::create(
      ctx, /*StaticLoc*/ SourceLoc(), StaticSpellingKind::None,
      /*VarLoc*/ SourceLoc(), varPattern, /*EqualLoc*/ SourceLoc(),
      /*InitExpr*/ nullptr, dc);
  if (isImplicit)
    patternBinding->setImplicit();

  return {var, patternBinding};
}

static FuncDecl *createFuncOrAccessor(ASTContext &ctx, SourceLoc funcLoc,
                                      Optional<AccessorInfo> accessorInfo,
                                      DeclName name, SourceLoc nameLoc,
                                      ParamDecl *selfDecl,
                                      ParameterList *bodyParams,
                                      Type resultTy,
                                      bool throws,
                                      DeclContext *dc,
                                      ClangNode clangNode) {
  TypeLoc resultTypeLoc = resultTy ? TypeLoc::withoutLoc(resultTy) : TypeLoc();
  if (accessorInfo) {
    return AccessorDecl::create(ctx, funcLoc,
                                /*accessorKeywordLoc*/ SourceLoc(),
                                accessorInfo->Kind,
                                AddressorKind::NotAddressor,
                                accessorInfo->Storage,
                                /*StaticLoc*/SourceLoc(),
                                StaticSpellingKind::None,
                                throws,
                                /*ThrowsLoc=*/SourceLoc(),
                                /*GenericParams=*/nullptr,
                                selfDecl,
                                bodyParams,
                                resultTypeLoc, dc, clangNode);
  } else {
    return FuncDecl::create(ctx, /*StaticLoc=*/SourceLoc(),
                            StaticSpellingKind::None,
                            funcLoc, name, nameLoc,
                            throws, /*ThrowsLoc=*/SourceLoc(),
                            /*GenericParams=*/nullptr,
                            selfDecl,
                            bodyParams,
                            resultTypeLoc, dc, clangNode);
  }
}

static void makeComputed(AbstractStorageDecl *storage,
                         AccessorDecl *getter, AccessorDecl *setter) {
  assert(getter);
  if (setter) {
    storage->setAccessors(StorageImplInfo::getMutableComputed(),
                          SourceLoc(), {getter, setter}, SourceLoc());
  } else {
    storage->setAccessors(StorageImplInfo::getImmutableComputed(),
                          SourceLoc(), {getter}, SourceLoc());
  }
}

#ifndef NDEBUG
static bool verifyNameMapping(MappedTypeNameKind NameMapping,
                              StringRef left, StringRef right) {
  return NameMapping == MappedTypeNameKind::DoNothing || left != right;
}
#endif

/// \brief Map a well-known C type to a swift type from the standard library.
///
/// \param IsError set to true when we know the corresponding swift type name,
/// but we could not find it.  (For example, the type was not defined in the
/// standard library or the required standard library module was not imported.)
/// This should be a hard error, we don't want to map the type only sometimes.
///
/// \returns A pair of a swift type and its name that corresponds to a given
/// C type.
static std::pair<Type, StringRef>
getSwiftStdlibType(const clang::TypedefNameDecl *D,
                   Identifier Name,
                   ClangImporter::Implementation &Impl,
                   bool *IsError, MappedTypeNameKind &NameMapping) {
  *IsError = false;

  MappedCTypeKind CTypeKind;
  unsigned Bitwidth;
  StringRef SwiftModuleName;
  bool IsSwiftModule; // True if SwiftModuleName == STDLIB_NAME.
  StringRef SwiftTypeName;
  bool CanBeMissing;

  do {
#define MAP_TYPE(C_TYPE_NAME, C_TYPE_KIND, C_TYPE_BITWIDTH,        \
                 SWIFT_MODULE_NAME, SWIFT_TYPE_NAME,               \
                 CAN_BE_MISSING, C_NAME_MAPPING)                   \
    if (Name.str() == C_TYPE_NAME) {                               \
      CTypeKind = MappedCTypeKind::C_TYPE_KIND;                    \
      Bitwidth = C_TYPE_BITWIDTH;                                  \
      if (StringRef(SWIFT_MODULE_NAME) == StringRef(STDLIB_NAME))  \
        IsSwiftModule = true;                                      \
      else {                                                       \
        IsSwiftModule = false;                                     \
        SwiftModuleName = SWIFT_MODULE_NAME;                       \
      }                                                            \
      SwiftTypeName = SWIFT_TYPE_NAME;                             \
      CanBeMissing = CAN_BE_MISSING;                               \
      NameMapping = MappedTypeNameKind::C_NAME_MAPPING;            \
      assert(verifyNameMapping(MappedTypeNameKind::C_NAME_MAPPING, \
                               C_TYPE_NAME, SWIFT_TYPE_NAME) &&    \
             "MappedTypes.def: Identical names must use DoNothing"); \
      break;                                                       \
    }
#include "MappedTypes.def"

    // We did not find this type, thus it is not mapped.
    return std::make_pair(Type(), "");
  } while (0);

  clang::ASTContext &ClangCtx = Impl.getClangASTContext();

  auto ClangType = D->getUnderlyingType();

  // If the C type does not have the expected size, don't import it as a stdlib
  // type.
  unsigned ClangTypeSize = ClangCtx.getTypeSize(ClangType);
  if (Bitwidth != 0 && Bitwidth != ClangTypeSize)
    return std::make_pair(Type(), "");

  // Check other expected properties of the C type.
  switch(CTypeKind) {
  case MappedCTypeKind::UnsignedInt:
    if (!ClangType->isUnsignedIntegerType())
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::SignedInt:
    if (!ClangType->isSignedIntegerType())
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::UnsignedWord:
    if (ClangTypeSize != 64 && ClangTypeSize != 32)
      return std::make_pair(Type(), "");
    if (!ClangType->isUnsignedIntegerType())
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::SignedWord:
    if (ClangTypeSize != 64 && ClangTypeSize != 32)
      return std::make_pair(Type(), "");
    if (!ClangType->isSignedIntegerType())
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::FloatIEEEsingle:
  case MappedCTypeKind::FloatIEEEdouble:
  case MappedCTypeKind::FloatX87DoubleExtended: {
    if (!ClangType->isFloatingType())
      return std::make_pair(Type(), "");

    const llvm::fltSemantics &Sem = ClangCtx.getFloatTypeSemantics(ClangType);
    switch(CTypeKind) {
    case MappedCTypeKind::FloatIEEEsingle:
      assert(Bitwidth == 32 && "FloatIEEEsingle should be 32 bits wide");
      if (&Sem != &APFloat::IEEEsingle())
        return std::make_pair(Type(), "");
      break;

    case MappedCTypeKind::FloatIEEEdouble:
      assert(Bitwidth == 64 && "FloatIEEEdouble should be 64 bits wide");
      if (&Sem != &APFloat::IEEEdouble())
        return std::make_pair(Type(), "");
      break;

    case MappedCTypeKind::FloatX87DoubleExtended:
      assert(Bitwidth == 80 && "FloatX87DoubleExtended should be 80 bits wide");
      if (&Sem != &APFloat::x87DoubleExtended())
        return std::make_pair(Type(), "");
      break;

    default:
      llvm_unreachable("should see only floating point types here");
    }
    }
    break;

  case MappedCTypeKind::VaList:
    if (ClangTypeSize != ClangCtx.getTypeSize(ClangCtx.VoidPtrTy))
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::ObjCBool:
    if (!ClangCtx.hasSameType(ClangType, ClangCtx.ObjCBuiltinBoolTy) &&
        !(ClangCtx.getBOOLDecl() &&
          ClangCtx.hasSameType(ClangType, ClangCtx.getBOOLType())))
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::ObjCSel:
    if (!ClangCtx.hasSameType(ClangType, ClangCtx.getObjCSelType()) &&
        !ClangCtx.hasSameType(ClangType,
                              ClangCtx.getObjCSelRedefinitionType()))
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::ObjCId:
    if (!ClangCtx.hasSameType(ClangType, ClangCtx.getObjCIdType()) &&
        !ClangCtx.hasSameType(ClangType,
                              ClangCtx.getObjCIdRedefinitionType()))
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::ObjCClass:
    if (!ClangCtx.hasSameType(ClangType, ClangCtx.getObjCClassType()) &&
        !ClangCtx.hasSameType(ClangType,
                              ClangCtx.getObjCClassRedefinitionType()))
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::CGFloat:
    if (!ClangType->isFloatingType())
      return std::make_pair(Type(), "");
    break;

  case MappedCTypeKind::Block:
    if (!ClangType->isBlockPointerType())
      return std::make_pair(Type(), "");
    break;
  }

  ModuleDecl *M;
  if (IsSwiftModule)
    M = Impl.getStdlibModule();
  else
    M = Impl.getNamedModule(SwiftModuleName);
  if (!M) {
    // User did not import the library module that contains the type we want to
    // substitute.
    *IsError = true;
    return std::make_pair(Type(), "");
  }

  Type SwiftType = Impl.getNamedSwiftType(M, SwiftTypeName);
  if (!SwiftType && !CanBeMissing) {
    // The required type is not defined in the standard library.
    *IsError = true;
    return std::make_pair(Type(), "");
  }
  return std::make_pair(SwiftType, SwiftTypeName);
}

static bool isNSDictionaryMethod(const clang::ObjCMethodDecl *MD,
                                 clang::Selector cmd) {
  if (MD->getSelector() != cmd)
    return false;
  if (isa<clang::ObjCProtocolDecl>(MD->getDeclContext()))
    return false;
  if (MD->getClassInterface()->getName() != "NSDictionary")
    return false;
  return true;
}

// Build the init(rawValue:) initializer for an imported NS_ENUM.
//   enum NSSomeEnum: RawType {
//     init?(rawValue: RawType) {
//       self = Builtin.reinterpretCast(rawValue)
//     }
//   }
// Unlike a standard init(rawValue:) enum initializer, this does a reinterpret
// cast in order to preserve unknown or future cases from C.
static ConstructorDecl *
makeEnumRawValueConstructor(ClangImporter::Implementation &Impl,
                            EnumDecl *enumDecl) {
  ASTContext &C = Impl.SwiftContext;
  auto rawTy = enumDecl->getRawType();
  auto enumTy = enumDecl->getDeclaredInterfaceType();
  
  auto selfDecl = ParamDecl::createSelf(SourceLoc(), enumDecl,
                                        /*static*/false, /*inout*/true);

  auto param = new (C) ParamDecl(VarDecl::Specifier::Default, SourceLoc(),
                                 SourceLoc(), C.Id_rawValue,
                                 SourceLoc(), C.Id_rawValue,
                                 rawTy,
                                 enumDecl);
  param->setInterfaceType(rawTy);
  param->setValidationToChecked();

  auto paramPL = ParameterList::createWithoutLoc(param);

  DeclName name(C, DeclBaseName::createConstructor(), paramPL);
  auto *ctorDecl =
    new (C) ConstructorDecl(name, enumDecl->getLoc(),
                            OTK_Optional, /*FailabilityLoc=*/SourceLoc(),
                            /*Throws=*/false, /*ThrowsLoc=*/SourceLoc(),
                            selfDecl, paramPL,
                            /*GenericParams=*/nullptr, enumDecl);
  ctorDecl->setImplicit();
  ctorDecl->setAccess(AccessLevel::Public);

  ctorDecl->computeType();
  ctorDecl->setValidationToChecked();

  // Don't bother synthesizing the body if we've already finished type-checking.
  if (Impl.hasFinishedTypeChecking())
    return ctorDecl;
  
  auto selfRef = new (C) DeclRefExpr(selfDecl, DeclNameLoc(), /*implicit*/true);
  selfRef->setType(LValueType::get(selfDecl->getType()));

  auto paramRef = new (C) DeclRefExpr(param, DeclNameLoc(),
                                      /*implicit*/ true);
  paramRef->setType(param->getType());

  auto reinterpretCast
    = cast<FuncDecl>(
        getBuiltinValueDecl(C, C.getIdentifier("reinterpretCast")));
  SubstitutionMap subMap =
    SubstitutionMap::get(reinterpretCast->getGenericSignature(),
                         { rawTy, enumTy }, { });
  ConcreteDeclRef concreteDeclRef(reinterpretCast, subMap);
  auto reinterpretCastRef
    = new (C) DeclRefExpr(concreteDeclRef, DeclNameLoc(), /*implicit*/ true);
  reinterpretCastRef->setType(FunctionType::get({rawTy}, enumTy));

  auto reinterpreted = CallExpr::createImplicit(C, reinterpretCastRef,
                                                { paramRef }, { Identifier() });
  reinterpreted->setType(enumTy);
  reinterpreted->setThrows(false);

  auto assign = new (C) AssignExpr(selfRef, SourceLoc(), reinterpreted,
                                   /*implicit*/ true);
  assign->setType(TupleType::getEmpty(C));

  auto result = TupleExpr::createEmpty(C, SourceLoc(), SourceLoc(),
                                       /*Implicit=*/true);
  auto ret = new (C) ReturnStmt(SourceLoc(), result, /*Implicit=*/true);

  auto body = BraceStmt::create(C, SourceLoc(), {assign, ret}, SourceLoc(),
                                /*implicit*/ true);
  
  ctorDecl->setBody(body);
  ctorDecl->setBodyTypeCheckedIfPresent();
  
  Impl.registerExternalDecl(ctorDecl);
  
  return ctorDecl;
}

// Build the rawValue getter for an imported NS_ENUM.
//   enum NSSomeEnum: RawType {
//     var rawValue: RawType {
//       return Builtin.reinterpretCast(self)
//     }
//   }
// Unlike a standard init(rawValue:) enum initializer, this does a reinterpret
// cast in order to preserve unknown or future cases from C.
static AccessorDecl *makeEnumRawValueGetter(ClangImporter::Implementation &Impl,
                                            EnumDecl *enumDecl,
                                            VarDecl *rawValueDecl) {
  ASTContext &C = Impl.SwiftContext;

  auto rawTy = enumDecl->getRawType();
  auto enumTy = enumDecl->getDeclaredType();

  auto *selfDecl = ParamDecl::createSelf(SourceLoc(), enumDecl);
  auto *params = ParameterList::createEmpty(C);

  auto getterDecl = AccessorDecl::create(C,
                     /*FuncLoc=*/SourceLoc(),
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Get,
                     AddressorKind::NotAddressor,
                     rawValueDecl,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr, selfDecl, params,
                     TypeLoc::withoutLoc(rawTy), enumDecl);
  getterDecl->setImplicit();
  getterDecl->setIsObjC(false);

  getterDecl->computeType();
  getterDecl->setValidationToChecked();

  getterDecl->setAccess(AccessLevel::Public);

  makeComputed(rawValueDecl, getterDecl, nullptr);

  // Don't bother synthesizing the body if we've already finished type-checking.
  if (Impl.hasFinishedTypeChecking())
    return getterDecl;
  
  auto selfRef = new (C) DeclRefExpr(selfDecl, DeclNameLoc(), /*implicit*/true);
  selfRef->setType(selfDecl->getType());

  auto reinterpretCast
    = cast<FuncDecl>(
        getBuiltinValueDecl(C, C.getIdentifier("reinterpretCast")));
  SubstitutionMap subMap =
    SubstitutionMap::get(reinterpretCast->getGenericSignature(),
                         { enumTy, rawTy }, { });
  ConcreteDeclRef concreteDeclRef(reinterpretCast, subMap);

  auto reinterpretCastRef
    = new (C) DeclRefExpr(concreteDeclRef, DeclNameLoc(), /*implicit*/ true);
  reinterpretCastRef->setType(FunctionType::get({enumTy}, rawTy));

  auto reinterpreted = CallExpr::createImplicit(C, reinterpretCastRef,
                                                { selfRef }, { Identifier() });
  reinterpreted->setType(rawTy);
  reinterpreted->setThrows(false);

  auto ret = new (C) ReturnStmt(SourceLoc(), reinterpreted);
  auto body = BraceStmt::create(C, SourceLoc(), ASTNode(ret), SourceLoc(),
                                /*implicit*/ true);
  
  getterDecl->setBody(body);
  getterDecl->setBodyTypeCheckedIfPresent();
  Impl.registerExternalDecl(getterDecl);
  return getterDecl;
}

// Build the rawValue getter for a struct type.
//
//   struct SomeType: RawRepresentable {
//     private var _rawValue: ObjCType
//     var rawValue: SwiftType {
//       return _rawValue as SwiftType
//     }
//   }
static AccessorDecl *makeStructRawValueGetter(
                   ClangImporter::Implementation &Impl,
                   StructDecl *structDecl,
                   VarDecl *computedVar,
                   VarDecl *storedVar) {
  assert(storedVar->hasStorage());

  ASTContext &C = Impl.SwiftContext;
  
  auto *selfDecl = ParamDecl::createSelf(SourceLoc(), structDecl);
  auto *params = ParameterList::createEmpty(C);

  auto computedType = computedVar->getInterfaceType();
  auto storedType = storedVar->getInterfaceType();

  auto getterDecl = AccessorDecl::create(C,
                     /*FuncLoc=*/SourceLoc(),
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Get,
                     AddressorKind::NotAddressor,
                     computedVar,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr, selfDecl, params,
                     TypeLoc::withoutLoc(computedType), structDecl);
  getterDecl->setImplicit();
  getterDecl->setIsObjC(false);

  getterDecl->computeType();
  getterDecl->setValidationToChecked();

  getterDecl->setAccess(AccessLevel::Public);

  // Don't bother synthesizing the body if we've already finished type-checking.
  if (Impl.hasFinishedTypeChecking())
    return getterDecl;

  auto selfRef = new (C) DeclRefExpr(selfDecl, DeclNameLoc(), /*implicit*/true);
  selfRef->setType(selfDecl->getType());

  auto storedRef = new (C) MemberRefExpr(selfRef, SourceLoc(), storedVar,
                                         DeclNameLoc(), /*Implicit=*/true,
                                         AccessSemantics::DirectToStorage);
  storedRef->setType(storedType);

  Expr *result = storedRef;

  if (!computedType->isEqual(storedType)) {
    auto bridge = new (C) BridgeFromObjCExpr(storedRef, computedType);
    bridge->setType(computedType);

    auto coerce = new (C) CoerceExpr(bridge, {}, {nullptr, computedType});
    coerce->setType(computedType);

    result = coerce;
  }

  auto ret = new (C) ReturnStmt(SourceLoc(), result);
  auto body = BraceStmt::create(C, SourceLoc(), ASTNode(ret), SourceLoc(),
                                /*implicit*/ true);
  
  getterDecl->setBody(body);
  getterDecl->setBodyTypeCheckedIfPresent();

  Impl.registerExternalDecl(getterDecl);
  return getterDecl;
}

static AccessorDecl *makeFieldGetterDecl(ClangImporter::Implementation &Impl,
                                         StructDecl *importedDecl,
                                         VarDecl *importedFieldDecl,
                                         ClangNode clangNode = ClangNode()) {
  auto &C = Impl.SwiftContext;
  auto selfDecl = ParamDecl::createSelf(SourceLoc(), importedDecl);

  auto *params = ParameterList::createEmpty(C);
  
  auto getterType = importedFieldDecl->getInterfaceType();
  auto getterDecl = AccessorDecl::create(C,
                     /*FuncLoc=*/importedFieldDecl->getLoc(),
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Get,
                     AddressorKind::NotAddressor,
                     importedFieldDecl,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr, selfDecl, params,
                     TypeLoc::withoutLoc(getterType), importedDecl, clangNode);
  getterDecl->setAccess(AccessLevel::Public);
  getterDecl->setIsObjC(false);

  getterDecl->computeType();
  getterDecl->setValidationToChecked();

  return getterDecl;
}

static AccessorDecl *makeFieldSetterDecl(ClangImporter::Implementation &Impl,
                                         StructDecl *importedDecl,
                                         VarDecl *importedFieldDecl,
                                         ClangNode clangNode = ClangNode()) {
  auto &C = Impl.SwiftContext;
  auto selfDecl = ParamDecl::createSelf(SourceLoc(), importedDecl,
                                        /*isStatic*/false, /*isInOut*/true);
  auto newValueDecl = new (C) ParamDecl(VarDecl::Specifier::Default,
                                        SourceLoc(), SourceLoc(),
                                        Identifier(), SourceLoc(), C.Id_value,
                                        importedFieldDecl->getType(),
                                        importedDecl);
  newValueDecl->setInterfaceType(importedFieldDecl->getInterfaceType());

  auto *params = ParameterList::createWithoutLoc(newValueDecl);

  auto voidTy = TupleType::getEmpty(C);

  auto setterDecl = AccessorDecl::create(C,
                     /*FuncLoc=*/SourceLoc(),
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Set,
                     AddressorKind::NotAddressor,
                     importedFieldDecl,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr, selfDecl, params,
                     TypeLoc::withoutLoc(voidTy), importedDecl, clangNode);
  setterDecl->setIsObjC(false);
  setterDecl->setSelfAccessKind(SelfAccessKind::Mutating);

  setterDecl->computeType();
  setterDecl->setValidationToChecked();

  setterDecl->setAccess(AccessLevel::Public);
  setterDecl->setSelfAccessKind(SelfAccessKind::Mutating);

  return setterDecl;
}

/// Build the indirect field getter and setter.
///
/// \code
/// struct SomeImportedIndirectField {
///   struct __Unnamed_struct___Anonymous_field_1 {
///     var myField : Int
///   }
///   var __Anonymous_field_1 : __Unnamed_struct___Anonymous_field_1
///   var myField : Int {
///     get {
///       __Anonymous_field_1.myField
///     }
///     set(newValue) {
///       __Anonymous_field_1.myField = newValue
///     }
///   }
/// }
/// \endcode
///
/// \returns a pair of getter and setter function decls.
static std::pair<AccessorDecl *, AccessorDecl *>
makeIndirectFieldAccessors(ClangImporter::Implementation &Impl,
                           const clang::IndirectFieldDecl *indirectField,
                           ArrayRef<VarDecl *> members,
                           StructDecl *importedStructDecl,
                           VarDecl *importedFieldDecl) {
  auto &C = Impl.SwiftContext;

  auto getterDecl = makeFieldGetterDecl(Impl,
                                        importedStructDecl,
                                        importedFieldDecl);

  auto setterDecl = makeFieldSetterDecl(Impl,
                                        importedStructDecl,
                                        importedFieldDecl);

  makeComputed(importedFieldDecl, getterDecl, setterDecl);

  auto containingField = indirectField->chain().front();
  VarDecl *anonymousFieldDecl = nullptr;

  // Reverse scan of the members because indirect field are generated just
  // after the corresponding anonymous type, so a reverse scan allows
  // switching from O(n) to O(1) here.
  for (auto decl : reverse(members)) {
    if (decl->getClangDecl() == containingField) {
      anonymousFieldDecl = cast<VarDecl>(decl);
      break;
    }
  }
  assert (anonymousFieldDecl && "anonymous field not generated");

  auto anonymousFieldType = anonymousFieldDecl->getInterfaceType();
  auto anonymousFieldTypeDecl = anonymousFieldType->getStructOrBoundGenericStruct();

  VarDecl *anonymousInnerFieldDecl = nullptr;
  for (auto decl : anonymousFieldTypeDecl->lookupDirect(importedFieldDecl->getName())) {
    if (isa<VarDecl>(decl)) {
      anonymousInnerFieldDecl = cast<VarDecl>(decl);
      break;
    }
  }
  assert (anonymousInnerFieldDecl && "cannot find field in anonymous generated structure");

  // Don't bother synthesizing the body if we've already finished type-checking.
  if (Impl.hasFinishedTypeChecking())
    return { getterDecl, setterDecl };

  // Synthesize the getter body
  {
    auto selfDecl = getterDecl->getImplicitSelfDecl();
    Expr *expr = new (C) DeclRefExpr(selfDecl, DeclNameLoc(),
                                     /*implicit*/true);
    expr = new (C) MemberRefExpr(expr, SourceLoc(), anonymousFieldDecl,
                                 DeclNameLoc(), /*implicit*/true);

    expr = new (C) MemberRefExpr(expr, SourceLoc(), anonymousInnerFieldDecl,
                                 DeclNameLoc(), /*implicit*/true);

    auto ret = new (C) ReturnStmt(SourceLoc(), expr);
    auto body = BraceStmt::create(C, SourceLoc(), ASTNode(ret), SourceLoc(),
                                  /*implicit*/ true);
    getterDecl->setBody(body);
    getterDecl->getAttrs().add(new (C) TransparentAttr(/*implicit*/ true));
    Impl.registerExternalDecl(getterDecl);
  }

  // Synthesize the setter body
  {
    auto selfDecl = setterDecl->getImplicitSelfDecl();
    Expr *lhs = new (C) DeclRefExpr(selfDecl, DeclNameLoc(),
                                     /*implicit*/true);
    lhs = new (C) MemberRefExpr(lhs, SourceLoc(), anonymousFieldDecl,
                                 DeclNameLoc(), /*implicit*/true);

    lhs = new (C) MemberRefExpr(lhs, SourceLoc(), anonymousInnerFieldDecl,
                                DeclNameLoc(), /*implicit*/true);

    auto newValueDecl = setterDecl->getParameters()->get(0);

    auto rhs = new (C) DeclRefExpr(newValueDecl, DeclNameLoc(),
                                   /*implicit*/ true);

    auto assign = new (C) AssignExpr(lhs, SourceLoc(), rhs, /*implicit*/true);

    auto body = BraceStmt::create(C, SourceLoc(), { assign }, SourceLoc(),
                                  /*implicit*/ true);
    setterDecl->setBody(body);
    setterDecl->getAttrs().add(new (C) TransparentAttr(/*implicit*/ true));
    Impl.registerExternalDecl(setterDecl);
  }

  return { getterDecl, setterDecl };
}

/// Build the union field getter and setter.
///
/// \code
/// struct SomeImportedUnion {
///   var myField: Int {
///     get {
///       return Builtin.reinterpretCast(self)
///     }
///     set(newValue) {
///       Builtin.initialize(Builtin.addressof(self), newValue))
///     }
///   }
/// }
/// \endcode
///
/// \returns a pair of the getter and setter function decls.
static std::pair<AccessorDecl *, AccessorDecl *>
makeUnionFieldAccessors(ClangImporter::Implementation &Impl,
                        StructDecl *importedUnionDecl,
                        VarDecl *importedFieldDecl) {
  auto &C = Impl.SwiftContext;

  auto getterDecl = makeFieldGetterDecl(Impl,
                                        importedUnionDecl,
                                        importedFieldDecl);

  auto setterDecl = makeFieldSetterDecl(Impl,
                                        importedUnionDecl,
                                        importedFieldDecl);

  makeComputed(importedFieldDecl, getterDecl, setterDecl);

  // Don't bother synthesizing the body if we've already finished type-checking.
  if (Impl.hasFinishedTypeChecking())
    return { getterDecl, setterDecl };

  // Synthesize the getter body
  {
    auto selfDecl = getterDecl->getImplicitSelfDecl();

    auto selfRef = new (C) DeclRefExpr(selfDecl, DeclNameLoc(),
                                       /*implicit*/ true);
    auto reinterpretCast = cast<FuncDecl>(getBuiltinValueDecl(
        C, C.getIdentifier("reinterpretCast")));
    auto reinterpretCastRef
      = new (C) DeclRefExpr(reinterpretCast, DeclNameLoc(), /*implicit*/ true);
    auto reinterpreted = CallExpr::createImplicit(C, reinterpretCastRef,
                                                  { selfRef },
                                                  { Identifier() });
    reinterpreted->setThrows(false);
    auto ret = new (C) ReturnStmt(SourceLoc(), reinterpreted);
    auto body = BraceStmt::create(C, SourceLoc(), ASTNode(ret), SourceLoc(),
                                  /*implicit*/ true);
    getterDecl->setBody(body);
    getterDecl->getAttrs().add(new (C) TransparentAttr(/*implicit*/ true));
    Impl.registerExternalDecl(getterDecl);
  }

  // Synthesize the setter body
  {
    auto inoutSelfDecl = setterDecl->getImplicitSelfDecl();

    auto inoutSelfRef = new (C) DeclRefExpr(inoutSelfDecl, DeclNameLoc(),
                                            /*implicit*/ true);
    auto inoutSelf = new (C) InOutExpr(SourceLoc(), inoutSelfRef,
      importedUnionDecl->getDeclaredType(), /*implicit*/ true);

    auto newValueDecl = setterDecl->getParameters()->get(0);

    auto newValueRef = new (C) DeclRefExpr(newValueDecl, DeclNameLoc(),
                                           /*implicit*/ true);
    auto addressofFn = cast<FuncDecl>(getBuiltinValueDecl(
      C, C.getIdentifier("addressof")));
    auto addressofFnRef
      = new (C) DeclRefExpr(addressofFn, DeclNameLoc(), /*implicit*/ true);
    auto selfPointer = CallExpr::createImplicit(C, addressofFnRef,
                                                { inoutSelf },
                                                { Identifier() });
    auto initializeFn = cast<FuncDecl>(getBuiltinValueDecl(
      C, C.getIdentifier("initialize")));
    auto initializeFnRef
      = new (C) DeclRefExpr(initializeFn, DeclNameLoc(), /*implicit*/ true);
    auto initialize = CallExpr::createImplicit(C, initializeFnRef,
                                               { newValueRef, selfPointer },
                                               { Identifier(), Identifier() });
    auto body = BraceStmt::create(C, SourceLoc(), { initialize }, SourceLoc(),
                                  /*implicit*/ true);
    setterDecl->setBody(body);
    setterDecl->getAttrs().add(new (C) TransparentAttr(/*implicit*/ true));
    Impl.registerExternalDecl(setterDecl);
  }

  return { getterDecl, setterDecl };
}

static clang::DeclarationName
getAccessorDeclarationName(clang::ASTContext &Ctx,
                           StructDecl *structDecl,
                           VarDecl *fieldDecl,
                           const char *suffix) {
  std::string id;
  llvm::raw_string_ostream IdStream(id);
  IdStream << "$" << structDecl->getName()
           << "$" << fieldDecl->getName()
           << "$" << suffix;

  return clang::DeclarationName(&Ctx.Idents.get(IdStream.str()));
}

/// Build the bitfield getter and setter using Clang.
///
/// \code
/// static inline int get(RecordType self) {
///   return self.field;
/// }
/// static inline void set(int newValue, RecordType *self) {
///   self->field = newValue;
/// }
/// \endcode
///
/// \returns a pair of the getter and setter function decls.
static std::pair<FuncDecl *, FuncDecl *>
makeBitFieldAccessors(ClangImporter::Implementation &Impl,
                      clang::RecordDecl *structDecl,
                      StructDecl *importedStructDecl,
                      clang::FieldDecl *fieldDecl,
                      VarDecl *importedFieldDecl) {
  clang::ASTContext &Ctx = Impl.getClangASTContext();

  // Getter: static inline FieldType get(RecordType self);
  auto recordType = Ctx.getRecordType(structDecl);
  auto recordPointerType = Ctx.getPointerType(recordType);
  auto fieldType = fieldDecl->getType();
  auto fieldNameInfo = clang::DeclarationNameInfo(fieldDecl->getDeclName(),
                                                  clang::SourceLocation());

  auto cGetterName = getAccessorDeclarationName(Ctx, importedStructDecl,
                                                importedFieldDecl, "getter");
  auto cGetterType = Ctx.getFunctionType(fieldDecl->getType(),
                                         recordType,
                                         clang::FunctionProtoType::ExtProtoInfo());
  auto cGetterTypeInfo = Ctx.getTrivialTypeSourceInfo(cGetterType);
  auto cGetterDecl = clang::FunctionDecl::Create(Ctx,
                                                 structDecl->getDeclContext(),
                                                 clang::SourceLocation(),
                                                 clang::SourceLocation(),
                                                 cGetterName,
                                                 cGetterType,
                                                 cGetterTypeInfo,
                                                 clang::SC_Static);
  cGetterDecl->setImplicitlyInline();
  assert(!cGetterDecl->isExternallyVisible());

  auto getterDecl = makeFieldGetterDecl(Impl,
                                        importedStructDecl,
                                        importedFieldDecl,
                                        cGetterDecl);

  // Setter: static inline void set(FieldType newValue, RecordType *self);
  SmallVector<clang::QualType, 8> cSetterParamTypes;
  cSetterParamTypes.push_back(fieldType);
  cSetterParamTypes.push_back(recordPointerType);

  auto cSetterName = getAccessorDeclarationName(Ctx, importedStructDecl,
                                                importedFieldDecl, "setter");
  auto cSetterType = Ctx.getFunctionType(Ctx.VoidTy,
                                         cSetterParamTypes,
                                         clang::FunctionProtoType::ExtProtoInfo());
  auto cSetterTypeInfo = Ctx.getTrivialTypeSourceInfo(cSetterType);
  
  auto cSetterDecl = clang::FunctionDecl::Create(Ctx,
                                                 structDecl->getDeclContext(),
                                                 clang::SourceLocation(),
                                                 clang::SourceLocation(),
                                                 cSetterName,
                                                 cSetterType,
                                                 cSetterTypeInfo,
                                                 clang::SC_Static);
  cSetterDecl->setImplicitlyInline();
  assert(!cSetterDecl->isExternallyVisible());

  auto setterDecl = makeFieldSetterDecl(Impl,
                                        importedStructDecl,
                                        importedFieldDecl,
                                        cSetterDecl);

  makeComputed(importedFieldDecl, getterDecl, setterDecl);

  // Don't bother synthesizing the body if we've already finished type-checking.
  if (Impl.hasFinishedTypeChecking())
    return { getterDecl, setterDecl };
  
  // Synthesize the getter body
  {
    auto cGetterSelfId = nullptr;
    auto recordTypeInfo = Ctx.getTrivialTypeSourceInfo(recordType);
    auto cGetterSelf = clang::ParmVarDecl::Create(Ctx, cGetterDecl,
                                                  clang::SourceLocation(),
                                                  clang::SourceLocation(),
                                                  cGetterSelfId,
                                                  recordType,
                                                  recordTypeInfo,
                                                  clang::SC_None,
                                                  nullptr);
    cGetterDecl->setParams(cGetterSelf);
    
    auto cGetterSelfExpr = new (Ctx) clang::DeclRefExpr(cGetterSelf, false,
                                                        recordType,
                                                        clang::VK_RValue,
                                                        clang::SourceLocation());
    auto cGetterExpr = new (Ctx) clang::MemberExpr(cGetterSelfExpr,
                                                   /*isarrow=*/ false,
                                                   clang::SourceLocation(),
                                                   fieldDecl,
                                                   fieldNameInfo,
                                                   fieldType,
                                                   clang::VK_RValue,
                                                   clang::OK_BitField);
    
    auto cGetterBody = new (Ctx) clang::ReturnStmt(clang::SourceLocation(),
                                                   cGetterExpr,
                                                   nullptr);
    cGetterDecl->setBody(cGetterBody);

    Impl.registerExternalDecl(getterDecl);
  }

  // Synthesize the setter body
  {
    SmallVector<clang::ParmVarDecl *, 2> cSetterParams;
    auto fieldTypeInfo = Ctx.getTrivialTypeSourceInfo(fieldType);
    auto cSetterValue = clang::ParmVarDecl::Create(Ctx, cSetterDecl,
                                                   clang::SourceLocation(),
                                                   clang::SourceLocation(),
                                                   /* nameID? */ nullptr,
                                                   fieldType,
                                                   fieldTypeInfo,
                                                   clang::SC_None,
                                                   nullptr);
    cSetterParams.push_back(cSetterValue);
    auto recordPointerTypeInfo = Ctx.getTrivialTypeSourceInfo(recordPointerType);
    auto cSetterSelf = clang::ParmVarDecl::Create(Ctx, cSetterDecl,
                                                  clang::SourceLocation(),
                                                  clang::SourceLocation(),
                                                  /* nameID? */ nullptr,
                                                  recordPointerType,
                                                  recordPointerTypeInfo,
                                                  clang::SC_None,
                                                  nullptr);
    cSetterParams.push_back(cSetterSelf);
    cSetterDecl->setParams(cSetterParams);
    
    auto cSetterSelfExpr = new (Ctx) clang::DeclRefExpr(cSetterSelf, false,
                                                        recordPointerType,
                                                        clang::VK_RValue,
                                                        clang::SourceLocation());
    
    auto cSetterMemberExpr = new (Ctx) clang::MemberExpr(cSetterSelfExpr,
                                                         /*isarrow=*/ true,
                                                         clang::SourceLocation(),
                                                         fieldDecl,
                                                         fieldNameInfo,
                                                         fieldType,
                                                         clang::VK_LValue,
                                                         clang::OK_BitField);
    
    auto cSetterValueExpr = new (Ctx) clang::DeclRefExpr(cSetterValue, false,
                                                         fieldType,
                                                         clang::VK_RValue,
                                                         clang::SourceLocation());
    
    auto cSetterExpr = new (Ctx) clang::BinaryOperator(cSetterMemberExpr,
                                                       cSetterValueExpr,
                                                       clang::BO_Assign,
                                                       fieldType,
                                                       clang::VK_RValue,
                                                       clang::OK_Ordinary,
                                                       clang::SourceLocation(),
                                                       clang::FPOptions());
    
    cSetterDecl->setBody(cSetterExpr);

    Impl.registerExternalDecl(setterDecl);
  }

  return { getterDecl, setterDecl };
}

/// Create a default constructor that initializes a struct to zero.
static ConstructorDecl *
createDefaultConstructor(ClangImporter::Implementation &Impl,
                         StructDecl *structDecl) {
  auto &context = Impl.SwiftContext;

  // Create the 'self' declaration.
  auto selfDecl = ParamDecl::createSelf(SourceLoc(), structDecl,
                                        /*static*/ false, /*inout*/ true);

  // self & param.
  auto emptyPL = ParameterList::createEmpty(context);

  // Create the constructor.
  DeclName name(context, DeclBaseName::createConstructor(), emptyPL);
  auto constructor = new (context) ConstructorDecl(
      name, structDecl->getLoc(), OTK_None, /*FailabilityLoc=*/SourceLoc(),
      /*Throws=*/false, /*ThrowsLoc=*/SourceLoc(), selfDecl, emptyPL,
      /*GenericParams=*/nullptr, structDecl);

  // Set the constructor's type.
  constructor->computeType();
  constructor->setValidationToChecked();

  constructor->setAccess(AccessLevel::Public);

  // Mark the constructor transparent so that we inline it away completely.
  constructor->getAttrs().add(new (context) TransparentAttr(/*implicit*/ true));

  if (Impl.hasFinishedTypeChecking())
    return constructor;

  // Use a builtin to produce a zero initializer, and assign it to self.

  // Construct the left-hand reference to self.
  Expr *lhs = new (context) DeclRefExpr(selfDecl,
                                        DeclNameLoc(), /*Implicit=*/true);
  auto selfType = structDecl->getDeclaredInterfaceType();
  lhs->setType(LValueType::get(selfType));

  auto emptyTuple = TupleType::getEmpty(context);

  // Construct the right-hand call to Builtin.zeroInitializer.
  Identifier zeroInitID = context.getIdentifier("zeroInitializer");
  auto zeroInitializerFunc =
    cast<FuncDecl>(getBuiltinValueDecl(context, zeroInitID));
  SubstitutionMap subMap =
    SubstitutionMap::get(zeroInitializerFunc->getGenericSignature(),
                         llvm::makeArrayRef(selfType), { });
  ConcreteDeclRef concreteDeclRef(zeroInitializerFunc, subMap);
  auto zeroInitializerRef =
    new (context) DeclRefExpr(concreteDeclRef, DeclNameLoc(),
                              /*implicit*/ true);
  zeroInitializerRef->setType(FunctionType::get(emptyTuple, selfType));

  auto call = CallExpr::createImplicit(context, zeroInitializerRef, {}, {});
  call->setType(selfType);
  call->setThrows(false);

  auto assign = new (context) AssignExpr(lhs, SourceLoc(), call,
                                         /*implicit*/ true);
  assign->setType(emptyTuple);

  auto result = TupleExpr::createEmpty(context, SourceLoc(), SourceLoc(),
                                       /*Implicit=*/true);
  result->setType(emptyTuple);

  auto ret = new (context) ReturnStmt(SourceLoc(), result, /*Implicit=*/true);

  // Create the function body.
  auto body = BraceStmt::create(context, SourceLoc(), {assign, ret},
                                SourceLoc());
  constructor->setBody(body);
  constructor->setBodyTypeCheckedIfPresent();

  // Add this as an external definition.
  Impl.registerExternalDecl(constructor);

  // We're done.
  return constructor;
}

/// \brief Create a constructor that initializes a struct from its members.
static ConstructorDecl *
createValueConstructor(ClangImporter::Implementation &Impl,
                       StructDecl *structDecl, ArrayRef<VarDecl *> members,
                       bool wantCtorParamNames, bool wantBody) {
  auto &context = Impl.SwiftContext;

  // Create the 'self' declaration.
  auto selfDecl = ParamDecl::createSelf(SourceLoc(), structDecl,
                                        /*static*/ false, /*inout*/ true);

  // Construct the set of parameters from the list of members.
  SmallVector<ParamDecl *, 8> valueParameters;
  for (auto var : members) {
    bool generateParamName = wantCtorParamNames;

    if (var->hasClangNode()) {
      // TODO create value constructor with indirect fields instead of the
      // generated __Anonymous_field.
      if (isa<clang::IndirectFieldDecl>(var->getClangDecl()))
        continue;

      if (auto clangField = dyn_cast<clang::FieldDecl>(var->getClangDecl()))
        if (clangField->isAnonymousStructOrUnion())
          generateParamName = false;
    }

    Identifier argName = generateParamName ? var->getName() : Identifier();
    auto param = new (context)
        ParamDecl(VarDecl::Specifier::Default, SourceLoc(), SourceLoc(), argName,
                  SourceLoc(), var->getName(), var->getType(), structDecl);
    param->setInterfaceType(var->getInterfaceType());
    param->setValidationToChecked();
    Impl.recordImplicitUnwrapForDecl(
        param, var->getAttrs().hasAttribute<ImplicitlyUnwrappedOptionalAttr>());
    valueParameters.push_back(param);
  }

  auto *paramList = ParameterList::create(context, valueParameters);

  // Create the constructor
  DeclName name(context, DeclBaseName::createConstructor(), paramList);
  auto constructor = new (context) ConstructorDecl(
      name, structDecl->getLoc(), OTK_None, /*FailabilityLoc=*/SourceLoc(),
      /*Throws=*/false, /*ThrowsLoc=*/SourceLoc(), selfDecl, paramList,
      /*GenericParams=*/nullptr, structDecl);

  // Set the constructor's type.
  constructor->computeType();
  constructor->setValidationToChecked();

  constructor->setAccess(AccessLevel::Public);

  // Make the constructor transparent so we inline it away completely.
  constructor->getAttrs().add(new (context) TransparentAttr(/*implicit*/ true));

  if (wantBody) {
    // Assign all of the member variables appropriately.
    SmallVector<ASTNode, 4> stmts;

    // To keep DI happy, initialize stored properties before computed.
    for (unsigned pass = 0; pass < 2; pass++) {
      unsigned paramPos = 0;

      for (unsigned i = 0, e = members.size(); i < e; i++) {
        auto var = members[i];

        if (var->hasClangNode() && isa<clang::IndirectFieldDecl>(var->getClangDecl()))
          continue;

        if (var->hasStorage() == (pass != 0)) {
          paramPos++;
          continue;
        }

        // Construct left-hand side.
        Expr *lhs = new (context) DeclRefExpr(selfDecl, DeclNameLoc(),
                                              /*Implicit=*/true);
        lhs->setType(LValueType::get(selfDecl->getType()));

        auto semantics = (var->hasStorage()
                          ? AccessSemantics::DirectToStorage
                          : AccessSemantics::Ordinary);

        lhs = new (context) MemberRefExpr(lhs, SourceLoc(), var, DeclNameLoc(),
                                          /*Implicit=*/true,
                                          semantics);
        lhs->setType(LValueType::get(var->getType()));

        // Construct right-hand side.
        auto rhs = new (context) DeclRefExpr(valueParameters[paramPos],
                                             DeclNameLoc(),
                                             /*Implicit=*/true);
        rhs->setType(valueParameters[paramPos]->getType());

        // Add assignment.
        auto assign = new (context) AssignExpr(lhs, SourceLoc(), rhs,
                                               /*Implicit=*/true);
        assign->setType(TupleType::getEmpty(context));

        stmts.push_back(assign);
        paramPos++;
      }
    }

    auto result = TupleExpr::createEmpty(context, SourceLoc(), SourceLoc(),
                                         /*Implicit=*/true);
    result->setType(TupleType::getEmpty(context));

    auto ret = new (context) ReturnStmt(SourceLoc(), result, /*Implicit=*/true);
    stmts.push_back(ret);

    // Create the function body.
    auto body = BraceStmt::create(context, SourceLoc(), stmts, SourceLoc());
    constructor->setBody(body);
    constructor->setBodyTypeCheckedIfPresent();
  }

  // Add this as an external definition.
  Impl.registerExternalDecl(constructor);

  // We're done.
  return constructor;
}

static void addSynthesizedProtocolAttrs(
    ClangImporter::Implementation &Impl,
    NominalTypeDecl *nominal,
    ArrayRef<KnownProtocolKind> synthesizedProtocolAttrs) {
  for (auto kind : synthesizedProtocolAttrs) {
    nominal->getAttrs().add(new (Impl.SwiftContext)
                                 SynthesizedProtocolAttr(kind, &Impl));
  }
}

/// Add a synthesized typealias to the given nominal type.
static void addSynthesizedTypealias(NominalTypeDecl *nominal, Identifier name,
                                    Type underlyingType) {
  auto &ctx = nominal->getASTContext();

  auto typealias = new (ctx) TypeAliasDecl(SourceLoc(), SourceLoc(),
                                           name, SourceLoc(),
                                           nullptr, nominal);
  typealias->setUnderlyingType(underlyingType);
  typealias->setEarlyAttrValidation(true);
  typealias->setAccess(AccessLevel::Public);
  typealias->setValidationToChecked();
  typealias->setImplicit();

  nominal->addMember(typealias);
}

/// Make a struct declaration into a raw-value-backed struct
///
/// \param structDecl the struct to make a raw value for
/// \param underlyingType the type of the raw value
/// \param synthesizedProtocolAttrs synthesized protocol attributes to add
/// \param setterAccess the access level of the raw value's setter
///
/// This will perform most of the work involved in making a new Swift struct
/// be backed by a raw value. This will populated derived protocols and
/// synthesized protocols, add the new variable and pattern bindings, and
/// create the inits parameterized over a raw value
///
static void makeStructRawValued(
    ClangImporter::Implementation &Impl, StructDecl *structDecl,
    Type underlyingType, ArrayRef<KnownProtocolKind> synthesizedProtocolAttrs,
    MakeStructRawValuedOptions options = getDefaultMakeStructRawValuedOptions(),
    AccessLevel setterAccess = AccessLevel::Private) {
  auto &ctx = Impl.SwiftContext;

  addSynthesizedProtocolAttrs(Impl, structDecl, synthesizedProtocolAttrs);

  // Create a variable to store the underlying value.
  VarDecl *var;
  PatternBindingDecl *patternBinding;
  auto specifier = options.contains(MakeStructRawValuedFlags::IsLet)
                 ? VarDecl::Specifier::Let
                 : VarDecl::Specifier::Var;
  std::tie(var, patternBinding) = createVarWithPattern(
      ctx, structDecl, ctx.Id_rawValue, underlyingType,
      specifier,
      options.contains(MakeStructRawValuedFlags::IsImplicit),
      AccessLevel::Public,
      setterAccess);

  // Create the getter for the computed value variable.
  auto varGetter = makeStructRawValueGetter(
      Impl, structDecl, var, var);

  var->setSynthesizedGetter(varGetter);
  assert(var->hasStorage());

  // Create constructors to initialize that value from a value of the
  // underlying type.
  if (options.contains(MakeStructRawValuedFlags::MakeUnlabeledValueInit))
    structDecl->addMember(
        createValueConstructor(Impl, structDecl, var,
                               /*wantCtorParamNames=*/false,
                               /*wantBody=*/!Impl.hasFinishedTypeChecking()));
  structDecl->addMember(
      createValueConstructor(Impl, structDecl, var,
                             /*wantCtorParamNames=*/true,
                             /*wantBody=*/!Impl.hasFinishedTypeChecking()));
  structDecl->addMember(patternBinding);
  structDecl->addMember(var);
  structDecl->addMember(varGetter);

  addSynthesizedTypealias(structDecl, ctx.Id_RawValue, underlyingType);
}

/// Create a rawValue-ed constructor that bridges to its underlying storage.
static ConstructorDecl *createRawValueBridgingConstructor(
    ClangImporter::Implementation &Impl, StructDecl *structDecl,
    VarDecl *computedRawValue, VarDecl *storedRawValue, bool wantLabel,
    bool wantBody) {
  auto &ctx = Impl.SwiftContext;
  auto init = createValueConstructor(Impl, structDecl, computedRawValue,
                                     /*wantCtorParamNames=*/wantLabel,
                                     /*wantBody=*/false);
  // Insert our custom init body
  if (wantBody) {
    auto selfDecl = init->getImplicitSelfDecl();
    auto storedType = storedRawValue->getInterfaceType();

    // Construct left-hand side.
    Expr *lhs = new (ctx) DeclRefExpr(selfDecl, DeclNameLoc(),
                                      /*Implicit=*/true);
    lhs->setType(LValueType::get(selfDecl->getType()));

    lhs = new (ctx) MemberRefExpr(lhs, SourceLoc(), storedRawValue,
                                  DeclNameLoc(), /*Implicit=*/true,
                                  AccessSemantics::DirectToStorage);
    lhs->setType(LValueType::get(storedType));

    // Construct right-hand side.
    // FIXME: get the parameter from the init, and plug it in here.
    auto *paramDecl = init->getParameters()->get(0);
    auto *paramRef = new (ctx) DeclRefExpr(
        paramDecl, DeclNameLoc(), /*Implicit=*/true);
    paramRef->setType(paramDecl->getType());

    Expr *rhs = paramRef;
    if (!storedRawValue->getInterfaceType()->isEqual(paramDecl->getType())) {
      auto bridge = new (ctx) BridgeToObjCExpr(paramRef, storedType);
      bridge->setType(storedType);

      auto coerce = new (ctx) CoerceExpr(bridge, SourceLoc(),
                                         {nullptr, storedType});
      coerce->setType(storedType);

      rhs = coerce;
    }

    // Add assignment.
    auto assign = new (ctx) AssignExpr(lhs, SourceLoc(), rhs,
                                       /*Implicit=*/true);
    assign->setType(TupleType::getEmpty(ctx));

    auto result = TupleExpr::createEmpty(ctx, SourceLoc(), SourceLoc(),
                                         /*Implicit=*/true);
    auto ret = new (ctx) ReturnStmt(SourceLoc(), result, /*Implicit=*/true);

    auto body = BraceStmt::create(ctx, SourceLoc(), {assign, ret}, SourceLoc());
    init->setBody(body);
    init->setBodyTypeCheckedIfPresent();
  }

  return init;
}

/// Make a struct declaration into a raw-value-backed struct, with
/// bridged computed rawValue property which differs from stored backing
///
/// \param structDecl the struct to make a raw value for
/// \param storedUnderlyingType the type of the stored raw value
/// \param bridgedType the type of the 'rawValue' computed property bridge
/// \param synthesizedProtocolAttrs synthesized protocol attributes to add
///
/// This will perform most of the work involved in making a new Swift struct
/// be backed by a stored raw value and computed raw value of bridged type.
/// This will populated derived protocols and synthesized protocols, add the
/// new variable and pattern bindings, and create the inits parameterized
/// over a bridged type that will cast to the stored type, as appropriate.
///
static void makeStructRawValuedWithBridge(
    ClangImporter::Implementation &Impl, StructDecl *structDecl,
    Type storedUnderlyingType, Type bridgedType,
    ArrayRef<KnownProtocolKind> synthesizedProtocolAttrs,
    bool makeUnlabeledValueInit = false) {
  auto &ctx = Impl.SwiftContext;

  addSynthesizedProtocolAttrs(Impl, structDecl, synthesizedProtocolAttrs);

  auto storedVarName = ctx.getIdentifier("_rawValue");
  auto computedVarName = ctx.Id_rawValue;

  // Create a variable to store the underlying value.
  VarDecl *storedVar;
  PatternBindingDecl *storedPatternBinding;
  std::tie(storedVar, storedPatternBinding) = createVarWithPattern(
      ctx, structDecl, storedVarName, storedUnderlyingType,
      VarDecl::Specifier::Var, /*isImplicit=*/true,
      AccessLevel::Private,
      AccessLevel::Private);

  // Create a computed value variable.
  auto computedVar = new (ctx) VarDecl(
      /*IsStatic*/false, VarDecl::Specifier::Var, /*IsCaptureList*/false,
      SourceLoc(), computedVarName, bridgedType, structDecl);
  computedVar->setInterfaceType(bridgedType);
  computedVar->setImplicit();
  computedVar->setAccess(AccessLevel::Public);
  computedVar->setSetterAccess(AccessLevel::Private);
  computedVar->setValidationToChecked();

  // Create the getter for the computed value variable.
  auto computedVarGetter = makeStructRawValueGetter(
      Impl, structDecl, computedVar, storedVar);
  makeComputed(computedVar, computedVarGetter, nullptr);

  // Create a pattern binding to describe the variable.
  Pattern *computedVarPattern = createTypedNamedPattern(computedVar);
  auto *computedPatternBinding = PatternBindingDecl::createImplicit(
      ctx, StaticSpellingKind::None, computedVarPattern, /*InitExpr*/ nullptr,
      structDecl);

  // Don't bother synthesizing the body if we've already finished
  // type-checking.
  bool wantBody = !Impl.hasFinishedTypeChecking();

  auto init = createRawValueBridgingConstructor(Impl, structDecl, computedVar,
                                                storedVar,
                                                /*wantLabel*/ true, wantBody);

  ConstructorDecl *unlabeledCtor = nullptr;
  if (makeUnlabeledValueInit)
    unlabeledCtor = createRawValueBridgingConstructor(
        Impl, structDecl, computedVar, storedVar,
        /*wantLabel*/ false, wantBody);

  if (unlabeledCtor)
    structDecl->addMember(unlabeledCtor);
  structDecl->addMember(init);
  structDecl->addMember(storedPatternBinding);
  structDecl->addMember(storedVar);
  structDecl->addMember(computedPatternBinding);
  structDecl->addMember(computedVar);
  structDecl->addMember(computedVarGetter);

  addSynthesizedTypealias(structDecl, ctx.Id_RawValue, bridgedType);
}

/// Build a declaration for an Objective-C subscript getter.
static AccessorDecl *
buildSubscriptGetterDecl(ClangImporter::Implementation &Impl,
                         SubscriptDecl *subscript, const FuncDecl *getter,
                         Type elementTy, DeclContext *dc, ParamDecl *index) {
  auto &C = Impl.SwiftContext;
  auto loc = getter->getLoc();

  // self & index.
  auto *selfDecl = ParamDecl::createSelf(SourceLoc(), dc);
  auto *params = ParameterList::create(C, index);

  // Create the getter thunk.
  auto thunk = AccessorDecl::create(C,
                     /*FuncLoc=*/loc,
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Get,
                     AddressorKind::NotAddressor,
                     subscript,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr,
                     selfDecl, params,
                     TypeLoc::withoutLoc(elementTy), dc,
                     getter->getClangNode());

  thunk->setGenericEnvironment(dc->getGenericEnvironmentOfContext());
  thunk->computeType();
  thunk->setValidationToChecked();

  thunk->setAccess(getOverridableAccessLevel(dc));

  if (auto objcAttr = getter->getAttrs().getAttribute<ObjCAttr>())
    thunk->getAttrs().add(objcAttr->clone(C));
  thunk->setIsObjC(getter->isObjC());
  // FIXME: Should we record thunks?

  return thunk;
}

/// Build a declaration for an Objective-C subscript setter.
static AccessorDecl *
buildSubscriptSetterDecl(ClangImporter::Implementation &Impl,
                         SubscriptDecl *subscript, const FuncDecl *setter,
                         Type elementInterfaceTy,
                         DeclContext *dc, ParamDecl *index) {
  auto &C = Impl.SwiftContext;
  auto loc = setter->getLoc();

  // Objective-C subscript setters are imported with a function type
  // such as:
  //
  //   (self) -> (value, index) -> ()
  //
  // Build a setter thunk with the latter signature that maps to the
  // former.
  auto valueIndex = setter->getParameters();

  // 'self'
  auto selfDecl = ParamDecl::createSelf(SourceLoc(), dc);
  auto elementTy = dc->mapTypeIntoContext(elementInterfaceTy);

  auto paramVarDecl =
      new (C) ParamDecl(VarDecl::Specifier::Default, SourceLoc(), SourceLoc(),
                        Identifier(), loc, valueIndex->get(0)->getName(),
                        elementTy, dc);
  paramVarDecl->setInterfaceType(elementInterfaceTy);
  paramVarDecl->setValidationToChecked();

  auto valueIndicesPL = ParameterList::create(C, {paramVarDecl, index});

  // Create the setter thunk.
  auto thunk = AccessorDecl::create(C,
                     /*FuncLoc=*/setter->getLoc(),
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Set,
                     AddressorKind::NotAddressor,
                     subscript,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr,
                     selfDecl, valueIndicesPL,
                     TypeLoc::withoutLoc(TupleType::getEmpty(C)), dc,
                     setter->getClangNode());
  thunk->setGenericEnvironment(dc->getGenericEnvironmentOfContext());
  thunk->computeType();
  thunk->setValidationToChecked();

  thunk->setAccess(getOverridableAccessLevel(dc));

  if (auto objcAttr = setter->getAttrs().getAttribute<ObjCAttr>())
    thunk->getAttrs().add(objcAttr->clone(C));
  thunk->setIsObjC(setter->isObjC());

  return thunk;
}

/// Retrieve the element interface type and key param decl of a subscript
/// setter.
static std::pair<Type, ParamDecl *> decomposeSubscriptSetter(FuncDecl *setter) {
  auto *PL = setter->getParameters();
  if (PL->size() != 2)
    return {nullptr, nullptr};

  // Setter type is (self) -> (elem_type, key_type) -> ()
  Type elementType = setter->getInterfaceType()
                         ->castTo<AnyFunctionType>()
                         ->getResult()
                         ->castTo<AnyFunctionType>()
                         ->getParams().front().getType();
  ParamDecl *keyDecl = PL->get(1);

  return {elementType, keyDecl};
}

/// Rectify the (possibly different) types determined by the
/// getter and setter for a subscript.
///
/// \param canUpdateType whether the type of subscript can be
/// changed from the getter type to something compatible with both
/// the getter and the setter.
///
/// \returns the type to be used for the subscript, or a null type
/// if the types cannot be rectified.
static ImportedType rectifySubscriptTypes(Type getterType, bool getterIsIUO,
                                          Type setterType, bool canUpdateType) {
  // If the caller couldn't provide a setter type, there is
  // nothing to rectify.
  if (!setterType)
    return {nullptr, false};

  // Trivial case: same type in both cases.
  if (getterType->isEqual(setterType))
    return {getterType, getterIsIUO};

  // The getter/setter types are different. If we cannot update
  // the type, we have to fail.
  if (!canUpdateType)
    return {nullptr, false};

  // Unwrap one level of optionality from each.
  if (Type getterObjectType = getterType->getOptionalObjectType())
    getterType = getterObjectType;
  if (Type setterObjectType = setterType->getOptionalObjectType())
    setterType = setterObjectType;

  // If they are still different, fail.
  // FIXME: We could produce the greatest common supertype of the
  // two types.
  if (!getterType->isEqual(setterType))
    return {nullptr, false};

  // Create an optional of the object type that can be implicitly
  // unwrapped which subsumes both behaviors.
  return {OptionalType::get(setterType), true};
}

/// Add an AvailableAttr to the declaration for the given
/// version range.
static void applyAvailableAttribute(Decl *decl, AvailabilityContext &info,
                                    ASTContext &C) {
  // If the range is "all", this is the same as not having an available
  // attribute.
  if (info.isAlwaysAvailable())
    return;

  clang::VersionTuple noVersion;
  auto AvAttr = new (C) AvailableAttr(SourceLoc(), SourceRange(),
                                      targetPlatform(C.LangOpts),
                                      /*Message=*/StringRef(),
                                      /*Rename=*/StringRef(),
                                      info.getOSVersion().getLowerEndpoint(),
                                      /*IntroducedRange*/SourceRange(),
                                      /*Deprecated=*/noVersion,
                                      /*DeprecatedRange*/SourceRange(),
                                      /*Obsoleted=*/noVersion,
                                      /*ObsoletedRange*/SourceRange(),
                                      PlatformAgnosticAvailabilityKind::None,
                                      /*Implicit=*/false);

  decl->getAttrs().add(AvAttr);
}

/// Synthesize availability attributes for protocol requirements
/// based on availability of the types mentioned in the requirements.
static void inferProtocolMemberAvailability(ClangImporter::Implementation &impl,
                                            DeclContext *dc, Decl *member) {
  // Don't synthesize attributes if there is already an
  // availability annotation.
  if (member->getAttrs().hasAttribute<AvailableAttr>())
    return;

  auto *valueDecl = dyn_cast<ValueDecl>(member);
  if (!valueDecl)
    return;

  AvailabilityContext requiredRange =
      AvailabilityInference::inferForType(valueDecl->getInterfaceType());

  ASTContext &C = impl.SwiftContext;

  const Decl *innermostDecl = dc->getInnermostDeclarationDeclContext();
  AvailabilityContext containingDeclRange =
      AvailabilityInference::availableRange(innermostDecl, C);

  requiredRange.intersectWith(containingDeclRange);

  applyAvailableAttribute(valueDecl, requiredRange, C);
}

/// Add a domain error member, as required by conformance to
/// _BridgedStoredNSError.
/// \returns true on success, false on failure
static bool addErrorDomain(NominalTypeDecl *swiftDecl,
                           clang::NamedDecl *errorDomainDecl,
                           ClangImporter::Implementation &importer) {
  auto &C = importer.SwiftContext;
  auto swiftValueDecl = dyn_cast_or_null<ValueDecl>(
      importer.importDecl(errorDomainDecl, importer.CurrentVersion));
  auto stringTy = C.getStringDecl()->getDeclaredType();
  assert(stringTy && "no string type available");
  if (!swiftValueDecl || !swiftValueDecl->getInterfaceType()->isEqual(stringTy)) {
    // Couldn't actually import it as an error enum, fall back to enum
    return false;
  }

  bool isStatic = true;
  bool isImplicit = true;

  // Make the property decl
  auto errorDomainPropertyDecl = new (C) VarDecl(
      /*IsStatic*/isStatic, VarDecl::Specifier::Var, /*IsCaptureList*/false,
      SourceLoc(), C.Id_errorDomain, stringTy, swiftDecl);
  errorDomainPropertyDecl->setInterfaceType(stringTy);
  errorDomainPropertyDecl->setValidationToChecked();
  errorDomainPropertyDecl->setAccess(AccessLevel::Public);

  DeclRefExpr *domainDeclRef = new (C)
      DeclRefExpr(ConcreteDeclRef(swiftValueDecl), {}, isImplicit);
  auto *selfDecl = ParamDecl::createSelf(SourceLoc(), swiftDecl, isStatic);
  auto *params = ParameterList::createEmpty(C);

  auto getterDecl = AccessorDecl::create(C,
                     /*FuncLoc=*/SourceLoc(),
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Get,
                     AddressorKind::NotAddressor,
                     errorDomainPropertyDecl,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr,
                     selfDecl, params,
                     TypeLoc::withoutLoc(stringTy), swiftDecl);
  getterDecl->setStatic(isStatic);
  getterDecl->computeType();
  getterDecl->setValidationToChecked();
  getterDecl->setIsObjC(false);

  swiftDecl->addMember(errorDomainPropertyDecl);
  swiftDecl->addMember(getterDecl);
  makeComputed(errorDomainPropertyDecl, getterDecl, nullptr);

  getterDecl->setImplicit();
  getterDecl->setStatic(isStatic);
  getterDecl->setAccess(AccessLevel::Public);

  auto ret = new (C) ReturnStmt(SourceLoc(), domainDeclRef);
  getterDecl->setBody(
      BraceStmt::create(C, SourceLoc(), {ret}, SourceLoc(), isImplicit));
  importer.registerExternalDecl(getterDecl);
  return true;
}

/// As addErrorDomain above, but performs a lookup
static bool addErrorDomain(NominalTypeDecl *swiftDecl,
                           StringRef errorDomainName,
                           ClangImporter::Implementation &importer) {
  auto &clangSema = importer.getClangSema();
  clang::IdentifierInfo *errorDomainDeclName =
    &clangSema.getASTContext().Idents.get(errorDomainName);
  clang::LookupResult lookupResult(
      clangSema, clang::DeclarationName(errorDomainDeclName),
      clang::SourceLocation(), clang::Sema::LookupNameKind::LookupOrdinaryName);

  if (!clangSema.LookupName(lookupResult, clangSema.TUScope)) {
    // Couldn't actually import it as an error enum, fall back to enum
    return false;
  }

  auto clangNamedDecl = lookupResult.getAsSingle<clang::NamedDecl>();
  if (!clangNamedDecl) {
    // Couldn't actually import it as an error enum, fall back to enum
    return false;
  }

  return addErrorDomain(swiftDecl, clangNamedDecl, importer);
}

/// Retrieve the property type as determined by the given accessor.
static clang::QualType
getAccessorPropertyType(const clang::FunctionDecl *accessor, bool isSetter,
                        Optional<unsigned> selfIndex) {
  // Simple case: the property type of the getter is in the return
  // type.
  if (!isSetter) return accessor->getReturnType();

  // For the setter, first check that we have the right number of
  // parameters.
  unsigned numExpectedParams = selfIndex ? 2 : 1;
  if (accessor->getNumParams() != numExpectedParams)
    return clang::QualType();

  // Dig out the parameter for the value.
  unsigned valueIdx = selfIndex ? (1 - *selfIndex) : 0;
  auto param = accessor->getParamDecl(valueIdx);
  return param->getType();
}

/// Whether we should suppress importing the Objective-C generic type params
/// of this class as Swift generic type params.
static bool
shouldSuppressGenericParamsImport(const LangOptions &langOpts,
                                  const clang::ObjCInterfaceDecl *decl) {
  if (decl->hasAttr<clang::SwiftImportAsNonGenericAttr>())
    return true;

  // FIXME: This check is only necessary to keep things working even without
  // the SwiftImportAsNonGeneric API note. Once we can guarantee that that
  // attribute is present in all contexts, we can remove this check.
  auto isFromFoundationModule = [](const clang::Decl *decl) -> bool {
    clang::Module *module = getClangSubmoduleForDecl(decl).getValue();
    if (!module)
      return false;
    return module->getTopLevelModuleName() == "Foundation";
  };

  if (isFromFoundationModule(decl)) {
    // In Swift 3 we used a hardcoded list of declarations, and made all of
    // their subclasses drop their generic parameters when imported.
    while (decl) {
      StringRef name = decl->getName();
      if (name == "NSArray" || name == "NSDictionary" || name == "NSSet" ||
          name == "NSOrderedSet" || name == "NSEnumerator" ||
          name == "NSMeasurement") {
        return true;
      }
      decl = decl->getSuperClass();
    }
  }

  return false;
}

/// Determine if the given Objective-C instance method should also
/// be imported as a class method.
///
/// Objective-C root class instance methods are also reflected as
/// class methods.
static bool shouldAlsoImportAsClassMethod(FuncDecl *method) {
  // Only instance methods.
  if (!method->isInstanceMember())
    return false;

  // Must be a method within a class or extension thereof.
  auto classDecl =
      method->getDeclContext()->getAsClassOrClassExtensionContext();
  if (!classDecl)
    return false;

  // The class must not have a superclass.
  if (classDecl->getSuperclass())
    return false;

  // There must not already be a class method with the same
  // selector.
  auto objcClass =
      cast_or_null<clang::ObjCInterfaceDecl>(classDecl->getClangDecl());
  if (!objcClass)
    return false;

  auto objcMethod = cast_or_null<clang::ObjCMethodDecl>(method->getClangDecl());
  if (!objcMethod)
    return false;
  return !objcClass->getClassMethod(objcMethod->getSelector(),
                                    /*AllowHidden=*/true);
}

static bool
classImplementsProtocol(const clang::ObjCInterfaceDecl *constInterface,
                        const clang::ObjCProtocolDecl *constProto,
                        bool checkCategories) {
  auto interface = const_cast<clang::ObjCInterfaceDecl *>(constInterface);
  auto proto = const_cast<clang::ObjCProtocolDecl *>(constProto);
  return interface->ClassImplementsProtocol(proto, checkCategories);
}

static void
applyPropertyOwnership(VarDecl *prop,
                       clang::ObjCPropertyDecl::PropertyAttributeKind attrs) {
  Type ty = prop->getInterfaceType();
  if (auto innerTy = ty->getOptionalObjectType())
    ty = innerTy;
  if (!ty->is<GenericTypeParamType>() && !ty->isAnyClassReferenceType())
    return;

  ASTContext &ctx = prop->getASTContext();
  if (attrs & clang::ObjCPropertyDecl::OBJC_PR_copy) {
    prop->getAttrs().add(new (ctx) NSCopyingAttr(false));
    return;
  }
  if (attrs & clang::ObjCPropertyDecl::OBJC_PR_weak) {
    prop->getAttrs().add(new (ctx)
                             ReferenceOwnershipAttr(ReferenceOwnership::Weak));
    prop->setType(WeakStorageType::get(prop->getType(), ctx));
    prop->setInterfaceType(WeakStorageType::get(
        prop->getInterfaceType(), ctx));
    return;
  }
  if ((attrs & clang::ObjCPropertyDecl::OBJC_PR_assign) ||
      (attrs & clang::ObjCPropertyDecl::OBJC_PR_unsafe_unretained)) {
    prop->getAttrs().add(
        new (ctx) ReferenceOwnershipAttr(ReferenceOwnership::Unmanaged));
    prop->setType(UnmanagedStorageType::get(prop->getType(), ctx));
    prop->setInterfaceType(UnmanagedStorageType::get(
        prop->getInterfaceType(), ctx));
    return;
  }
}

/// Does this name refer to a method that might shadow Swift.print?
///
/// As a heuristic, methods that have a base name of 'print' but more than
/// one argument are left alone. These can still shadow Swift.print but are
/// less likely to be confused for it, at least.
static bool isPrintLikeMethod(DeclName name, const DeclContext *dc) {
  if (!name || name.isSpecial() || name.isSimpleName())
    return false;
  if (name.getBaseIdentifier().str() != "print")
    return false;
  if (!dc->isTypeContext())
    return false;
  if (name.getArgumentNames().size() > 1)
    return false;
  return true;
}

using MirroredMethodEntry =
  std::pair<const clang::ObjCMethodDecl*, ProtocolDecl*>;

namespace {
  /// Customized llvm::DenseMapInfo for storing borrowed APSInts.
  struct APSIntRefDenseMapInfo {
    static inline const llvm::APSInt *getEmptyKey() {
      return llvm::DenseMapInfo<const llvm::APSInt *>::getEmptyKey();
    }
    static inline const llvm::APSInt *getTombstoneKey() {
      return llvm::DenseMapInfo<const llvm::APSInt *>::getTombstoneKey();
    }
    static unsigned getHashValue(const llvm::APSInt *ptrVal) {
      assert(ptrVal != getEmptyKey() && ptrVal != getTombstoneKey());
      return llvm::hash_value(*ptrVal);
    }
    static bool isEqual(const llvm::APSInt *lhs, const llvm::APSInt *rhs) {
      if (lhs == rhs) return true;
      if (lhs == getEmptyKey() || rhs == getEmptyKey()) return false;
      if (lhs == getTombstoneKey() || rhs == getTombstoneKey()) return false;
      return *lhs == *rhs;
    }
  };

  /// \brief Convert Clang declarations into the corresponding Swift
  /// declarations.
  class SwiftDeclConverter
    : public clang::ConstDeclVisitor<SwiftDeclConverter, Decl *>
  {
    ClangImporter::Implementation &Impl;
    bool forwardDeclaration = false;
    ImportNameVersion version;

    /// The version that we're being asked to import for. May not be the version
    /// the user requested, as we may be forming an alternate for diagnostic
    /// purposes.
    ImportNameVersion getVersion() const { return version; }

    /// The actual language version the user requested we compile for.
    ImportNameVersion getActiveSwiftVersion() const {
      return Impl.CurrentVersion;
    }

    /// Whether the names we're importing are from the language version the user
    /// requested, or if these are decls from another version
    bool isActiveSwiftVersion() const {
      return getVersion() == getActiveSwiftVersion();
    }

    /// Import the name of the given entity.
    ///
    /// This version of importFullName introduces any context-specific
    /// name importing options (e.g., if we're importing the Swift 2 version).
    ///
    /// Note: Use this rather than calling Impl.importFullName directly!
    ImportedName importFullName(const clang::NamedDecl *D,
                                Optional<ImportedName> &correctSwiftName) {
      ImportNameVersion canonicalVersion = getActiveSwiftVersion();
      if (isa<clang::TypeDecl>(D) || isa<clang::ObjCContainerDecl>(D)) {
        canonicalVersion = ImportNameVersion::forTypes();
      }
      correctSwiftName = None;

      // First, import based on the Swift name of the canonical declaration:
      // the latest version for types and the current version for non-type
      // values. If that fails, we won't do anything.
      auto canonicalName = Impl.importFullName(D, canonicalVersion);
      if (!canonicalName)
        return ImportedName();

      if (getVersion() == canonicalVersion) {
        // Make sure we don't try to import the same type twice as canonical.
        if (canonicalVersion != getActiveSwiftVersion()) {
          auto activeName = Impl.importFullName(D, getActiveSwiftVersion());
          if (activeName &&
              activeName.getDeclName() == canonicalName.getDeclName() &&
              activeName.getEffectiveContext().equalsWithoutResolving(
                  canonicalName.getEffectiveContext())) {
            return ImportedName();
          }
        }

        return canonicalName;
      }

      // Special handling when we import using the older Swift name.
      //
      // Import using the alternate Swift name. If that fails, or if it's
      // identical to the active Swift name, we won't introduce an alternate
      // Swift name stub declaration.
      auto alternateName = Impl.importFullName(D, getVersion());
      if (!alternateName)
        return ImportedName();

      if (alternateName.getDeclName() == canonicalName.getDeclName() &&
          alternateName.getEffectiveContext().equalsWithoutResolving(
              canonicalName.getEffectiveContext())) {
        if (getVersion() == getActiveSwiftVersion()) {
          assert(canonicalVersion != getActiveSwiftVersion());
          return alternateName;
        }
        return ImportedName();
      }

      // Always use the active version as the preferred name, even if the
      // canonical name is a different version.
      correctSwiftName = Impl.importFullName(D, getActiveSwiftVersion());
      assert(correctSwiftName);

      return alternateName;
    }

    /// \brief Create a declaration name for anonymous enums, unions and
    /// structs.
    ///
    /// Since Swift does not natively support these features, we fake them by
    /// importing them as declarations with generated names. The generated name
    /// is derived from the name of the field in the outer type. Since the
    /// anonymous type is imported as a nested type of the outer type, this
    /// generated name will most likely be unique.
    ImportedName getClangDeclName(const clang::TagDecl *decl,
                                  Optional<ImportedName> &correctSwiftName) {
      // If we have a name for this declaration, use it.
      if (auto name = importFullName(decl, correctSwiftName))
        return name;

      // If that didn't succeed, check whether this is an anonymous tag declaration
      // with a corresponding typedef-name declaration.
      if (decl->getDeclName().isEmpty()) {
        if (auto *typedefForAnon = decl->getTypedefNameForAnonDecl())
          return importFullName(typedefForAnon, correctSwiftName);
      }

      if (!decl->isRecord())
        return ImportedName();

      // If the type has no name and no structure name, but is not anonymous,
      // generate a name for it. Specifically this is for cases like:
      //   struct a {
      //     struct {} z;
      //   }
      // Where the member z is an unnamed struct, but does have a member-name
      // and is accessible as a member of struct a.
      correctSwiftName = None;
      if (auto recordDecl = dyn_cast<clang::RecordDecl>(
                              decl->getLexicalDeclContext())) {
        for (auto field : recordDecl->fields()) {
          if (field->getType()->getAsTagDecl() == decl) {
            // Create a name for the declaration from the field name.
            std::string Id;
            llvm::raw_string_ostream IdStream(Id);

            const char *kind;
            if (decl->isStruct())
              kind = "struct";
            else if (decl->isUnion())
              kind = "union";
            else
              llvm_unreachable("unknown decl kind");

            IdStream << "__Unnamed_" << kind << "_";
            if (field->isAnonymousStructOrUnion()) {
              IdStream << "__Anonymous_field" << field->getFieldIndex();
            } else {
              IdStream << field->getName();
            }
            ImportedName Result;
            Result.setDeclName(Impl.SwiftContext.getIdentifier(IdStream.str()));
            Result.setEffectiveContext(decl->getDeclContext());
            return Result;
          }
        }
      }
      
      return ImportedName();
    }

    bool isFactoryInit(ImportedName &name) {
      return name &&
             name.getDeclName().getBaseName() == DeclBaseName::createConstructor() &&
             (name.getInitKind() == CtorInitializerKind::Factory ||
              name.getInitKind() == CtorInitializerKind::ConvenienceFactory);
    }

  public:
    explicit SwiftDeclConverter(ClangImporter::Implementation &impl,
                                ImportNameVersion vers)
      : Impl(impl), version(vers) { }

    bool hadForwardDeclaration() const {
      return forwardDeclaration;
    }

    Decl *VisitDecl(const clang::Decl *decl) {
      return nullptr;
    }

    Decl *VisitTranslationUnitDecl(const clang::TranslationUnitDecl *decl) {
      // Note: translation units are handled specially by importDeclContext.
      return nullptr;
    }

    Decl *VisitNamespaceDecl(const clang::NamespaceDecl *decl) {
      // FIXME: Implement once Swift has namespaces.
      return nullptr;
    }

    Decl *VisitUsingDirectiveDecl(const clang::UsingDirectiveDecl *decl) {
      // Never imported.
      return nullptr;
    }

    Decl *VisitNamespaceAliasDecl(const clang::NamespaceAliasDecl *decl) {
      // FIXME: Implement once Swift has namespaces.
      return nullptr;
    }

    Decl *VisitLabelDecl(const clang::LabelDecl *decl) {
      // Labels are function-local, and therefore never imported.
      return nullptr;
    }

    ClassDecl *importCFClassType(const clang::TypedefNameDecl *decl,
                                 Identifier className, CFPointeeInfo info,
                                 EffectiveClangContext effectiveContext);

    /// Mark the given declaration as an older Swift version variant of the
    /// current name.
    void markAsVariant(Decl *decl, ImportedName correctSwiftName) {
      // Types always import using the latest version. Make sure all names up
      // to that version are considered available.
      if (isa<TypeDecl>(decl)) {
        cast<TypeAliasDecl>(decl)->markAsCompatibilityAlias();

        if (getVersion() >= getActiveSwiftVersion())
          return;
      }

      // TODO: some versions should be deprecated instead of unavailable

      ASTContext &ctx = decl->getASTContext();
      llvm::SmallString<64> renamed;
      {
        // Render a swift_name string.
        llvm::raw_svector_ostream os(renamed);

        // If we're importing a global as a member, we need to provide the
        // effective context.
        Impl.printSwiftName(
            correctSwiftName, getActiveSwiftVersion(),
            /*fullyQualified=*/correctSwiftName.importAsMember(), os);
      }

      DeclAttribute *attr;
      if (isActiveSwiftVersion() || getVersion() == ImportNameVersion::raw()) {
        // "Raw" is the Objective-C name, which was never available in Swift.
        // Variants within the active version are usually declarations that
        // have been superseded, like the accessors of a property.
        attr = AvailableAttr::createPlatformAgnostic(
            ctx, /*Message*/StringRef(), ctx.AllocateCopy(renamed.str()),
            PlatformAgnosticAvailabilityKind::UnavailableInSwift);
      } else {
        unsigned majorVersion = getVersion().majorVersionNumber();
        unsigned minorVersion = getVersion().minorVersionNumber();
        if (getVersion() < getActiveSwiftVersion()) {
          // A Swift 2 name, for example, was obsoleted in Swift 3.
          // However, a Swift 4 name is obsoleted in Swift 4.2.
          // FIXME: it would be better to have a unified place
          // to represent Swift versions for API versioning.
          clang::VersionTuple obsoletedVersion =
            (majorVersion == 4 && minorVersion < 2)
                ? clang::VersionTuple(4, 2)
                : clang::VersionTuple(majorVersion + 1);
          attr = AvailableAttr::createPlatformAgnostic(
              ctx, /*Message*/StringRef(), ctx.AllocateCopy(renamed.str()),
              PlatformAgnosticAvailabilityKind::SwiftVersionSpecific,
              obsoletedVersion);
        } else {
          // Future names are introduced in their future version.
          assert(getVersion() > getActiveSwiftVersion());
          clang::VersionTuple introducedVersion =
            (majorVersion == 4 && minorVersion == 2)
                ? clang::VersionTuple(4, 2)
                : clang::VersionTuple(majorVersion);
          attr = new (ctx) AvailableAttr(
              SourceLoc(), SourceRange(), PlatformKind::none,
              /*Message*/StringRef(), ctx.AllocateCopy(renamed.str()),
              /*Introduced*/introducedVersion, SourceRange(),
              /*Deprecated*/clang::VersionTuple(), SourceRange(),
              /*Obsoleted*/clang::VersionTuple(), SourceRange(),
              PlatformAgnosticAvailabilityKind::SwiftVersionSpecific,
              /*Implicit*/false);
        }
      }

      decl->getAttrs().add(attr);
      decl->setImplicit();
    }

    /// Create a typealias for the name of a Clang type declaration in an
    /// alternate version of Swift.
    Decl *importCompatibilityTypeAlias(const clang::NamedDecl *decl,
                                       ImportedName compatibilityName,
                                       ImportedName correctSwiftName);

    /// Create a swift_newtype struct corresponding to a typedef. Returns
    /// nullptr if unable.
    Decl *importSwiftNewtype(const clang::TypedefNameDecl *decl,
                             clang::SwiftNewtypeAttr *newtypeAttr,
                             DeclContext *dc, Identifier name);

    Decl *VisitTypedefNameDecl(const clang::TypedefNameDecl *Decl) {
      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(Decl, correctSwiftName);
      auto Name = importedName.getDeclName().getBaseIdentifier();
      if (Name.empty())
        return nullptr;

      // If we've been asked to produce a compatibility stub, handle it via a
      // typealias.
      if (correctSwiftName)
        return importCompatibilityTypeAlias(Decl, importedName,
                                            *correctSwiftName);

      Type SwiftType;
      if (Decl->getDeclContext()->getRedeclContext()->isTranslationUnit()) {
        bool IsError;
        StringRef StdlibTypeName;
        MappedTypeNameKind NameMapping;
        std::tie(SwiftType, StdlibTypeName) =
            getSwiftStdlibType(Decl, Name, Impl, &IsError, NameMapping);

        if (IsError)
          return nullptr;

        // Import 'typedef struct __Blah *BlahRef;' and
        // 'typedef const void *FooRef;' as CF types if they have the
        // right attributes or match our list of known types.
        if (!SwiftType) {
          auto DC = Impl.importDeclContextOf(
              Decl, importedName.getEffectiveContext());
          if (!DC)
            return nullptr;

          if (auto pointee = CFPointeeInfo::classifyTypedef(Decl)) {
            // If the pointee is a record, consider creating a class type.
            if (pointee.isRecord()) {
              auto swiftClass = importCFClassType(
                  Decl, Name, pointee, importedName.getEffectiveContext());
              if (!swiftClass) return nullptr;

              Impl.SpecialTypedefNames[Decl->getCanonicalDecl()] =
                MappedTypeNameKind::DefineAndUse;
              return swiftClass;
            }

            // If the pointee is another CF typedef, create an extra typealias
            // for the name without "Ref", but not a separate type.
            if (pointee.isTypedef()) {
              auto underlying = cast_or_null<TypeDecl>(Impl.importDecl(
                  pointee.getTypedef(), getActiveSwiftVersion()));
              if (!underlying)
                return nullptr;

              // Check for a newtype
              if (auto newtypeAttr =
                      getSwiftNewtypeAttr(Decl, getVersion()))
                if (auto newtype =
                        importSwiftNewtype(Decl, newtypeAttr, DC, Name))
                  return newtype;

              // Create a typealias for this CF typedef.
              TypeAliasDecl *typealias = nullptr;
              typealias = Impl.createDeclWithClangNode<TypeAliasDecl>(
                            Decl, AccessLevel::Public,
                            Impl.importSourceLoc(Decl->getLocStart()),
                            SourceLoc(), Name,
                            Impl.importSourceLoc(Decl->getLocation()),
                            /*genericparams*/nullptr, DC);
              typealias->setUnderlyingType(
                  underlying->getDeclaredInterfaceType());

              Impl.SpecialTypedefNames[Decl->getCanonicalDecl()] =
                MappedTypeNameKind::DefineAndUse;
              return typealias;
            }

            // If the pointee is 'void', 'CFTypeRef', bring it
            // in specifically as AnyObject.
            if (pointee.isVoid()) {
              // Create a typealias for this CF typedef.
              TypeAliasDecl *typealias = nullptr;
              typealias = Impl.createDeclWithClangNode<TypeAliasDecl>(
                            Decl, AccessLevel::Public,
                            Impl.importSourceLoc(Decl->getLocStart()),
                            SourceLoc(), Name,
                            Impl.importSourceLoc(Decl->getLocation()),
                            /*genericparams*/nullptr, DC);
              typealias->setUnderlyingType(
                Impl.SwiftContext.getAnyObjectType());

              Impl.SpecialTypedefNames[Decl->getCanonicalDecl()] =
                MappedTypeNameKind::DefineAndUse;
              return typealias;
            }
          }
        }

        if (SwiftType) {
          // Note that this typedef-name is special.
          Impl.SpecialTypedefNames[Decl->getCanonicalDecl()] = NameMapping;

          if (NameMapping == MappedTypeNameKind::DoNothing) {
            // Record the remapping using the name of the Clang declaration.
            // This will be useful for type checker diagnostics when
            // a user tries to use the Objective-C/C type instead of the
            // Swift type.
            Impl.SwiftContext.RemappedTypes[Decl->getNameAsString()]
              = SwiftType;

            // Don't create an extra typealias in the imported module because
            // doing so will cause confusion (or even lookup ambiguity) between
            // the name in the imported module and the same name in the
            // standard library.
            if (auto *NAT =
                  dyn_cast<NameAliasType>(SwiftType.getPointer()))
              return NAT->getDecl();

            auto *NTD = SwiftType->getAnyNominal();
            assert(NTD);
            return NTD;
          }
        }
      }

      auto DC =
          Impl.importDeclContextOf(Decl, importedName.getEffectiveContext());
      if (!DC)
        return nullptr;

      // Check for swift_newtype
      if (!SwiftType)
        if (auto newtypeAttr = getSwiftNewtypeAttr(Decl, getVersion()))
          if (auto newtype = importSwiftNewtype(Decl, newtypeAttr, DC, Name))
            return newtype;

      if (!SwiftType) {
        // Import typedefs of blocks as their fully-bridged equivalent Swift
        // type. That matches how we want to use them in most cases. All other
        // types should be imported in a non-bridged way.
        clang::QualType ClangType = Decl->getUnderlyingType();
        SwiftType = Impl.importTypeIgnoreIUO(
            ClangType, ImportTypeKind::Typedef, isInSystemModule(DC),
            getTypedefBridgeability(Decl, ClangType), OTK_Optional);
      }

      if (!SwiftType)
        return nullptr;

      auto Loc = Impl.importSourceLoc(Decl->getLocation());
      auto Result = Impl.createDeclWithClangNode<TypeAliasDecl>(Decl,
                                      AccessLevel::Public,
                                      Impl.importSourceLoc(Decl->getLocStart()),
                                      SourceLoc(), Name,
                                      Loc,
                                      /*genericparams*/nullptr, DC);
      Result->setUnderlyingType(SwiftType);

      // Make Objective-C's 'id' unavailable.
      if (Impl.SwiftContext.LangOpts.EnableObjCInterop && isObjCId(Decl)) {
        auto attr = AvailableAttr::createPlatformAgnostic(
                      Impl.SwiftContext,
                      "'id' is not available in Swift; use 'Any'", "",
                      PlatformAgnosticAvailabilityKind::UnavailableInSwift);
        Result->getAttrs().add(attr);
      }

      return Result;
    }

    Decl *
    VisitUnresolvedUsingTypenameDecl(const
                                     clang::UnresolvedUsingTypenameDecl *decl) {
      // Note: only occurs in templates.
      return nullptr;
    }

    /// Import an NS_ENUM constant as a case of a Swift enum.
    Decl *importEnumCase(const clang::EnumConstantDecl *decl,
                         const clang::EnumDecl *clangEnum,
                         EnumDecl *theEnum,
                         Decl *swift3Decl = nullptr);

    /// Import an NS_OPTIONS constant as a static property of a Swift struct.
    ///
    /// This is also used to import enum case aliases.
    Decl *importOptionConstant(const clang::EnumConstantDecl *decl,
                               const clang::EnumDecl *clangEnum,
                               NominalTypeDecl *theStruct);

    /// Import \p alias as an alias for the imported constant \p original.
    ///
    /// This builds the getter in a way that's compatible with switch
    /// statements. Changing the body here may require changing
    /// TypeCheckPattern.cpp as well.
    Decl *importEnumCaseAlias(Identifier name,
                              const clang::EnumConstantDecl *alias,
                              ValueDecl *original,
                              const clang::EnumDecl *clangEnum,
                              NominalTypeDecl *importedEnum,
                              DeclContext *importIntoDC = nullptr);

    NominalTypeDecl *importAsOptionSetType(DeclContext *dc,
                                           Identifier name,
                                           const clang::EnumDecl *decl);

    Decl *VisitEnumDecl(const clang::EnumDecl *decl) {
      decl = decl->getDefinition();
      if (!decl) {
        forwardDeclaration = true;
        return nullptr;
      }

      Optional<ImportedName> correctSwiftName;
      auto importedName = getClangDeclName(decl, correctSwiftName);
      if (!importedName)
        return nullptr;

      // If we've been asked to produce a compatibility stub, handle it via a
      // typealias.
      if (correctSwiftName)
        return importCompatibilityTypeAlias(decl, importedName,
                                            *correctSwiftName);

      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;
      
      auto name = importedName.getDeclName().getBaseIdentifier();

      // Create the enum declaration and record it.
      StructDecl *errorWrapper = nullptr;
      NominalTypeDecl *result;
      auto enumInfo = Impl.getEnumInfo(decl);
      auto enumKind = enumInfo.getKind();
      switch (enumKind) {
      case EnumKind::Constants: {
        // There is no declaration. Rather, the type is mapped to the
        // underlying type.
        return nullptr;
      }

      case EnumKind::Unknown: {
        // Compute the underlying type of the enumeration.
        auto underlyingType = Impl.importTypeIgnoreIUO(
            decl->getIntegerType(), ImportTypeKind::Enum, isInSystemModule(dc),
            Bridgeability::None);
        if (!underlyingType)
          return nullptr;

        auto Loc = Impl.importSourceLoc(decl->getLocation());
        auto structDecl = Impl.createDeclWithClangNode<StructDecl>(decl,
          AccessLevel::Public, Loc, name, Loc, None, nullptr, dc);
        structDecl->computeType();
        structDecl->setAddedImplicitInitializers();

        auto options = getDefaultMakeStructRawValuedOptions();
        options |= MakeStructRawValuedFlags::MakeUnlabeledValueInit;
        options -= MakeStructRawValuedFlags::IsLet;
        options -= MakeStructRawValuedFlags::IsImplicit;

        makeStructRawValued(Impl, structDecl, underlyingType,
                            {KnownProtocolKind::RawRepresentable,
                             KnownProtocolKind::Equatable},
                            options, /*setterAccess=*/AccessLevel::Public);

        result = structDecl;
        break;
      }

      case EnumKind::NonFrozenEnum:
      case EnumKind::FrozenEnum: {
        auto &C = Impl.SwiftContext;
        EnumDecl *nativeDecl;
        bool declaredNative = hasNativeSwiftDecl(decl, name, dc, nativeDecl);
        if (declaredNative && nativeDecl)
          return nativeDecl;

        // Compute the underlying type.
        auto underlyingType = Impl.importTypeIgnoreIUO(
            decl->getIntegerType(), ImportTypeKind::Enum, isInSystemModule(dc),
            Bridgeability::None);
        if (!underlyingType)
          return nullptr;

        /// Basic information about the enum type we're building.
        Identifier enumName = name;
        DeclContext *enumDC = dc;
        SourceLoc loc = Impl.importSourceLoc(decl->getLocStart());

        // If this is an error enum, form the error wrapper type,
        // which is a struct containing an NSError instance.
        ProtocolDecl *bridgedNSError = nullptr;
        ClassDecl *nsErrorDecl = nullptr;
        ProtocolDecl *errorCodeProto = nullptr;
        if (enumInfo.isErrorEnum() && 
            (bridgedNSError =
               C.getProtocol(KnownProtocolKind::BridgedStoredNSError)) &&
            (nsErrorDecl = C.getNSErrorDecl()) &&
            (errorCodeProto =
               C.getProtocol(KnownProtocolKind::ErrorCodeProtocol))) {
          // Create the wrapper struct.
          errorWrapper = new (C) StructDecl(loc, name, loc, None, nullptr, dc);
          errorWrapper->computeType();
          errorWrapper->setValidationToChecked();
          errorWrapper->setAddedImplicitInitializers();
          errorWrapper->setAccess(AccessLevel::Public);
          errorWrapper->getAttrs().add(
            new (Impl.SwiftContext) FixedLayoutAttr(/*IsImplicit*/true));

          StringRef nameForMangling;
          ClangImporterSynthesizedTypeAttr::Kind relatedEntityKind;
          if (decl->getDeclName().isEmpty()) {
            nameForMangling = decl->getTypedefNameForAnonDecl()->getName();
            relatedEntityKind =
                ClangImporterSynthesizedTypeAttr::Kind::NSErrorWrapperAnon;
          } else {
            nameForMangling = decl->getName();
            relatedEntityKind =
                ClangImporterSynthesizedTypeAttr::Kind::NSErrorWrapper;
          }
          errorWrapper->getAttrs().add(new (C) ClangImporterSynthesizedTypeAttr(
              nameForMangling, relatedEntityKind));

          // Add inheritance clause.
          addSynthesizedProtocolAttrs(Impl, errorWrapper,
                                      {KnownProtocolKind::BridgedStoredNSError});

          // Create the _nsError member.
          //   public let _nsError: NSError
          auto nsErrorType = nsErrorDecl->getDeclaredInterfaceType();
          auto nsErrorProp = new (C) VarDecl(/*IsStatic*/false,
                                             VarDecl::Specifier::Let,
                                             /*IsCaptureList*/false,
                                             loc, C.Id_nsError, nsErrorType,
                                             errorWrapper);
          nsErrorProp->setImplicit();
          nsErrorProp->setAccess(AccessLevel::Public);
          nsErrorProp->setInterfaceType(nsErrorType);
          nsErrorProp->setValidationToChecked();

          // Create a pattern binding to describe the variable.
          Pattern *nsErrorPattern = createTypedNamedPattern(nsErrorProp);

          auto *nsErrorBinding = PatternBindingDecl::createImplicit(
              C, StaticSpellingKind::None, nsErrorPattern, /*InitExpr*/ nullptr,
              /*ParentDC*/ errorWrapper, /*VarLoc*/ loc);
          errorWrapper->addMember(nsErrorProp);
          errorWrapper->addMember(nsErrorBinding);

          // Create the _nsError initializer.
          //   public init(_nsError error: NSError)
          VarDecl *members[1] = { nsErrorProp };
          auto nsErrorInit = createValueConstructor(Impl, errorWrapper, members,
                                                    /*wantCtorParamNames=*/true,
                                                    /*wantBody=*/true);
          errorWrapper->addMember(nsErrorInit);

          // Add the domain error member.
          //   public static var errorDomain: String { return error-domain }
          addErrorDomain(errorWrapper, enumInfo.getErrorDomain(), Impl);

          // Note: the Code will be added after it's created.

          // The enum itself will be nested within the error wrapper,
          // and be named Code.
          enumDC = errorWrapper;
          enumName = C.Id_Code;
        }

        // Create the enumeration.
        auto enumDecl = Impl.createDeclWithClangNode<EnumDecl>(
            decl, AccessLevel::Public, loc, enumName,
            Impl.importSourceLoc(decl->getLocation()), None, nullptr, enumDC);
        enumDecl->computeType();

        // Annotate as 'frozen' if appropriate.
        assert((DeclAttribute::getOptions(DAK_Frozen) &
                DeclAttribute::UserInaccessible) &&
               "Once 'frozen' is supported, the attribute should not be "
               "implicit (below)");
        if (enumKind == EnumKind::FrozenEnum)
          enumDecl->getAttrs().add(new (C) FrozenAttr(/*implicit*/true));

        // Set up the C underlying type as its Swift raw type.
        enumDecl->setRawType(underlyingType);

        // Add the C name.
        addObjCAttribute(enumDecl,
                         Impl.importIdentifier(decl->getIdentifier()));

        // Add protocol declarations to the enum declaration.
        SmallVector<TypeLoc, 2> inheritedTypes;
        inheritedTypes.push_back(TypeLoc::withoutLoc(underlyingType));
        enumDecl->setInherited(C.AllocateCopy(inheritedTypes));

        if (errorWrapper) {
          addSynthesizedProtocolAttrs(Impl, enumDecl,
                                      {KnownProtocolKind::ErrorCodeProtocol,
                                       KnownProtocolKind::RawRepresentable});
        } else {
          addSynthesizedProtocolAttrs(Impl, enumDecl,
                                      {KnownProtocolKind::RawRepresentable});
        }

        // Provide custom implementations of the init(rawValue:) and rawValue
        // conversions that just do a bitcast. We can't reliably filter a
        // C enum without additional knowledge that the type has no
        // undeclared values, and won't ever add cases.
        auto rawValueConstructor = makeEnumRawValueConstructor(Impl, enumDecl);

        auto varName = C.Id_rawValue;
        auto rawValue = new (C) VarDecl(/*IsStatic*/false,
                                        VarDecl::Specifier::Var,
                                        /*IsCaptureList*/false,
                                        SourceLoc(), varName, underlyingType,
                                        enumDecl);
        rawValue->setImplicit();
        rawValue->setAccess(AccessLevel::Public);
        rawValue->setSetterAccess(AccessLevel::Private);
        rawValue->setInterfaceType(underlyingType);
        rawValue->setValidationToChecked();

        // Create a pattern binding to describe the variable.
        Pattern *varPattern = createTypedNamedPattern(rawValue);

        auto *rawValueBinding = PatternBindingDecl::createImplicit(
            C, StaticSpellingKind::None, varPattern, /*InitExpr*/ nullptr,
            enumDecl);

        auto rawValueGetter = makeEnumRawValueGetter(Impl, enumDecl, rawValue);

        enumDecl->addMember(rawValueConstructor);
        enumDecl->addMember(rawValueGetter);
        enumDecl->addMember(rawValue);
        enumDecl->addMember(rawValueBinding);

        addSynthesizedTypealias(enumDecl, C.Id_RawValue, underlyingType);

        // If we have an error wrapper, finish it up now that its
        // nested enum has been constructed.
        if (errorWrapper) {
          // Add the ErrorType alias:
          //   public typealias ErrorType
          auto alias = Impl.createDeclWithClangNode<TypeAliasDecl>(
                         decl,
                         AccessLevel::Public, loc, SourceLoc(),
                         C.Id_ErrorType, loc,
                         /*genericparams=*/nullptr, enumDecl);
          alias->setUnderlyingType(Impl.getSugaredTypeReference(errorWrapper));
          enumDecl->addMember(alias);

          // Add the 'Code' enum to the error wrapper.
          errorWrapper->addMember(enumDecl);
          Impl.addAlternateDecl(enumDecl, errorWrapper);
        }

        // The enumerators go into this enumeration.
        result = enumDecl;
        break;
      }

      case EnumKind::Options: {
        result = importAsOptionSetType(dc, name, decl);
        if (!result)
          return nullptr;

        // HACK: Make sure PrintAsObjC always omits the 'enum' tag for
        // option set enums.
        Impl.DeclsWithSuperfluousTypedefs.insert(decl);
        break;
      }
      }

      const clang::EnumDecl *canonicalClangDecl = decl->getCanonicalDecl();
      Impl.ImportedDecls[{canonicalClangDecl, getVersion()}] = result;

      // Import each of the enumerators.
      
      bool addEnumeratorsAsMembers;
      switch (enumKind) {
      case EnumKind::Constants:
      case EnumKind::Unknown:
        addEnumeratorsAsMembers = false;
        break;
      case EnumKind::Options:
      case EnumKind::NonFrozenEnum:
      case EnumKind::FrozenEnum:
        addEnumeratorsAsMembers = true;
        break;
      }

      llvm::SmallDenseMap<const llvm::APSInt *,
                          PointerUnion<const clang::EnumConstantDecl *,
                                       EnumElementDecl *>, 8,
                          APSIntRefDenseMapInfo> canonicalEnumConstants;

      if (enumKind == EnumKind::NonFrozenEnum ||
          enumKind == EnumKind::FrozenEnum) {
        for (auto constant : decl->enumerators()) {
          if (Impl.isUnavailableInSwift(constant))
            continue;
          canonicalEnumConstants.insert({&constant->getInitVal(), constant});
        }
      }

      auto contextIsEnum = [&](const ImportedName &name) -> bool {
        EffectiveClangContext importContext = name.getEffectiveContext();
        switch (importContext.getKind()) {
        case EffectiveClangContext::DeclContext:
          return importContext.getAsDeclContext() == canonicalClangDecl;
        case EffectiveClangContext::TypedefContext: {
          auto *typedefName = importContext.getTypedefName();
          clang::QualType underlyingTy = typedefName->getUnderlyingType();
          return underlyingTy->getAsTagDecl() == canonicalClangDecl;
        }
        case EffectiveClangContext::UnresolvedContext:
          // Assume this is a context other than the enum.
          return false;
        }
      };

      for (auto constant : decl->enumerators()) {
        Decl *enumeratorDecl = nullptr;
        TinyPtrVector<Decl *> variantDecls;
        switch (enumKind) {
        case EnumKind::Constants:
        case EnumKind::Unknown:
          Impl.forEachDistinctName(constant,
                                   [&](ImportedName newName,
                                       ImportNameVersion nameVersion) -> bool {
            Decl *imported = Impl.importDecl(constant, nameVersion);
            if (!imported)
              return false;
            if (nameVersion == getActiveSwiftVersion())
              enumeratorDecl = imported;
            else
              variantDecls.push_back(imported);
            return true;
          });
          break;
        case EnumKind::Options:
          Impl.forEachDistinctName(constant,
                                   [&](ImportedName newName,
                                       ImportNameVersion nameVersion) -> bool {
            if (!contextIsEnum(newName))
              return true;
            SwiftDeclConverter converter(Impl, nameVersion);
            Decl *imported =
                converter.importOptionConstant(constant, decl, result);
            if (!imported)
              return false;
            if (nameVersion == getActiveSwiftVersion())
              enumeratorDecl = imported;
            else
              variantDecls.push_back(imported);
            return true;
          });
          break;
        case EnumKind::NonFrozenEnum:
        case EnumKind::FrozenEnum: {
          auto canonicalCaseIter =
            canonicalEnumConstants.find(&constant->getInitVal());

          if (canonicalCaseIter == canonicalEnumConstants.end()) {
            // Unavailable declarations get no special treatment.
            enumeratorDecl =
                SwiftDeclConverter(Impl, getActiveSwiftVersion())
                    .importEnumCase(constant, decl, cast<EnumDecl>(result));
          } else {
            const clang::EnumConstantDecl *unimported =
                canonicalCaseIter->
                  second.dyn_cast<const clang::EnumConstantDecl *>();

            // Import the canonical enumerator for this case first.
            if (unimported) {
              enumeratorDecl = SwiftDeclConverter(Impl, getActiveSwiftVersion())
                  .importEnumCase(unimported, decl, cast<EnumDecl>(result));
              if (enumeratorDecl) {
                canonicalCaseIter->getSecond() =
                    cast<EnumElementDecl>(enumeratorDecl);
              }
            } else {
              enumeratorDecl =
                  canonicalCaseIter->second.get<EnumElementDecl *>();
            }

            if (unimported != constant && enumeratorDecl) {
              ImportedName importedName =
                  Impl.importFullName(constant, getActiveSwiftVersion());
              Identifier name = importedName.getDeclName().getBaseIdentifier();
              if (name.empty()) {
                // Clear the existing declaration so we don't try to process it
                // twice later.
                enumeratorDecl = nullptr;
              } else {
                auto original = cast<ValueDecl>(enumeratorDecl);
                enumeratorDecl = importEnumCaseAlias(name, constant, original,
                                                     decl, result);
              }
            }
          }

          Impl.forEachDistinctName(constant,
                                   [&](ImportedName newName,
                                       ImportNameVersion nameVersion) -> bool {
            if (nameVersion == getActiveSwiftVersion())
              return true;
            if (!contextIsEnum(newName))
              return true;
            SwiftDeclConverter converter(Impl, nameVersion);
            Decl *imported =
                converter.importEnumCase(constant, decl, cast<EnumDecl>(result),
                                         enumeratorDecl);
            if (!imported)
              return false;
            variantDecls.push_back(imported);
            return true;
          });
          break;
        }
        }
        if (!enumeratorDecl)
          continue;

        if (addEnumeratorsAsMembers) {
          // Add a member enumerator to the given nominal type.
          auto addDecl = [&](NominalTypeDecl *nominal, Decl *decl) {
            if (!decl) return;
            nominal->addMember(decl);
            if (auto *var = dyn_cast<VarDecl>(decl))
              nominal->addMember(var->getGetter());
          };

          addDecl(result, enumeratorDecl);
          for (auto *variant : variantDecls)
            addDecl(result, variant);
          
          // If there is an error wrapper, add an alias within the
          // wrapper to the corresponding value within the enumerator
          // context.
          if (errorWrapper) {
            auto enumeratorValue = cast<ValueDecl>(enumeratorDecl);
            auto name = enumeratorValue->getBaseName().getIdentifier();
            auto alias = importEnumCaseAlias(name,
                                             constant,
                                             enumeratorValue,
                                             decl,
                                             result,
                                             errorWrapper);
            addDecl(errorWrapper, alias);
          }
        }
      }

      // Add the type decl to ExternalDefinitions so that we can type-check
      // raw values and SILGen can emit witness tables for derived conformances.
      // FIXME: There might be better ways to do this.
      Impl.registerExternalDecl(result);
      if (errorWrapper)
        Impl.registerExternalDecl(errorWrapper);
      return result;
    }

    Decl *VisitRecordDecl(const clang::RecordDecl *decl) {
      // Track whether this record contains fields we can't reference in Swift
      // as stored properties.
      bool hasUnreferenceableStorage = false;
      
      // Track whether this record contains fields that can't be zero-
      // initialized.
      bool hasZeroInitializableStorage = true;

      // Track whether all fields in this record can be referenced in Swift,
      // either as stored or computed properties, in which case the record type
      // gets a memberwise initializer.
      bool hasMemberwiseInitializer = true;

      if (decl->isUnion()) {
        hasUnreferenceableStorage = true;

        // We generate initializers specially for unions below.
        hasMemberwiseInitializer = false;
      }

      // FIXME: Skip Microsoft __interfaces.
      if (decl->isInterface())
        return nullptr;

      // FIXME: Figure out how to deal with incomplete types, since that
      // notion doesn't exist in Swift.
      decl = decl->getDefinition();
      if (!decl) {
        forwardDeclaration = true;
        return nullptr;
      }

      // FIXME: We should actually support strong ARC references and similar in
      // C structs. That'll require some SIL and IRGen work, though.
      if (decl->isNonTrivialToPrimitiveCopy() ||
          decl->isNonTrivialToPrimitiveDestroy()) {
        // Note that there is a third predicate related to these,
        // isNonTrivialToPrimitiveDefaultInitialize. That one's not important
        // for us because Swift never "trivially default-initializes" a struct
        // (i.e. uses whatever bits were lying around as an initial value).

        // FIXME: It would be nice to instead import the declaration but mark
        // it as unavailable, but then it might get used as a type for an
        // imported function and the developer would be able to use it without
        // referencing the name, which would sidestep our availability
        // diagnostics.
        return nullptr;
      }

      // Import the name.
      Optional<ImportedName> correctSwiftName;
      auto importedName = getClangDeclName(decl, correctSwiftName);
      if (!importedName)
        return nullptr;

      // If we've been asked to produce a compatibility stub, handle it via a
      // typealias.
      if (correctSwiftName)
        return importCompatibilityTypeAlias(decl, importedName,
                                            *correctSwiftName);

      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;

      // Create the struct declaration and record it.
      auto name = importedName.getDeclName().getBaseIdentifier();
      auto result = Impl.createDeclWithClangNode<StructDecl>(decl,
                                 AccessLevel::Public,
                                 Impl.importSourceLoc(decl->getLocStart()),
                                 name,
                                 Impl.importSourceLoc(decl->getLocation()),
                                 None, nullptr, dc);
      result->computeType();
      result->setAddedImplicitInitializers();
      Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;

      // FIXME: Figure out what to do with superclasses in C++. One possible
      // solution would be to turn them into members and add conversion
      // functions.

      // Import each of the members.
      SmallVector<VarDecl *, 4> members;
      SmallVector<ConstructorDecl *, 4> ctors;

      // FIXME: Import anonymous union fields and support field access when
      // it is nested in a struct.

      for (auto m : decl->decls()) {
        auto nd = dyn_cast<clang::NamedDecl>(m);
        if (!nd) {
          // We couldn't import the member, so we can't reference it in Swift.
          hasUnreferenceableStorage = true;
          hasMemberwiseInitializer = false;
          continue;
        }

        if (auto field = dyn_cast<clang::FieldDecl>(nd)) {
          // Non-nullable pointers can't be zero-initialized.
          if (auto nullability = field->getType()
                ->getNullability(Impl.getClangASTContext())) {
            if (*nullability == clang::NullabilityKind::NonNull)
              hasZeroInitializableStorage = false;
          }
          // TODO: If we had the notion of a closed enum with no private
          // cases or resilience concerns, then complete NS_ENUMs with
          // no case corresponding to zero would also not be zero-
          // initializable.

          // Unnamed bitfields are just for padding and should not
          // inhibit creation of a memberwise initializer.
          if (field->isUnnamedBitfield()) {
            hasUnreferenceableStorage = true;
            continue;
          }
        }

        auto member = Impl.importDecl(nd, getActiveSwiftVersion());
        if (!member) {
          if (!isa<clang::TypeDecl>(nd)) {
            // We don't know what this field is.
            // Assume it may be important in C.
            hasUnreferenceableStorage = true;
            hasMemberwiseInitializer = false;
          }
          continue;
        }

        if (isa<TypeDecl>(member)) {
          // A struct nested inside another struct will either be logically
          // a sibling of the outer struct, or contained inside of it, depending
          // on if it has a declaration name or not.
          //
          // struct foo { struct bar { ... } baz; } // sibling
          // struct foo { struct { ... } baz; } // child
          //
          // In the latter case, we add the imported type as a nested type
          // of the parent.
          //
          // TODO: C++ types have different rules.
          if (auto nominalDecl = dyn_cast<NominalTypeDecl>(member->getDeclContext())) {
            assert(nominalDecl == result && "interesting nesting of C types?");
            nominalDecl->addMember(member);
          }
          continue;
        }

        auto VD = cast<VarDecl>(member);

        if (isa<clang::IndirectFieldDecl>(nd) || decl->isUnion()) {
          // Don't import unavailable fields that have no associated storage.
          if (VD->getAttrs().isUnavailable(Impl.SwiftContext)) {
            continue;
          }
        }

        members.push_back(VD);

        // Bitfields are imported as computed properties with Clang-generated
        // accessors.
        bool isBitField = false;
        if (auto field = dyn_cast<clang::FieldDecl>(nd)) {
          if (field->isBitField()) {
            // We can't represent this struct completely in SIL anymore,
            // but we're still able to define a memberwise initializer.
            hasUnreferenceableStorage = true;
            isBitField = true;

            makeBitFieldAccessors(Impl,
                                  const_cast<clang::RecordDecl *>(decl),
                                  result,
                                  const_cast<clang::FieldDecl *>(field),
                                  VD);
          }
        }

        if (auto ind = dyn_cast<clang::IndirectFieldDecl>(nd)) {
          // Indirect fields are created as computed property accessible the
          // fields on the anonymous field from which they are injected.
          makeIndirectFieldAccessors(Impl, ind, members, result, VD);
        } else if (decl->isUnion() && !isBitField) {
          // Union fields should only be available indirectly via a computed
          // property. Since the union is made of all of the fields at once,
          // this is a trivial accessor that casts self to the correct
          // field type.
          makeUnionFieldAccessors(Impl, result, VD);

          // Create labeled initializers for unions that take one of the
          // fields, which only initializes the data for that field.
          auto valueCtor =
              createValueConstructor(Impl, result, VD,
                                     /*want param names*/true,
                                     /*wantBody=*/!Impl.hasFinishedTypeChecking());
          ctors.push_back(valueCtor);
        }
      }

      bool hasReferenceableFields = !members.empty();

      if (hasZeroInitializableStorage) {
        // Add constructors for the struct.
        ctors.push_back(createDefaultConstructor(Impl, result));
      }

      if (hasReferenceableFields && hasMemberwiseInitializer) {
        // The default zero initializer suppresses the implicit value
        // constructor that would normally be formed, so we have to add that
        // explicitly as well.
        //
        // If we can completely represent the struct in SIL, leave the body
        // implicit, otherwise synthesize one to call property setters.
        bool wantBody = (hasUnreferenceableStorage &&
                         !Impl.hasFinishedTypeChecking());
        auto valueCtor = createValueConstructor(Impl, result, members,
                                                /*want param names*/true,
                                                /*want body*/wantBody);
        if (!hasUnreferenceableStorage)
          valueCtor->setIsMemberwiseInitializer();

        ctors.push_back(valueCtor);
      }

      for (auto member : members) {
        result->addMember(member);
      }
      
      for (auto ctor : ctors) {
        result->addMember(ctor);
      }

      result->setHasUnreferenceableStorage(hasUnreferenceableStorage);

      // Add the struct decl to ExternalDefinitions so that IRGen can emit
      // metadata for it.
      // FIXME: There might be better ways to do this.
      Impl.registerExternalDecl(result);

      return result;
    }

    Decl *VisitClassTemplateSpecializationDecl(
                 const clang::ClassTemplateSpecializationDecl *decl) {
      // FIXME: We could import specializations, but perhaps only as unnamed
      // structural types.
      return nullptr;
    }

    Decl *VisitClassTemplatePartialSpecializationDecl(
                 const clang::ClassTemplatePartialSpecializationDecl *decl) {
      // Note: templates are not imported.
      return nullptr;
    }

    Decl *VisitTemplateTypeParmDecl(const clang::TemplateTypeParmDecl *decl) {
      // Note: templates are not imported.
      return nullptr;
    }

    Decl *VisitEnumConstantDecl(const clang::EnumConstantDecl *decl) {
      auto clangEnum = cast<clang::EnumDecl>(decl->getDeclContext());

      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(decl, correctSwiftName);
      if (!importedName) return nullptr;

      auto name = importedName.getDeclName().getBaseIdentifier();
      if (name.empty())
        return nullptr;

      switch (Impl.getEnumKind(clangEnum)) {
      case EnumKind::Constants: {
        // The enumeration was simply mapped to an integral type. Create a
        // constant with that integral type.

        // The context where the constant will be introduced.
        auto dc =
            Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
        if (!dc)
          return nullptr;

        // Enumeration type.
        auto &clangContext = Impl.getClangASTContext();
        auto type = Impl.importTypeIgnoreIUO(
            clangContext.getTagDeclType(clangEnum), ImportTypeKind::Value,
            isInSystemModule(dc), Bridgeability::None);
        if (!type)
          return nullptr;
        // FIXME: Importing the type will recursively revisit this same
        // EnumConstantDecl. Short-circuit out if we already emitted the import
        // for this decl.
        if (auto Known = Impl.importDeclCached(decl, getVersion()))
          return Known;

        // Create the global constant.
        auto result = Impl.createConstant(name, dc, type,
                                          clang::APValue(decl->getInitVal()),
                                          ConstantConvertKind::Coerce,
                                          /*static*/dc->isTypeContext(), decl);
        Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;

        // If this is a compatibility stub, mark it as such.
        if (correctSwiftName)
          markAsVariant(result, *correctSwiftName);

        return result;
      }

      case EnumKind::Unknown: {
        // The enumeration was mapped to a struct containing the integral
        // type. Create a constant with that struct type.

        // The context where the constant will be introduced.
        auto dc =
            Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
        if (!dc)
          return nullptr;

        // Import the enumeration type.
        auto enumType = Impl.importTypeIgnoreIUO(
            Impl.getClangASTContext().getTagDeclType(clangEnum),
            ImportTypeKind::Value, isInSystemModule(dc), Bridgeability::None);
        if (!enumType)
          return nullptr;

        // FIXME: Importing the type will can recursively revisit this same
        // EnumConstantDecl. Short-circuit out if we already emitted the import
        // for this decl.
        if (auto Known = Impl.importDeclCached(decl, getVersion()))
          return Known;

        // Create the global constant.
        auto result = Impl.createConstant(name, dc, enumType,
                                          clang::APValue(decl->getInitVal()),
                                          ConstantConvertKind::Construction,
                                          /*static*/ false, decl);
        Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;

        // If this is a compatibility stub, mark it as such.
        if (correctSwiftName)
          markAsVariant(result, *correctSwiftName);

        return result;
      }

      case EnumKind::NonFrozenEnum:
      case EnumKind::FrozenEnum:
      case EnumKind::Options: {
        // The enumeration was mapped to a high-level Swift type, and its
        // elements were created as children of that enum. They aren't available
        // independently.

        // FIXME: This is gross. We shouldn't have to import
        // everything to get at the individual constants.
        return nullptr;
      }
      }
      
      llvm_unreachable("Invalid EnumKind.");
    }


    Decl *
    VisitUnresolvedUsingValueDecl(const clang::UnresolvedUsingValueDecl *decl) {
      // Note: templates are not imported.
      return nullptr;
    }

    Decl *VisitIndirectFieldDecl(const clang::IndirectFieldDecl *decl) {
      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(decl, correctSwiftName);
      if (!importedName) return nullptr;

      auto name = importedName.getDeclName().getBaseIdentifier();

      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;

      auto importedType =
          Impl.importType(decl->getType(), ImportTypeKind::Variable,
                          isInSystemModule(dc), Bridgeability::None);
      if (!importedType)
        return nullptr;

      auto type = importedType.getType();

      // Map this indirect field to a Swift variable.
      auto result = Impl.createDeclWithClangNode<VarDecl>(decl,
                       AccessLevel::Public,
                       /*IsStatic*/false,
                       VarDecl::Specifier::Var,
                       /*IsCaptureList*/false,
                       Impl.importSourceLoc(decl->getLocStart()),
                       name, dc->mapTypeIntoContext(type), dc);
      result->setInterfaceType(type);
      result->setIsObjC(false);
      Impl.recordImplicitUnwrapForDecl(result,
                                       importedType.isImplicitlyUnwrapped());

      // If this is a compatibility stub, mark is as such.
      if (correctSwiftName)
        markAsVariant(result, *correctSwiftName);

      return result;
    }

    ParameterList *getNonSelfParamList(
        DeclContext *dc, const clang::FunctionDecl *decl,
        Optional<unsigned> selfIdx, ArrayRef<Identifier> argNames,
        bool allowNSUIntegerAsInt, bool isAccessor) {
      if (bool(selfIdx)) {
        assert(((decl->getNumParams() == argNames.size() + 1) || isAccessor) &&
               (*selfIdx < decl->getNumParams()) && "where's self?");
      } else {
        assert(decl->getNumParams() == argNames.size() || isAccessor);
      }

      SmallVector<const clang::ParmVarDecl *, 4> nonSelfParams;
      for (unsigned i = 0; i < decl->getNumParams(); ++i) {
        if (selfIdx && i == *selfIdx)
          continue;
        nonSelfParams.push_back(decl->getParamDecl(i));
      }
      return Impl.importFunctionParameterList(dc, decl, nonSelfParams,
                                              decl->isVariadic(),
                                              allowNSUIntegerAsInt, argNames);
    }

    Decl *importGlobalAsInitializer(const clang::FunctionDecl *decl,
                                    DeclName name, DeclContext *dc,
                                    CtorInitializerKind initKind,
                                    Optional<ImportedName> correctSwiftName);

    Decl *importGlobalAsMethod(const clang::FunctionDecl *decl, DeclName name,
                               DeclContext *dc, Optional<unsigned> selfIdx,
                               Optional<ImportedName> correctSwiftName,
                               Optional<AccessorInfo> accessorInfo);

    /// Create an implicit property given the imported name of one of
    /// the accessors.
    VarDecl *getImplicitProperty(ImportedName importedName,
                                 const clang::FunctionDecl *accessor);

    Decl *VisitFunctionDecl(const clang::FunctionDecl *decl) {
      // Import the name of the function.
      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(decl, correctSwiftName);
      if (!importedName)
        return nullptr;

      AbstractStorageDecl *owningStorage;
      switch (importedName.getAccessorKind()) {
      case ImportedAccessorKind::None:
        owningStorage = nullptr;
        break;

      case ImportedAccessorKind::SubscriptGetter:
      case ImportedAccessorKind::SubscriptSetter:
        llvm_unreachable("Not possible for a function");

      case ImportedAccessorKind::PropertyGetter: {
        auto property = getImplicitProperty(importedName, decl);
        if (!property) return nullptr;
        return property->getGetter();
      }

      case ImportedAccessorKind::PropertySetter:
        auto property = getImplicitProperty(importedName, decl);
        if (!property) return nullptr;
        return property->getSetter();
      }

      return importFunctionDecl(decl, importedName, correctSwiftName, None);
    }

    Decl *importFunctionDecl(const clang::FunctionDecl *decl,
                             ImportedName importedName,
                             Optional<ImportedName> correctSwiftName,
                             Optional<AccessorInfo> accessorInfo) {
      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;

      DeclName name = accessorInfo ? DeclName() : importedName.getDeclName();
      if (importedName.importAsMember()) {
        // Handle initializers.
        if (name.getBaseName() == DeclBaseName::createConstructor()) {
          assert(!accessorInfo);
          return importGlobalAsInitializer(decl, name, dc,
                                           importedName.getInitKind(),
                                           correctSwiftName);
        }

        // Everything else is a method.
        return importGlobalAsMethod(decl, name, dc,
                                    importedName.getSelfIndex(),
                                    correctSwiftName, accessorInfo);
      }

      // Import the function type. If we have parameters, make sure their names
      // get into the resulting function type.
      ParameterList *bodyParams = nullptr;
      auto importedType = Impl.importFunctionType(
          dc, decl, {decl->param_begin(), decl->param_size()},
          decl->isVariadic(), isInSystemModule(dc), name, bodyParams);
      if (!importedType)
        return nullptr;

      Type type = importedType.getType();

      auto resultTy = type->castTo<FunctionType>()->getResult();
      auto loc = Impl.importSourceLoc(decl->getLocation());

      if (name && name.isSimpleName()) {
        assert(importedName.hasCustomName() &&
               "imported function with simple name?");
        // Just fill in empty argument labels.
        name = DeclName(Impl.SwiftContext, name.getBaseName(), bodyParams);
      }

      // FIXME: Poor location info.
      auto nameLoc = Impl.importSourceLoc(decl->getLocation());
      FuncDecl *result = createFuncOrAccessor(Impl.SwiftContext, loc,
                                              accessorInfo, name, nameLoc,
                                              /*selfDecl*/ nullptr, bodyParams,
                                              resultTy, /*throws*/ false,
                                              dc, decl);

      result->setInterfaceType(type);
      result->setValidationToChecked();
      result->setIsObjC(false);
      Impl.recordImplicitUnwrapForDecl(result,
                                       importedType.isImplicitlyUnwrapped());

      // Someday, maybe this will need to be 'open' for C++ virtual methods.
      result->setAccess(AccessLevel::Public);
      finishFuncDecl(decl, result);

      // If this is a compatibility stub, mark it as such.
      if (correctSwiftName)
        markAsVariant(result, *correctSwiftName);

      return result;
    }

    void finishFuncDecl(const clang::FunctionDecl *decl,
                        AbstractFunctionDecl *result) {
      // Keep track of inline function bodies so that we can generate
      // IR from them using Clang's IR generator.
      if ((decl->isInlined() || decl->hasAttr<clang::AlwaysInlineAttr>() ||
           !decl->isExternallyVisible()) &&
          decl->hasBody()) {
        Impl.registerExternalDecl(result);
      }

      // Set availability.
      if (decl->isVariadic()) {
        Impl.markUnavailable(result, "Variadic function is unavailable");
      }

      if (decl->hasAttr<clang::ReturnsTwiceAttr>()) {
        // The Clang 'returns_twice' attribute is used for functions like
        // 'vfork' or 'setjmp'. Because these functions may return control flow
        // of a Swift program to an arbitrary point, Swift's guarantees of
        // definitive initialization of variables cannot be upheld. As a result,
        // functions like these cannot be used in Swift.
        Impl.markUnavailable(
          result,
          "Functions that may return more than one time (annotated with the "
          "'returns_twice' attribute) are unavailable in Swift");
      }

      recordObjCOverride(result);
    }

    Decl *VisitCXXMethodDecl(const clang::CXXMethodDecl *decl) {
      // FIXME: Import C++ member functions as methods.
      return nullptr;
    }

    Decl *VisitFieldDecl(const clang::FieldDecl *decl) {
      // Fields are imported as variables.
      Optional<ImportedName> correctSwiftName;
      ImportedName importedName;

      if (!decl->isAnonymousStructOrUnion()) {
        importedName = importFullName(decl, correctSwiftName);
        if (!importedName) {
          return nullptr;
        }
      } else {
        // Generate a field name for anonymous fields, this will be used in
        // order to be able to expose the indirect fields injected from there
        // as computed properties forwarding the access to the subfield.
        std::string Id;
        llvm::raw_string_ostream IdStream(Id);

        IdStream << "__Anonymous_field" << decl->getFieldIndex();
        importedName.setDeclName(Impl.SwiftContext.getIdentifier(IdStream.str()));
        importedName.setEffectiveContext(decl->getDeclContext());
      }

      auto name = importedName.getDeclName().getBaseIdentifier();

      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;

      auto importedType =
          Impl.importType(decl->getType(), ImportTypeKind::RecordField,
                          isInSystemModule(dc), Bridgeability::None);
      if (!importedType)
        return nullptr;

      auto type = importedType.getType();

      auto result =
        Impl.createDeclWithClangNode<VarDecl>(decl, AccessLevel::Public,
                              /*IsStatic*/ false,
                              VarDecl::Specifier::Var,
                              /*IsCaptureList*/false,
                              Impl.importSourceLoc(decl->getLocation()),
                              name, dc->mapTypeIntoContext(type), dc);
      result->setIsObjC(false);
      result->setInterfaceType(type);
      Impl.recordImplicitUnwrapForDecl(result,
                                       importedType.isImplicitlyUnwrapped());

      // Handle attributes.
      if (decl->hasAttr<clang::IBOutletAttr>())
        result->getAttrs().add(
            new (Impl.SwiftContext) IBOutletAttr(/*IsImplicit=*/false));
      // FIXME: Handle IBOutletCollection.

      // If this is a compatibility stub, handle it as such.
      if (correctSwiftName)
        markAsVariant(result, *correctSwiftName);

      return result;
    }

    Decl *VisitObjCIvarDecl(const clang::ObjCIvarDecl *decl) {
      // Disallow direct ivar access (and avoid conflicts with property names).
      return nullptr;
    }

    Decl *VisitObjCAtDefsFieldDecl(const clang::ObjCAtDefsFieldDecl *decl) {
      // @defs is an anachronism; ignore it.
      return nullptr;
    }

    Decl *VisitVarDecl(const clang::VarDecl *decl) {
      // FIXME: Swift does not have static variables in structs/classes yet.
      if (decl->getDeclContext()->isRecord())
        return nullptr;

      // Variables are imported as... variables.
      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(decl, correctSwiftName);
      if (!importedName) return nullptr;

      auto name = importedName.getDeclName().getBaseIdentifier();
      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;

      // If the declaration is const, consider it audited.
      // We can assume that loading a const global variable doesn't
      // involve an ownership transfer.
      bool isAudited = decl->getType().isConstQualified();

      auto declType = decl->getType();

      // Special case: NS Notifications
      if (isNSNotificationGlobal(decl))
        if (auto newtypeDecl = findSwiftNewtype(decl, Impl.getClangSema(),
                                                Impl.CurrentVersion))
          declType = Impl.getClangASTContext().getTypedefType(newtypeDecl);

      // Note that we deliberately don't bridge most globals because we want to
      // preserve pointer identity.
      auto importedType =
          Impl.importType(declType,
                          (isAudited ? ImportTypeKind::AuditedVariable
                                     : ImportTypeKind::Variable),
                          isInSystemModule(dc), Bridgeability::None);

      if (!importedType)
        return nullptr;

      auto type = importedType.getType();

      // If we've imported this variable as a member, it's a static
      // member.
      bool isStatic = false;
      if (dc->isTypeContext())
        isStatic = true;

      auto specifier = Impl.shouldImportGlobalAsLet(decl->getType())
                     ? VarDecl::Specifier::Let
                     : VarDecl::Specifier::Var;
      auto result = Impl.createDeclWithClangNode<VarDecl>(decl,
                       AccessLevel::Public,
                       /*IsStatic*/isStatic,
                       specifier,
                       /*IsCaptureList*/false,
                       Impl.importSourceLoc(decl->getLocation()),
                       name, dc->mapTypeIntoContext(type), dc);
      result->setIsObjC(false);
      result->setInterfaceType(type);
      Impl.recordImplicitUnwrapForDecl(result,
                                       importedType.isImplicitlyUnwrapped());

      // If imported as member, the member should be final.
      if (dc->getAsClassOrClassExtensionContext())
        result->getAttrs().add(new (Impl.SwiftContext)
                                 FinalAttr(/*IsImplicit=*/true));

      if (!decl->hasExternalStorage())
        Impl.registerExternalDecl(result);

      // If this is a compatibility stub, mark it as such.
      if (correctSwiftName)
        markAsVariant(result, *correctSwiftName);

      return result;
    }

    Decl *VisitImplicitParamDecl(const clang::ImplicitParamDecl *decl) {
      // Parameters are never directly imported.
      return nullptr;
    }

    Decl *VisitParmVarDecl(const clang::ParmVarDecl *decl) {
      // Parameters are never directly imported.
      return nullptr;
    }

    Decl *
    VisitNonTypeTemplateParmDecl(const clang::NonTypeTemplateParmDecl *decl) {
      // Note: templates are not imported.
      return nullptr;
    }

    Decl *VisitTemplateDecl(const clang::TemplateDecl *decl) {
      // Note: templates are not imported.
      return nullptr;
    }

    Decl *VisitUsingDecl(const clang::UsingDecl *decl) {
      // Using declarations are not imported.
      return nullptr;
    }

    Decl *VisitUsingShadowDecl(const clang::UsingShadowDecl *decl) {
      // Using shadow declarations are not imported; rather, name lookup just
      // looks through them.
      return nullptr;
    }

    /// Add an @objc(name) attribute with the given, optional name expressed as
    /// selector.
    ///
    /// The importer should use this rather than adding the attribute directly.
    void addObjCAttribute(ValueDecl *decl, Optional<ObjCSelector> name) {
      auto &ctx = Impl.SwiftContext;
      if (name) {
        decl->getAttrs().add(ObjCAttr::create(ctx, name,
                                              /*implicitName=*/true));
      }
      decl->setIsObjC(true);

      // If the declaration we attached the 'objc' attribute to is within a
      // class, record it in the class.
      if (auto contextTy = decl->getDeclContext()->getDeclaredInterfaceType()) {
        if (auto classDecl = contextTy->getClassOrBoundGenericClass()) {
          if (auto method = dyn_cast<AbstractFunctionDecl>(decl)) {
            classDecl->recordObjCMethod(method);
          }
        }
      }
    }

    /// Add an @objc(name) attribute with the given, optional name expressed as
    /// selector.
    ///
    /// The importer should use this rather than adding the attribute directly.
    void addObjCAttribute(ValueDecl *decl, Identifier name) {
      addObjCAttribute(decl, ObjCSelector(Impl.SwiftContext, 0, name));
    }

    Decl *VisitObjCMethodDecl(const clang::ObjCMethodDecl *decl) {
      auto dc = Impl.importDeclContextOf(decl, decl->getDeclContext());
      if (!dc)
        return nullptr;

      // While importing the DeclContext, we might have imported the decl
      // itself.
      if (auto Known = Impl.importDeclCached(decl, getVersion()))
        return Known;

      return importObjCMethodDecl(decl, dc, None);
    }

    /// Check whether we have already imported a method with the given
    /// selector in the given context.
    bool isMethodAlreadyImported(ObjCSelector selector, bool isInstance,
                                 DeclContext *dc,
                    llvm::function_ref<bool(AbstractFunctionDecl *fn)> filter) {
      // We only need to perform this check for classes.
      auto classDecl
        = dc->getDeclaredInterfaceType()->getClassOrBoundGenericClass();
      if (!classDecl)
        return false;

      // Make sure we don't search in Clang modules for this method.
      ++Impl.ActiveSelectors[{selector, isInstance}];

      // Look for a matching imported or deserialized member.
      bool result = false;
      for (auto decl : classDecl->lookupDirect(selector, isInstance)) {
        if ((decl->getClangDecl()
             || !decl->getDeclContext()->getParentSourceFile())
            && filter(decl)) {
          result = true;
          break;
        }
      }

      // Restore the previous active count in the active-selector mapping.
      auto activeCount = Impl.ActiveSelectors.find({selector, isInstance});
      --activeCount->second;
      if (activeCount->second == 0)
        Impl.ActiveSelectors.erase(activeCount);

      return result;
    }

    Decl *importObjCMethodDecl(const clang::ObjCMethodDecl *decl,
                              DeclContext *dc,
                              Optional<AccessorInfo> accessorInfo) {
      return importObjCMethodDecl(decl, dc, false, accessorInfo);
    }

  private:
    static bool isAcceptableResult(Decl *fn,
                                   Optional<AccessorInfo> accessorInfo) {
      // We can't safely re-use the same declaration if it disagrees
      // in accessor-ness.
      auto accessor = dyn_cast<AccessorDecl>(fn);
      if (!accessorInfo)
        return accessor == nullptr;

      // For consistency with previous behavior, allow it even if it's been
      // imported for some other property.
      return (accessor && accessor->getAccessorKind() == accessorInfo->Kind);
    }

    Decl *importObjCMethodDecl(const clang::ObjCMethodDecl *decl,
                              DeclContext *dc,
                              bool forceClassMethod,
                              Optional<AccessorInfo> accessorInfo) {
      // If we have an init method, import it as an initializer.
      if (isInitMethod(decl)) {
        // Cannot import initializers as accessors.
        if (accessorInfo)
          return nullptr;

        // Cannot force initializers into class methods.
        if (forceClassMethod)
          return nullptr;

        return importConstructor(decl, dc, /*implicit=*/false, None,
                                 /*required=*/false);
      }

      // Check whether we already imported this method.
      if (!forceClassMethod &&
          dc == Impl.importDeclContextOf(decl, decl->getDeclContext())) {
        // FIXME: Should also be able to do this for forced class
        // methods.
        auto known = Impl.ImportedDecls.find({decl->getCanonicalDecl(),
                                              getVersion()});
        if (known != Impl.ImportedDecls.end()) {
          auto decl = known->second;
          if (isAcceptableResult(decl, accessorInfo))
            return decl;
        }
      }

      // Check whether another method with the same selector has already been
      // imported into this context.
      ObjCSelector selector = Impl.importSelector(decl->getSelector());
      bool isInstance = decl->isInstanceMethod() && !forceClassMethod;
      if (isActiveSwiftVersion()) {
        if (isMethodAlreadyImported(selector, isInstance, dc,
                                    [&](AbstractFunctionDecl *fn) {
              return isAcceptableResult(fn, accessorInfo);
            })) {
          return nullptr;
        }
      }

      ImportedName importedName;
      Optional<ImportedName> correctSwiftName;
      importedName = importFullName(decl, correctSwiftName);
      if (!importedName)
        return nullptr;

      // Normal case applies when we're importing an older name, or when we're
      // not an init
      if (!isFactoryInit(importedName)) {
        auto result = importNonInitObjCMethodDecl(decl, dc, importedName,
                                                  selector, forceClassMethod,
                                                  accessorInfo);

        if (!isActiveSwiftVersion() && result)
          markAsVariant(result, *correctSwiftName);

        return result;
      }

      // We can't import a factory-initializer as an accessor.
      if (accessorInfo)
        return nullptr;

      // We don't want to suppress init formation in Swift 3 names. Instead, we
      // want the normal Swift 3 name, and a "raw" name for diagnostics. The
      // "raw" name will be imported as unavailable with a more helpful and
      // specific message.
      ++NumFactoryMethodsAsInitializers;
      bool redundant = false;
      auto result =
          importConstructor(decl, dc, false, importedName.getInitKind(),
                            /*required=*/false, selector, importedName,
                            {decl->param_begin(), decl->param_size()},
                            decl->isVariadic(), redundant);

      if (!isActiveSwiftVersion() && result)
        markAsVariant(result, *correctSwiftName);

      return result;
    }

    Decl *importNonInitObjCMethodDecl(const clang::ObjCMethodDecl *decl,
                                      DeclContext *dc,
                                      ImportedName importedName,
                                      ObjCSelector selector,
                                      bool forceClassMethod,
                                      Optional<AccessorInfo> accessorInfo) {
      assert(dc->isTypeContext() && "Method in non-type context?");
      assert(isa<ClangModuleUnit>(dc->getModuleScopeContext()) &&
             "Clang method in Swift context?");

      // FIXME: We should support returning "Self.Type" for a root class
      // instance method mirrored as a class method, but it currently causes
      // problems for the type checker.
      if (forceClassMethod && decl->hasRelatedResultType())
        return nullptr;

      // Hack: avoid importing methods named "print" that aren't available in
      // the current version of Swift. We'd rather just let the user use
      // Swift.print in that case.
      if (!isActiveSwiftVersion() &&
          isPrintLikeMethod(importedName.getDeclName(), dc)) {
        return nullptr;
      }

      // Add the implicit 'self' parameter patterns.
      bool isInstance = decl->isInstanceMethod() && !forceClassMethod;
      ParameterList *bodyParams = nullptr;
      auto selfVar =
        ParamDecl::createSelf(SourceLoc(), dc, /*isStatic*/!isInstance);

      SpecialMethodKind kind = SpecialMethodKind::Regular;
      if (isNSDictionaryMethod(decl, Impl.objectForKeyedSubscript))
        kind = SpecialMethodKind::NSDictionarySubscriptGetter;

      // Import the type that this method will have.
      Optional<ForeignErrorConvention> errorConvention;

      // If we have a property accessor, find the corresponding property
      // declaration.
      const clang::ObjCPropertyDecl *prop = nullptr;
      if (decl->isPropertyAccessor()) {
        prop = decl->findPropertyDecl();
        if (!prop) return nullptr;

        // If we're importing just the accessors (not the property), ignore
        // the property.
        if (shouldImportPropertyAsAccessors(prop))
          prop = nullptr;
      }

      // If we have an accessor-import request but didn't find a property,
      // reject the import request.
      if (accessorInfo && !prop) {
        return nullptr;
      }

      ImportedType importedType;
      if (prop) {
        // If the matching property is in a superclass, or if the getter and
        // setter are redeclared in a potentially incompatible way, bail out.
        if (prop->getGetterMethodDecl() != decl &&
            prop->getSetterMethodDecl() != decl)
          return nullptr;
        importedType =
            Impl.importAccessorMethodType(dc, prop, decl, isInSystemModule(dc),
                                          importedName, &bodyParams);
      } else {
        importedType = Impl.importMethodType(
            dc, decl, decl->parameters(), decl->isVariadic(),
            isInSystemModule(dc), &bodyParams, importedName,
            errorConvention, kind);
      }
      if (!importedType)
        return nullptr;

      // Check whether we recursively imported this method
      if (!forceClassMethod &&
          dc == Impl.importDeclContextOf(decl, decl->getDeclContext())) {
        // FIXME: Should also be able to do this for forced class
        // methods.
        auto known = Impl.ImportedDecls.find({decl->getCanonicalDecl(),
                                              getVersion()});
        if (known != Impl.ImportedDecls.end()) {
          auto decl = known->second;
          if (isAcceptableResult(decl, accessorInfo))
            return decl;
        }
      }

      auto result = createFuncOrAccessor(Impl.SwiftContext,
                                         /*funcLoc*/SourceLoc(),
                                         accessorInfo,
                                         importedName.getDeclName(),
                                         /*nameLoc*/SourceLoc(),
                                         selfVar, bodyParams, Type(),
                                         importedName.getErrorInfo().hasValue(),
                                         dc, decl);

      result->setAccess(getOverridableAccessLevel(dc));

      auto resultTy = importedType.getType()
          ->castTo<FunctionType>()->getResult();

      auto isIUO = importedType.isImplicitlyUnwrapped();

      // If the method has a related result type that is representable
      // in Swift as DynamicSelf, do so.
      if (!prop && decl->hasRelatedResultType()) {
        result->setDynamicSelf(true);
        resultTy = DynamicSelfType::get(dc->getSelfInterfaceType(),
                                        Impl.SwiftContext);
        assert(!dc->getSelfInterfaceType()->getOptionalObjectType());
        isIUO = false;

        OptionalTypeKind nullability = OTK_ImplicitlyUnwrappedOptional;
        if (auto typeNullability = decl->getReturnType()->getNullability(
                                     Impl.getClangASTContext())) {
          // If the return type has nullability, use it.
          nullability = translateNullability(*typeNullability);
        }
        if (nullability != OTK_None && !errorConvention.hasValue()) {
          resultTy = OptionalType::get(resultTy);
          isIUO = nullability == OTK_ImplicitlyUnwrappedOptional;
        }
      }

      // Record the return type.
      result->getBodyResultTypeLoc().setType(resultTy);

      result->setGenericEnvironment(dc->getGenericEnvironmentOfContext());

      // Optional methods in protocols.
      if (decl->getImplementationControl() == clang::ObjCMethodDecl::Optional &&
          isa<ProtocolDecl>(dc))
        result->getAttrs().add(new (Impl.SwiftContext)
                                      OptionalAttr(/*implicit*/false));

      // Mark class methods as static.
      if (decl->isClassMethod() || forceClassMethod)
        result->setStatic();
      if (forceClassMethod)
        result->setImplicit();

      // Compute the interface type.
      result->computeType();
      result->setValidationToChecked();

      Impl.recordImplicitUnwrapForDecl(result, isIUO);

      // Mark this method @objc.
      addObjCAttribute(result, selector);

      // If this method overrides another method, mark it as such.
      recordObjCOverride(result);

      // Record the error convention.
      if (errorConvention) {
        result->setForeignErrorConvention(*errorConvention);
      }

      // Handle attributes.
      if (decl->hasAttr<clang::IBActionAttr>() &&
          isa<FuncDecl>(result) &&
          cast<FuncDecl>(result)->isPotentialIBActionTarget()) {
        result->getAttrs().add(
            new (Impl.SwiftContext) IBActionAttr(/*IsImplicit=*/false));
      }

      // Check whether there's some special method to import.
      if (!forceClassMethod) {
        if (dc == Impl.importDeclContextOf(decl, decl->getDeclContext()) &&
            !Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}])
          Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}]
            = result;

        if (importedName.isSubscriptAccessor()) {
          // If this was a subscript accessor, try to create a
          // corresponding subscript declaration.
          (void)importSubscript(result, decl);
        } else if (shouldAlsoImportAsClassMethod(result)) {
          // If we should import this instance method also as a class
          // method, do so and mark the result as an alternate
          // declaration.
          if (auto imported = importObjCMethodDecl(decl, dc,
                                                  /*forceClassMethod=*/true,
                                                  /*accessor*/None))
            Impl.addAlternateDecl(result, cast<ValueDecl>(imported));
        }
      }

      return result;
    }

  public:
    /// Record the function or initializer overridden by the given Swift method.
    void recordObjCOverride(AbstractFunctionDecl *decl);

    /// \brief Given an imported method, try to import it as a constructor.
    ///
    /// Objective-C methods in the 'init' family are imported as
    /// constructors in Swift, enabling object construction syntax, e.g.,
    ///
    /// \code
    /// // in objc: [[NSArray alloc] initWithCapacity:1024]
    /// NSArray(capacity: 1024)
    /// \endcode
    ConstructorDecl *importConstructor(const clang::ObjCMethodDecl *objcMethod,
                                       DeclContext *dc,
                                       bool implicit,
                                       Optional<CtorInitializerKind> kind,
                                       bool required);

    /// Returns the latest "introduced" version on the current platform for
    /// \p D.
    clang::VersionTuple findLatestIntroduction(const clang::Decl *D);

    /// Returns true if importing \p objcMethod will produce a "better"
    /// initializer than \p existingCtor.
    bool
    existingConstructorIsWorse(const ConstructorDecl *existingCtor,
                               const clang::ObjCMethodDecl *objcMethod,
                               CtorInitializerKind kind);

    /// \brief Given an imported method, try to import it as a constructor.
    ///
    /// Objective-C methods in the 'init' family are imported as
    /// constructors in Swift, enabling object construction syntax, e.g.,
    ///
    /// \code
    /// // in objc: [[NSArray alloc] initWithCapacity:1024]
    /// NSArray(capacity: 1024)
    /// \endcode
    ///
    /// This variant of the function is responsible for actually binding the
    /// constructor declaration appropriately.
    ConstructorDecl *importConstructor(const clang::ObjCMethodDecl *objcMethod,
                                       DeclContext *dc,
                                       bool implicit,
                                       Optional<CtorInitializerKind> kindIn,
                                       bool required,
                                       ObjCSelector selector,
                                       ImportedName importedName,
                                       ArrayRef<const clang::ParmVarDecl*> args,
                                       bool variadic,
                                       bool &redundant);

    void recordObjCOverride(SubscriptDecl *subscript);

    /// \brief Given either the getter or setter for a subscript operation,
    /// create the Swift subscript declaration.
    SubscriptDecl *importSubscript(Decl *decl,
                                   const clang::ObjCMethodDecl *objcMethod);

    /// Import the accessor and its attributes.
    AccessorDecl *importAccessor(clang::ObjCMethodDecl *clangAccessor,
                                 AbstractStorageDecl *storage,
                                 AccessorKind accessorKind,
                                 DeclContext *dc);

  public:

    /// Recursively add the given protocol and its inherited protocols to the
    /// given vector, guarded by the known set of protocols.
    void addProtocols(ProtocolDecl *protocol,
                      SmallVectorImpl<ProtocolDecl *> &protocols,
                      llvm::SmallPtrSetImpl<ProtocolDecl *> &known);

    // Import the given Objective-C protocol list, along with any
    // implicitly-provided protocols, and attach them to the given
    // declaration.
    void importObjCProtocols(Decl *decl,
                             const clang::ObjCProtocolList &clangProtocols,
                             SmallVectorImpl<TypeLoc> &inheritedTypes);

    /// Add conformances to the given Objective-C protocols to the
    /// given declaration.
    void addObjCProtocolConformances(Decl *decl,
                                     ArrayRef<ProtocolDecl*> protocols);

    // Returns None on error. Returns nullptr if there is no type param list to
    // import or we suppress its import, as in the case of NSArray, NSSet, and
    // NSDictionary.
    Optional<GenericParamList *>
    importObjCGenericParams(const clang::ObjCInterfaceDecl *decl,
                            DeclContext *dc);

    /// \brief Import the members of all of the protocols to which the given
    /// Objective-C class, category, or extension explicitly conforms into
    /// the given list of members, so long as the method was not already
    /// declared in the class.
    ///
    /// FIXME: This whole thing is a hack, because name lookup should really
    /// just find these members when it looks in the protocol. Unfortunately,
    /// that's not something the name lookup code can handle right now, and
    /// it may still be necessary when the protocol's instance methods become
    /// class methods on a root class (e.g. NSObject-the-protocol's instance
    /// methods become class methods on NSObject).
    void importMirroredProtocolMembers(const clang::ObjCContainerDecl *decl,
                                       DeclContext *dc,
                                       ArrayRef<ProtocolDecl *> protocols,
                                       SmallVectorImpl<Decl *> &members,
                                       ASTContext &Ctx);

    void importNonOverriddenMirroredMethods(DeclContext *dc,
                                  MutableArrayRef<MirroredMethodEntry> entries,
                                  SmallVectorImpl<Decl *> &newMembers);

    /// \brief Import constructors from our superclasses (and their
    /// categories/extensions), effectively "inheriting" constructors.
    void importInheritedConstructors(ClassDecl *classDecl,
                                     SmallVectorImpl<Decl *> &newMembers);

    Decl *VisitObjCCategoryDecl(const clang::ObjCCategoryDecl *decl) {
      // If the declaration is invalid, fail.
      if (decl->isInvalidDecl()) return nullptr;

      // Objective-C categories and extensions map to Swift extensions.
      if (importer::hasNativeSwiftDecl(decl))
        return nullptr;

      // Find the Swift class being extended.
      auto objcClass = castIgnoringCompatibilityAlias<ClassDecl>(
          Impl.importDecl(decl->getClassInterface(), getActiveSwiftVersion()));
      if (!objcClass)
        return nullptr;

      auto dc = Impl.importDeclContextOf(decl, decl->getDeclContext());
      if (!dc)
        return nullptr;

      auto loc = Impl.importSourceLoc(decl->getLocStart());
      auto result = ExtensionDecl::create(
                      Impl.SwiftContext, loc,
                      TypeLoc::withoutLoc(objcClass->getDeclaredType()),
                      { }, dc, nullptr, decl);

      // Determine the type and generic args of the extension.
      if (objcClass->getGenericParams()) {
        // Clone generic parameters.
        SmallVector<GenericTypeParamDecl *, 2> toGenericParams;
        for (auto fromGP : *objcClass->getGenericParams()) {
          // Create the new generic parameter.
          auto toGP = new (Impl.SwiftContext) GenericTypeParamDecl(
              result, fromGP->getName(), SourceLoc(), fromGP->getDepth(),
              fromGP->getIndex());
          toGP->setImplicit(true);
          toGP->setInherited(
              Impl.SwiftContext.AllocateCopy(fromGP->getInherited()));
          // Record new generic parameter.
          toGenericParams.push_back(toGP);
        }

        auto genericParams = GenericParamList::create(Impl.SwiftContext,
            SourceLoc(), toGenericParams, SourceLoc());
        result->setGenericParams(genericParams);

        auto *env = Impl.buildGenericEnvironment(genericParams, result);
        result->setGenericEnvironment(env);

        // Calculate the correct bound-generic extended type.
        SmallVector<Type, 2> genericArgs;
        for (auto paramTy :
             env->getGenericSignature()->getInnermostGenericParams()) {
          genericArgs.push_back(env->mapTypeIntoContext(paramTy));
        }
        Type extendedType =
          BoundGenericClassType::get(objcClass, nullptr, genericArgs);
        result->getExtendedTypeLoc().setType(extendedType);
      }

      // Create the extension declaration and record it.
      objcClass->addExtension(result);
      Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;
      SmallVector<TypeLoc, 4> inheritedTypes;
      importObjCProtocols(result, decl->getReferencedProtocols(),
                          inheritedTypes);
      result->setValidationToChecked();
      result->setInherited(Impl.SwiftContext.AllocateCopy(inheritedTypes));
      result->setMemberLoader(&Impl, 0);

      return result;
    }

    template <typename T, typename U>
    T *resolveSwiftDeclImpl(const U *decl, Identifier name, ModuleDecl *adapter) {
      const auto &languageVersion =
          Impl.SwiftContext.LangOpts.EffectiveLanguageVersion;

      SmallVector<ValueDecl *, 4> results;
      adapter->lookupValue({}, name, NLKind::QualifiedLookup, results);
      T *found = nullptr;
      for (auto result : results) {
        if (auto singleResult = dyn_cast<T>(result)) {
          // Skip versioned variants.
          const DeclAttributes &attrs = singleResult->getAttrs();
          if (attrs.isUnavailableInSwiftVersion(languageVersion))
            continue;

          if (found)
            return nullptr;

          found = singleResult;
        }
      }

      if (found)
        Impl.ImportedDecls[{decl->getCanonicalDecl(),
                            getActiveSwiftVersion()}] = found;

      return found;
    }

    template <typename T, typename U>
    T *resolveSwiftDecl(const U *decl, Identifier name,
                        ClangModuleUnit *clangModule) {
      if (auto adapter = clangModule->getAdapterModule())
        return resolveSwiftDeclImpl<T>(decl, name, adapter);
      if (clangModule == Impl.ImportedHeaderUnit) {
        // Use an index-based loop because new owners can come in as we're
        // iterating.
        for (size_t i = 0; i < Impl.ImportedHeaderOwners.size(); ++i) {
          ModuleDecl *owner = Impl.ImportedHeaderOwners[i];
          if (T *result = resolveSwiftDeclImpl<T>(decl, name, owner))
            return result;
        }
      }
      return nullptr;
    }

    template <typename T, typename U>
    bool hasNativeSwiftDecl(const U *decl, Identifier name,
                            const DeclContext *dc, T *&swiftDecl) {
      if (!importer::hasNativeSwiftDecl(decl))
        return false;
      auto wrapperUnit = cast<ClangModuleUnit>(dc->getModuleScopeContext());
      swiftDecl = resolveSwiftDecl<T>(decl, name, wrapperUnit);
      return true;
    }

    void markMissingSwiftDecl(ValueDecl *VD) {
      const char *message;
      if (isa<ClassDecl>(VD))
        message = "cannot find Swift declaration for this class";
      else if (isa<ProtocolDecl>(VD))
        message = "cannot find Swift declaration for this protocol";
      else
        llvm_unreachable("unknown bridged decl kind");
      auto attr = AvailableAttr::createPlatformAgnostic(Impl.SwiftContext,
                                                        message);
      VD->getAttrs().add(attr);
    }

    Decl *VisitObjCProtocolDecl(const clang::ObjCProtocolDecl *decl) {
      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(decl, correctSwiftName);
      if (!importedName) return nullptr;

      // If we've been asked to produce a compatibility stub, handle it via a
      // typealias.
      if (correctSwiftName)
        return importCompatibilityTypeAlias(decl, importedName,
                                            *correctSwiftName);

      Identifier name = importedName.getDeclName().getBaseIdentifier();

      // FIXME: Figure out how to deal with incomplete protocols, since that
      // notion doesn't exist in Swift.
      if (!decl->hasDefinition()) {
        // Check if this protocol is implemented in its adapter.
        if (auto clangModule = Impl.getClangModuleForDecl(decl, true))
          if (auto native = resolveSwiftDecl<ProtocolDecl>(decl, name,
                                                           clangModule))
            return native;

        forwardDeclaration = true;
        return nullptr;
      }

      decl = decl->getDefinition();

      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;

      ProtocolDecl *nativeDecl;
      bool declaredNative = hasNativeSwiftDecl(decl, name, dc, nativeDecl);
      if (declaredNative && nativeDecl)
        return nativeDecl;

      // Create the protocol declaration and record it.
      auto result = Impl.createDeclWithClangNode<ProtocolDecl>(
          decl, AccessLevel::Public, dc,
          Impl.importSourceLoc(decl->getLocStart()),
          Impl.importSourceLoc(decl->getLocation()), name, None,
          /*TrailingWhere=*/nullptr);
      result->computeType();

      // FIXME: Kind of awkward that we have to do this here
      result->getGenericParams()->getParams()[0]->setDepth(0);

      addObjCAttribute(result, Impl.importIdentifier(decl->getIdentifier()));

      if (declaredNative)
        markMissingSwiftDecl(result);

      Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;

      result->setCircularityCheck(CircularityCheck::Checked);

      // Import protocols this protocol conforms to.
      SmallVector<TypeLoc, 4> inheritedTypes;
      importObjCProtocols(result, decl->getReferencedProtocols(),
                          inheritedTypes);
      result->setInherited(Impl.SwiftContext.AllocateCopy(inheritedTypes));

      // Compute the requirement signature.
      if (!result->isRequirementSignatureComputed())
        result->computeRequirementSignature();

      auto *env = Impl.buildGenericEnvironment(result->getGenericParams(), dc);
      result->setGenericEnvironment(env);

      result->setMemberLoader(&Impl, 0);

      // Add the protocol decl to ExternalDefinitions so that IRGen can emit
      // metadata for it.
      // FIXME: There might be better ways to do this.
      Impl.registerExternalDecl(result);

      return result;
    }

    // Add inferred attributes.
    void addInferredAttributes(Decl *decl, unsigned attributes) {
      using namespace inferred_attributes;
      if (attributes & requires_stored_property_inits) {
        auto a = new (Impl.SwiftContext)
          RequiresStoredPropertyInitsAttr(/*IsImplicit=*/true);
        decl->getAttrs().add(a);
        cast<ClassDecl>(decl)->setRequiresStoredPropertyInits(true);
      }
    }

    Decl *VisitObjCInterfaceDecl(const clang::ObjCInterfaceDecl *decl) {
      auto createFakeRootClass = [=](Identifier name,
                                     DeclContext *dc = nullptr) -> ClassDecl * {
        if (!dc) {
          dc = Impl.getClangModuleForDecl(decl->getCanonicalDecl(),
                                          /*allowForwardDeclaration=*/true);
        }

        auto result = Impl.createDeclWithClangNode<ClassDecl>(decl,
                                                        AccessLevel::Public,
                                                        SourceLoc(), name,
                                                        SourceLoc(), None,
                                                        nullptr, dc);
        result->computeType();
        Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;
        result->setCircularityCheck(CircularityCheck::Checked);
        result->setSuperclass(Type());
        result->setAddedImplicitInitializers(); // suppress all initializers
        addObjCAttribute(result, Impl.importIdentifier(decl->getIdentifier()));
        result->addImplicitDestructor();
        return result;
      };

      // Special case for Protocol, which gets forward-declared as an ObjC
      // class which is hidden in modern Objective-C runtimes.
      // We treat it as a foreign class (like a CF type) because it doesn't
      // have a real public class object.
      clang::ASTContext &clangCtx = Impl.getClangASTContext();
      if (decl->getCanonicalDecl() ==
          clangCtx.getObjCProtocolDecl()->getCanonicalDecl()) {
        Type nsObjectTy = Impl.getNSObjectType();
        if (!nsObjectTy)
          return nullptr;
        const ClassDecl *nsObjectDecl =
          nsObjectTy->getClassOrBoundGenericClass();

        auto result = createFakeRootClass(Impl.SwiftContext.Id_Protocol,
                                      nsObjectDecl->getDeclContext());
        result->setForeignClassKind(ClassDecl::ForeignKind::RuntimeOnly);
        return result;
      }

      if (auto *definition = decl->getDefinition())
        decl = definition;

      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(decl, correctSwiftName);
      if (!importedName) return nullptr;

      // If we've been asked to produce a compatibility stub, handle it via a
      // typealias.
      if (correctSwiftName)
        return importCompatibilityTypeAlias(decl, importedName,
                                            *correctSwiftName);

      auto name = importedName.getDeclName().getBaseIdentifier();

      if (!decl->hasDefinition()) {
        // Check if this class is implemented in its adapter.
        if (auto clangModule = Impl.getClangModuleForDecl(decl, true)) {
          if (auto native = resolveSwiftDecl<ClassDecl>(decl, name,
                                                        clangModule)) {
            return native;
          }
        }

        if (Impl.ImportForwardDeclarations) {
          // Fake it by making an unavailable opaque @objc root class.
          auto result = createFakeRootClass(name);
          result->setImplicit();
          auto attr = AvailableAttr::createPlatformAgnostic(Impl.SwiftContext,
              "This Objective-C class has only been forward-declared; "
              "import its owning module to use it");
          result->getAttrs().add(attr);
          result->getAttrs().add(
              new (Impl.SwiftContext) ForbidSerializingReferenceAttr(true));
          return result;
        }

        forwardDeclaration = true;
        return nullptr;
      }

      auto dc =
          Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
      if (!dc)
        return nullptr;

      ClassDecl *nativeDecl;
      bool declaredNative = hasNativeSwiftDecl(decl, name, dc, nativeDecl);
      if (declaredNative && nativeDecl)
        return nativeDecl;

      auto access = AccessLevel::Open;
      if (decl->hasAttr<clang::ObjCSubclassingRestrictedAttr>() &&
          Impl.SwiftContext.isSwiftVersionAtLeast(5)) {
        access = AccessLevel::Public;
      }

      // Create the class declaration and record it.
      auto result = Impl.createDeclWithClangNode<ClassDecl>(
          decl, access, Impl.importSourceLoc(decl->getLocStart()), name,
          Impl.importSourceLoc(decl->getLocation()), None, nullptr, dc);

      // Import generic arguments, if any.
      if (auto gpImportResult = importObjCGenericParams(decl, dc)) {
        auto genericParams = *gpImportResult;
        if (genericParams) {
          result->setGenericParams(genericParams);

          auto *env = Impl.buildGenericEnvironment(genericParams, dc);
          result->setGenericEnvironment(env);
        }
      } else {
        return nullptr;
      }

      result->computeType();

      Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = result;
      result->setCircularityCheck(CircularityCheck::Checked);
      addObjCAttribute(result, Impl.importIdentifier(decl->getIdentifier()));

      if (declaredNative)
        markMissingSwiftDecl(result);
      if (decl->getAttr<clang::ObjCRuntimeVisibleAttr>()) {
        result->setForeignClassKind(ClassDecl::ForeignKind::RuntimeOnly);
      }

      // If this Objective-C class has a supertype, import it.
      SmallVector<TypeLoc, 4> inheritedTypes;
      Type superclassType;
      if (decl->getSuperClass()) {
        clang::QualType clangSuperclassType =
          decl->getSuperClassType()->stripObjCKindOfTypeAndQuals(clangCtx);
        clangSuperclassType =
          clangCtx.getObjCObjectPointerType(clangSuperclassType);
        superclassType = Impl.importTypeIgnoreIUO(
            clangSuperclassType, ImportTypeKind::Abstract, isInSystemModule(dc),
            Bridgeability::None);
        if (superclassType) {
          assert(superclassType->is<ClassType>() ||
                 superclassType->is<BoundGenericClassType>());
          inheritedTypes.push_back(TypeLoc::withoutLoc(superclassType));
        }
      }
      result->setSuperclass(superclassType);

      // Mark the class as runtime-only if it is named 'OS_object', even
      // if it doesn't have the runtime-only Clang attribute. This is a
      // targeted fix allowing IRGen to emit convenience initializers
      // correctly.
      //
      // FIXME: Remove this once SILGen gets proper support for factory
      // initializers.
      if (decl->getName() == "OS_object" ||
          decl->getName() == "OS_os_log") {
        result->setForeignClassKind(ClassDecl::ForeignKind::RuntimeOnly);
      }

      // If the superclass is runtime-only, our class is also. This only
      // matters in the case above.
      if (superclassType) {
        auto superclassDecl = cast<ClassDecl>(superclassType->getAnyNominal());
        auto kind = superclassDecl->getForeignClassKind();
        if (kind != ClassDecl::ForeignKind::Normal)
          result->setForeignClassKind(kind);
      }

      // Import protocols this class conforms to.
      importObjCProtocols(result, decl->getReferencedProtocols(),
                          inheritedTypes);
      result->setInherited(Impl.SwiftContext.AllocateCopy(inheritedTypes));

      // Add inferred attributes.
#define INFERRED_ATTRIBUTES(ModuleName, ClassName, AttributeSet)               \
  if (name.str().equals(#ClassName) &&                                         \
      result->getParentModule()->getName().str().equals(#ModuleName)) {        \
    using namespace inferred_attributes;                                       \
    addInferredAttributes(result, AttributeSet);                               \
  }
#include "InferredAttributes.def"

      result->setMemberLoader(&Impl, 0);
      result->addImplicitDestructor();

      return result;
    }

    Decl *VisitObjCImplDecl(const clang::ObjCImplDecl *decl) {
      // Implementations of Objective-C classes and categories are not
      // reflected into Swift.
      return nullptr;
    }

    Decl *VisitObjCPropertyDecl(const clang::ObjCPropertyDecl *decl) {
      auto dc = Impl.importDeclContextOf(decl, decl->getDeclContext());
      if (!dc)
        return nullptr;

      // While importing the DeclContext, we might have imported the decl
      // itself.
      if (auto Known = Impl.importDeclCached(decl, getVersion()))
        return Known;

      return importObjCPropertyDecl(decl, dc);
    }

    /// Hack: Handle the case where a property is declared \c readonly in the
    /// main class interface (either explicitly or because of an adopted
    /// protocol) and then \c readwrite in a category/extension.
    ///
    /// \see VisitObjCPropertyDecl
    void handlePropertyRedeclaration(VarDecl *original,
                                     const clang::ObjCPropertyDecl *redecl) {
      // If the property isn't from Clang, we can't safely update it.
      if (!original->hasClangNode())
        return;

      // If the original declaration was implicit, we may want to change that.
      if (original->isImplicit() && !redecl->isImplicit() &&
          !isa<clang::ObjCProtocolDecl>(redecl->getDeclContext()))
        original->setImplicit(false);

      if (!original->getAttrs().hasAttribute<ReferenceOwnershipAttr>() &&
          !original->getAttrs().hasAttribute<NSCopyingAttr>()) {
        applyPropertyOwnership(original,
                               redecl->getPropertyAttributesAsWritten());
      }

      auto clangSetter = redecl->getSetterMethodDecl();
      if (!clangSetter)
        return;

      // The only other transformation we know how to do safely is add a
      // setter. If the property is already settable, we're done.
      if (original->isSettable(nullptr))
        return;

      AccessorDecl *setter = importAccessor(clangSetter,
                                            original, AccessorKind::Set,
                                            original->getDeclContext());
      if (!setter)
        return;

      // Check that the redeclared property's setter uses the same type as the
      // original property. Objective-C can get away with the types being
      // different (usually in something like nullability), but for Swift it's
      // an AST invariant that's assumed and asserted elsewhere. If the type is
      // different, just drop the setter, and leave the property as get-only.
      assert(setter->getParameters()->size() == 1);
      const ParamDecl *param = setter->getParameters()->get(0);
      if (!param->getInterfaceType()->isEqual(original->getInterfaceType()))
        return;

      original->setComputedSetter(setter);
    }

    Decl *importObjCPropertyDecl(const clang::ObjCPropertyDecl *decl,
                                DeclContext *dc) {
      assert(dc);

      Optional<ImportedName> correctSwiftName;
      auto name = importFullName(decl, correctSwiftName)
                      .getDeclName()
                      .getBaseIdentifier();
      if (name.empty())
        return nullptr;

      if (shouldImportPropertyAsAccessors(decl))
        return nullptr;

      // Check whether there is a function with the same name as this
      // property. If so, suppress the property; the user will have to use
      // the methods directly, to avoid ambiguities.
      Type containerTy = dc->getDeclaredInterfaceType();
      Type lookupContextTy = containerTy;
      if (auto *classDecl = dyn_cast<ClassDecl>(dc)) {
        // If we're importing into the primary @interface for something, as
        // opposed to an extension, make sure we don't try to load any
        // categories...by just looking into the super type.
        lookupContextTy = classDecl->getSuperclass();
      }

      VarDecl *overridden = nullptr;
      if (lookupContextTy) {
        SmallVector<ValueDecl *, 2> lookup;
        dc->lookupQualified(lookupContextTy, name,
                            NL_QualifiedDefault | NL_KnownNoDependency,
                            Impl.getTypeResolver(), lookup);
        for (auto result : lookup) {
          if (isa<FuncDecl>(result) &&
              result->isInstanceMember() == decl->isInstanceProperty() &&
              result->getFullName().getArgumentNames().empty())
            return nullptr;

          if (auto var = dyn_cast<VarDecl>(result)) {
            // If the selectors of the getter match in Objective-C, we have an
            // override.
            if (var->isInstanceMember() == decl->isInstanceProperty() &&
                var->getObjCGetterSelector() ==
                  Impl.importSelector(decl->getGetterName()))
              overridden = var;
          }
        }
      }

      if (overridden) {
        const DeclContext *overrideContext = overridden->getDeclContext();
        // It's okay to compare interface types directly because Objective-C
        // does not have constrained extensions.
        if (overrideContext != dc && overridden->hasClangNode() &&
            overrideContext->getDeclaredInterfaceType()->isEqual(containerTy)) {
          // We've encountered a redeclaration of the property.
          // HACK: Just update the original declaration instead of importing a
          // second property.
          handlePropertyRedeclaration(overridden, decl);
          return nullptr;
        }
      }

      auto importedType = Impl.importPropertyType(decl, isInSystemModule(dc));
      if (!importedType)
        return nullptr;

      // Check whether the property already got imported.
      if (dc == Impl.importDeclContextOf(decl, decl->getDeclContext())) {
        auto known = Impl.ImportedDecls.find({decl->getCanonicalDecl(),
                                              getVersion()});
        if (known != Impl.ImportedDecls.end())
          return known->second;
      }

      auto type = importedType.getType();
      auto result = Impl.createDeclWithClangNode<VarDecl>(decl,
          getOverridableAccessLevel(dc),
          /*IsStatic*/decl->isClassProperty(), VarDecl::Specifier::Var,
          /*IsCaptureList*/false, Impl.importSourceLoc(decl->getLocation()),
          name, dc->mapTypeIntoContext(type), dc);
      result->setInterfaceType(type);
      Impl.recordImplicitUnwrapForDecl(result,
                                       importedType.isImplicitlyUnwrapped());

      // Recover from a missing getter in no-asserts builds. We're still not
      // sure under what circumstances this occurs, but we shouldn't crash.
      auto clangGetter = decl->getGetterMethodDecl();
      assert(clangGetter && "ObjC property without getter");
      if (!clangGetter)
        return nullptr;

      // Import the getter.
      AccessorDecl *getter = importAccessor(clangGetter, result,
                                            AccessorKind::Get, dc);
      if (!getter)
        return nullptr;

      // Import the setter, if there is one.
      AccessorDecl *setter = nullptr;
      if (auto clangSetter = decl->getSetterMethodDecl()) {
        setter = importAccessor(clangSetter, result, AccessorKind::Set, dc);
        if (!setter)
          return nullptr;
      }

      // Turn this into a computed property.
      // FIXME: Fake locations for '{' and '}'?
      result->setIsSetterMutating(false);
      makeComputed(result, getter, setter);
      addObjCAttribute(result, Impl.importIdentifier(decl->getIdentifier()));
      applyPropertyOwnership(result, decl->getPropertyAttributesAsWritten());

      // Handle attributes.
      if (decl->hasAttr<clang::IBOutletAttr>())
        result->getAttrs().add(
            new (Impl.SwiftContext) IBOutletAttr(/*IsImplicit=*/false));
      if (decl->getPropertyImplementation() == clang::ObjCPropertyDecl::Optional
          && isa<ProtocolDecl>(dc) &&
          !result->getAttrs().hasAttribute<OptionalAttr>())
        result->getAttrs().add(new (Impl.SwiftContext)
                                      OptionalAttr(/*implicit*/false));
      // FIXME: Handle IBOutletCollection.

      if (overridden) {
        result->setOverriddenDecl(overridden);
        getter->setOverriddenDecl(overridden->getGetter());
        if (auto parentSetter = overridden->getSetter())
          if (setter)
            setter->setOverriddenDecl(parentSetter);
      }

      // If this is a compatibility stub, mark it as such.
      if (correctSwiftName)
        markAsVariant(result, *correctSwiftName);

      return result;
    }

    Decl *
    VisitObjCCompatibleAliasDecl(const clang::ObjCCompatibleAliasDecl *decl) {
      // Import Objective-C's @compatibility_alias as typealias.
      EffectiveClangContext effectiveContext(decl->getDeclContext()->getRedeclContext());
      auto dc = Impl.importDeclContextOf(decl, effectiveContext);
      if (!dc) return nullptr;

      Optional<ImportedName> correctSwiftName;
      auto importedName = importFullName(decl, correctSwiftName);
      auto name = importedName.getDeclName().getBaseIdentifier();

      if (name.empty()) return nullptr;

      auto importedDecl =
          Impl.importDecl(decl->getClassInterface(), getActiveSwiftVersion());
      auto typeDecl = dyn_cast_or_null<TypeDecl>(importedDecl);
      if (!typeDecl) return nullptr;

      // Create typealias.
      TypeAliasDecl *typealias = nullptr;
      typealias = Impl.createDeclWithClangNode<TypeAliasDecl>(
                    decl, AccessLevel::Public,
                    Impl.importSourceLoc(decl->getLocStart()),
                    SourceLoc(), name,
                    Impl.importSourceLoc(decl->getLocation()),
                    /*genericparams=*/nullptr, dc);

      if (auto *GTD = dyn_cast<GenericTypeDecl>(typeDecl)) {
        typealias->setGenericEnvironment(GTD->getGenericEnvironment());
        if (GTD->isGeneric())
          typealias->setGenericParams(GTD->getGenericParams()->clone(typealias));
      }

      typealias->setUnderlyingType(Impl.getSugaredTypeReference(typeDecl));
      return typealias;
    }

    Decl *VisitLinkageSpecDecl(const clang::LinkageSpecDecl *decl) {
      // Linkage specifications are not imported.
      return nullptr;
    }

    Decl *VisitObjCPropertyImplDecl(const clang::ObjCPropertyImplDecl *decl) {
      // @synthesize and @dynamic are not imported, since they are not part
      // of the interface to a class.
      return nullptr;
    }

    Decl *VisitFileScopeAsmDecl(const clang::FileScopeAsmDecl *decl) {
      return nullptr;
    }

    Decl *VisitAccessSpecDecl(const clang::AccessSpecDecl *decl) {
      return nullptr;
    }

    Decl *VisitFriendDecl(const clang::FriendDecl *decl) {
      // Friends are not imported; Swift has a different access control
      // mechanism.
      return nullptr;
    }

    Decl *VisitFriendTemplateDecl(const clang::FriendTemplateDecl *decl) {
      // Friends are not imported; Swift has a different access control
      // mechanism.
      return nullptr;
    }

    Decl *VisitStaticAssertDecl(const clang::StaticAssertDecl *decl) {
      // Static assertions are an implementation detail.
      return nullptr;
    }

    Decl *VisitBlockDecl(const clang::BlockDecl *decl) {
      // Blocks are not imported (although block types can be imported).
      return nullptr;
    }

    Decl *VisitClassScopeFunctionSpecializationDecl(
                 const clang::ClassScopeFunctionSpecializationDecl *decl) {
      // Note: templates are not imported.
      return nullptr;
    }

    Decl *VisitImportDecl(const clang::ImportDecl *decl) {
      // Transitive module imports are not handled at the declaration level.
      // Rather, they are understood from the module itself.
      return nullptr;
    }
  };
} // end anonymous namespace

/// Try to strip "Mutable" out of a type name.
static clang::IdentifierInfo *
getImmutableCFSuperclassName(const clang::TypedefNameDecl *decl, clang::ASTContext &ctx) {
  StringRef name = decl->getName();

  // Split at the first occurrence of "Mutable".
  StringRef _mutable = "Mutable";
  auto mutableIndex = camel_case::findWord(name, _mutable);
  if (mutableIndex == StringRef::npos)
    return nullptr;

  StringRef namePrefix = name.substr(0, mutableIndex);
  StringRef nameSuffix = name.substr(mutableIndex + _mutable.size());

  // Abort if "Mutable" appears twice.
  if (camel_case::findWord(nameSuffix, _mutable) != StringRef::npos)
    return nullptr;

  llvm::SmallString<128> buffer;
  buffer += namePrefix;
  buffer += nameSuffix;
  return &ctx.Idents.get(buffer.str());
}

/// Check whether this CF typedef is a Mutable type, and if so,
/// look for a non-Mutable typedef.
///
/// If the "subclass" is:
///   typedef struct __foo *XXXMutableYYY;
/// then we look for a "superclass" that matches:
///   typedef const struct __foo *XXXYYY;
static Type findImmutableCFSuperclass(ClangImporter::Implementation &impl,
                                      const clang::TypedefNameDecl *decl,
                                      CFPointeeInfo subclassInfo) {
  // If this type is already immutable, it has no immutable
  // superclass.
  if (subclassInfo.isConst())
    return Type();

  // If this typedef name does not contain "Mutable", it has no
  // immutable superclass.
  auto superclassName =
      getImmutableCFSuperclassName(decl, impl.getClangASTContext());
  if (!superclassName)
    return Type();

  // Look for a typedef that successfully classifies as a CF
  // typedef with the same underlying record.
  auto superclassTypedef = impl.lookupTypedef(superclassName);
  if (!superclassTypedef)
    return Type();
  auto superclassInfo = CFPointeeInfo::classifyTypedef(superclassTypedef);
  if (!superclassInfo || !superclassInfo.isRecord() ||
      !declaresSameEntity(superclassInfo.getRecord(), subclassInfo.getRecord()))
    return Type();

  // Try to import the superclass.
  Decl *importedSuperclassDecl =
      impl.importDeclReal(superclassTypedef, impl.CurrentVersion);
  if (!importedSuperclassDecl)
    return Type();

  auto importedSuperclass =
      cast<TypeDecl>(importedSuperclassDecl)->getDeclaredInterfaceType();
  assert(importedSuperclass->is<ClassType>() && "must have class type");
  return importedSuperclass;
}

/// Attempt to find a superclass for the given CF typedef.
static Type findCFSuperclass(ClangImporter::Implementation &impl,
                             const clang::TypedefNameDecl *decl,
                             CFPointeeInfo info) {
  if (Type immutable = findImmutableCFSuperclass(impl, decl, info))
    return immutable;

  // TODO: use NSObject if it exists?
  return Type();
}

ClassDecl *
SwiftDeclConverter::importCFClassType(const clang::TypedefNameDecl *decl,
                                      Identifier className, CFPointeeInfo info,
                                      EffectiveClangContext effectiveContext) {
  auto dc = Impl.importDeclContextOf(decl, effectiveContext);
  if (!dc)
    return nullptr;

  Type superclass = findCFSuperclass(Impl, decl, info);

  // TODO: maybe use NSObject as the superclass if we can find it?
  // TODO: try to find a non-mutable type to use as the superclass.

  auto theClass = Impl.createDeclWithClangNode<ClassDecl>(
      decl, AccessLevel::Public, SourceLoc(), className, SourceLoc(), None,
      nullptr, dc);
  theClass->computeType();
  theClass->setCircularityCheck(CircularityCheck::Checked);
  theClass->setSuperclass(superclass);
  theClass->setAddedImplicitInitializers(); // suppress all initializers
  theClass->setForeignClassKind(ClassDecl::ForeignKind::CFType);
  addObjCAttribute(theClass, None);
  Impl.registerExternalDecl(theClass);

  if (superclass) {
    SmallVector<TypeLoc, 4> inheritedTypes;
    inheritedTypes.push_back(TypeLoc::withoutLoc(superclass));
    theClass->setInherited(Impl.SwiftContext.AllocateCopy(inheritedTypes));
  }

  addSynthesizedProtocolAttrs(Impl, theClass, {KnownProtocolKind::CFObject});

  // Look for bridging attributes on the clang record.  We can
  // just check the most recent redeclaration, which will inherit
  // any attributes from earlier declarations.
  auto record = info.getRecord()->getMostRecentDecl();
  if (info.isConst()) {
    if (auto attr = record->getAttr<clang::ObjCBridgeAttr>()) {
      // Record the Objective-C class to which this CF type is toll-free
      // bridged.
      if (ClassDecl *objcClass = dynCastIgnoringCompatibilityAlias<ClassDecl>(
              Impl.importDeclByName(attr->getBridgedType()->getName()))) {
        theClass->getAttrs().add(new (Impl.SwiftContext)
                                     ObjCBridgedAttr(objcClass));
      }
    }
  } else {
    if (auto attr = record->getAttr<clang::ObjCBridgeMutableAttr>()) {
      // Record the Objective-C class to which this CF type is toll-free
      // bridged.
      if (ClassDecl *objcClass = dynCastIgnoringCompatibilityAlias<ClassDecl>(
              Impl.importDeclByName(attr->getBridgedType()->getName()))) {
        theClass->getAttrs().add(new (Impl.SwiftContext)
                                     ObjCBridgedAttr(objcClass));
      }
    }
  }

  theClass->addImplicitDestructor();
  return theClass;
}

Decl *SwiftDeclConverter::importCompatibilityTypeAlias(
    const clang::NamedDecl *decl,
    ImportedName compatibilityName,
    ImportedName correctSwiftName) {
  // Import the referenced declaration. If it doesn't come in as a type,
  // we don't care.
  Decl *importedDecl = nullptr;
  if (getVersion() >= getActiveSwiftVersion())
    importedDecl = Impl.importDecl(decl, ImportNameVersion::forTypes());
  if (!importedDecl && getVersion() != getActiveSwiftVersion())
    importedDecl = Impl.importDecl(decl, getActiveSwiftVersion());
  auto typeDecl = dyn_cast_or_null<TypeDecl>(importedDecl);
  if (!typeDecl)
    return nullptr;

  auto dc = Impl.importDeclContextOf(decl,
                                     compatibilityName.getEffectiveContext());
  if (!dc)
    return nullptr;

  // Create the type alias.
  auto alias = Impl.createDeclWithClangNode<TypeAliasDecl>(
      decl, AccessLevel::Public, Impl.importSourceLoc(decl->getLocStart()),
      SourceLoc(), compatibilityName.getDeclName().getBaseIdentifier(),
      Impl.importSourceLoc(decl->getLocation()), /*generic params*/nullptr, dc);

  auto *GTD = dyn_cast<GenericTypeDecl>(typeDecl);
  if (GTD && !isa<ProtocolDecl>(GTD)) {
    alias->setGenericEnvironment(GTD->getGenericEnvironment());
    if (GTD->isGeneric())
      alias->setGenericParams(GTD->getGenericParams()->clone(alias));
  }

  alias->setUnderlyingType(Impl.getSugaredTypeReference(typeDecl));

  // Record that this is the official version of this declaration.
  Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = alias;
  markAsVariant(alias, correctSwiftName);
  return alias;
}

namespace {
  template<typename D>
  bool inheritanceListContainsProtocol(D decl, const ProtocolDecl *proto) {
    return llvm::any_of(range(decl->getInherited().size()),
                        [decl, proto](unsigned index) -> bool {
      Type type = decl->getInheritedType(index);
      if (!type || !type->isExistentialType())
        return false;

      auto layout = type->getExistentialLayout();
      for (auto protoTy : layout.getProtocols()) {
        auto *protoDecl = protoTy->getDecl();
        if (protoDecl == proto || protoDecl->inheritsFrom(proto))
          return true;
      }

      return false;
    });
  }
}

static bool conformsToProtocolInOriginalModule(NominalTypeDecl *nominal,
                                               const ProtocolDecl *proto,
                                               ModuleDecl *foundationModule,
                                               LazyResolver *resolver) {
  auto &ctx = nominal->getASTContext();

  if (inheritanceListContainsProtocol(nominal, proto))
    return true;

  for (auto attr : nominal->getAttrs().getAttributes<SynthesizedProtocolAttr>())
    if (auto *otherProto = ctx.getProtocol(attr->getProtocolKind()))
      if (otherProto == proto || otherProto->inheritsFrom(proto))
        return true;

  // Only consider extensions from the original module...or from an overlay
  // or the Swift half of a mixed-source framework.
  const DeclContext *containingFile = nominal->getModuleScopeContext();
  ModuleDecl *originalModule = containingFile->getParentModule();

  ModuleDecl *adapterModule = nullptr;
  if (auto *clangUnit = dyn_cast<ClangModuleUnit>(containingFile))
    adapterModule = clangUnit->getAdapterModule();

  for (ExtensionDecl *extension : nominal->getExtensions()) {
    ModuleDecl *extensionModule = extension->getParentModule();
    if (extensionModule != originalModule && extensionModule != adapterModule &&
        extensionModule != foundationModule) {
      continue;
    }
    if (inheritanceListContainsProtocol(extension, proto))
      return true;
  }

  return false;
}

Decl *
SwiftDeclConverter::importSwiftNewtype(const clang::TypedefNameDecl *decl,
                                       clang::SwiftNewtypeAttr *newtypeAttr,
                                       DeclContext *dc, Identifier name) {
  // The only (current) difference between swift_newtype(struct) and
  // swift_newtype(enum), until we can get real enum support, is that enums
  // have no un-labeled inits(). This is because enums are to be considered
  // closed, and if constructed from a rawValue, should be very explicit.
  bool unlabeledCtor = false;

  switch (newtypeAttr->getNewtypeKind()) {
  case clang::SwiftNewtypeAttr::NK_Enum:
    unlabeledCtor = false;
    // TODO: import as enum instead
    break;

  case clang::SwiftNewtypeAttr::NK_Struct:
    unlabeledCtor = true;
    break;
    // No other cases yet
  }

  auto &ctx = Impl.SwiftContext;
  auto Loc = Impl.importSourceLoc(decl->getLocation());

  auto structDecl = Impl.createDeclWithClangNode<StructDecl>(
      decl, AccessLevel::Public, Loc, name, Loc, None, nullptr, dc);
  structDecl->computeType();
  structDecl->setAddedImplicitInitializers();

  // Import the type of the underlying storage
  auto storedUnderlyingType = Impl.importTypeIgnoreIUO(
      decl->getUnderlyingType(), ImportTypeKind::Value, isInSystemModule(dc),
      Bridgeability::None, OTK_None);

  if (!storedUnderlyingType)
    return nullptr;

  if (auto objTy = storedUnderlyingType->getOptionalObjectType())
    storedUnderlyingType = objTy;

  // If the type is Unmanaged, that is it is not CF ARC audited,
  // we will store the underlying type and leave it up to the use site
  // to determine whether to use this new_type, or an Unmanaged<CF...> type.
  if (auto genericType = storedUnderlyingType->getAs<BoundGenericType>()) {
    if (genericType->getDecl() == Impl.SwiftContext.getUnmanagedDecl()) {
      assert(genericType->getGenericArgs().size() == 1 && "other args?");
      storedUnderlyingType = genericType->getGenericArgs()[0];
    }
  }

  // Find a bridged type, which may be different
  auto computedPropertyUnderlyingType = Impl.importTypeIgnoreIUO(
      decl->getUnderlyingType(), ImportTypeKind::Property, isInSystemModule(dc),
      Bridgeability::Full, OTK_None);
  if (auto objTy = computedPropertyUnderlyingType->getOptionalObjectType())
    computedPropertyUnderlyingType = objTy;

  bool isBridged =
      !storedUnderlyingType->isEqual(computedPropertyUnderlyingType);

  // Determine the set of protocols to which the synthesized
  // type will conform.
  SmallVector<KnownProtocolKind, 4> synthesizedProtocols;

  // Local function to add a known protocol.
  auto addKnown = [&](KnownProtocolKind kind) {
    synthesizedProtocols.push_back(kind);
  };

  // Add conformances that are always available.
  addKnown(KnownProtocolKind::RawRepresentable);
  addKnown(KnownProtocolKind::SwiftNewtypeWrapper);

  // Local function to add a known protocol only when the
  // underlying type conforms to it.
  auto computedNominal = computedPropertyUnderlyingType->getAnyNominal();
  auto transferKnown = [&](KnownProtocolKind kind) {
    if (!computedNominal)
      return false;

    auto proto = ctx.getProtocol(kind);
    if (!proto)
      return false;

    // Break circularity by only looking for declared conformances in the
    // original module, or possibly its adapter.
    if (conformsToProtocolInOriginalModule(computedNominal, proto,
                                           Impl.tryLoadFoundationModule(),
                                           Impl.getTypeResolver())) {
      synthesizedProtocols.push_back(kind);
      return true;
    }

    return false;
  };

  // Transfer conformances. Each of these needs a forwarding
  // implementation in the standard library.
  transferKnown(KnownProtocolKind::Equatable);
  transferKnown(KnownProtocolKind::Hashable);
  bool hasObjCBridgeable =
      transferKnown(KnownProtocolKind::ObjectiveCBridgeable);
  bool wantsObjCBridgeableTypealias = hasObjCBridgeable && isBridged;

  // Wrappers around ObjC classes and protocols are also bridgeable.
  if (!hasObjCBridgeable) {
    if (isBridged) {
      if (auto *proto = dyn_cast_or_null<ProtocolDecl>(computedNominal))
        if (proto->getKnownProtocolKind() == KnownProtocolKind::Error)
          hasObjCBridgeable = true;
    } else {
      if (auto *objcClass = dyn_cast_or_null<ClassDecl>(computedNominal)) {
        switch (objcClass->getForeignClassKind()) {
        case ClassDecl::ForeignKind::Normal:
        case ClassDecl::ForeignKind::RuntimeOnly:
          if (objcClass->hasClangNode())
            hasObjCBridgeable = true;
          break;
        case ClassDecl::ForeignKind::CFType:
          break;
        }
      } else if (storedUnderlyingType->isObjCExistentialType()) {
        hasObjCBridgeable = true;
      }
    }

    if (hasObjCBridgeable) {
      addKnown(KnownProtocolKind::ObjectiveCBridgeable);
      wantsObjCBridgeableTypealias = true;
    }
  }

  if (!isBridged) {
    // Simple, our stored type is equivalent to our computed
    // type.
    auto options = getDefaultMakeStructRawValuedOptions();
    if (unlabeledCtor)
      options |= MakeStructRawValuedFlags::MakeUnlabeledValueInit;

    makeStructRawValued(Impl, structDecl, storedUnderlyingType,
                        synthesizedProtocols, options);
  } else {
    // We need to make a stored rawValue or storage type, and a
    // computed one of bridged type.
    makeStructRawValuedWithBridge(Impl, structDecl, storedUnderlyingType,
                                  computedPropertyUnderlyingType,
                                  synthesizedProtocols,
                                  /*makeUnlabeledValueInit=*/unlabeledCtor);
  }

  if (wantsObjCBridgeableTypealias) {
    addSynthesizedTypealias(structDecl, ctx.Id_ObjectiveCType,
                            storedUnderlyingType);
  }

  Impl.ImportedDecls[{decl->getCanonicalDecl(), getVersion()}] = structDecl;
  Impl.registerExternalDecl(structDecl);
  return structDecl;
}

Decl *SwiftDeclConverter::importEnumCase(const clang::EnumConstantDecl *decl,
                                         const clang::EnumDecl *clangEnum,
                                         EnumDecl *theEnum,
                                         Decl *correctDecl) {
  auto &context = Impl.SwiftContext;
  Optional<ImportedName> correctSwiftName;
  auto name =
      importFullName(decl, correctSwiftName).getDeclName().getBaseIdentifier();
  if (name.empty())
    return nullptr;

  if (correctSwiftName) {
    // We're creating a compatibility stub. Treat it as an enum case alias.
    auto correctCase = dyn_cast_or_null<EnumElementDecl>(correctDecl);
    if (!correctCase)
      return nullptr;

    // If the correct declaration was unavailable, don't map to it.
    // FIXME: This eliminates spurious errors, but affects QoI.
    if (correctCase->getAttrs().isUnavailable(Impl.SwiftContext))
      return nullptr;

    auto compatibilityCase =
        importEnumCaseAlias(name, decl, correctCase, clangEnum, theEnum);
    if (compatibilityCase)
      markAsVariant(compatibilityCase, *correctSwiftName);

    return compatibilityCase;
  }

  // Use the constant's underlying value as its raw value in Swift.
  bool negative = false;
  llvm::APSInt rawValue = decl->getInitVal();

  if (clangEnum->getIntegerType()->isSignedIntegerOrEnumerationType() &&
      rawValue.slt(0)) {
    rawValue = -rawValue;
    negative = true;
  }
  llvm::SmallString<12> rawValueText;
  rawValue.toString(rawValueText, 10, /*signed*/ false);
  StringRef rawValueTextC = context.AllocateCopy(StringRef(rawValueText));
  auto rawValueExpr =
      new (context) IntegerLiteralExpr(rawValueTextC, SourceLoc(),
                                       /*implicit*/ false);
  if (negative)
    rawValueExpr->setNegative(SourceLoc());

  auto element = Impl.createDeclWithClangNode<EnumElementDecl>(
      decl, AccessLevel::Public, SourceLoc(), name, nullptr,
      SourceLoc(), rawValueExpr, theEnum);

  // Give the enum element the appropriate type.
  element->computeType();

  Impl.importAttributes(decl, element);

  return element;
}

Decl *
SwiftDeclConverter::importOptionConstant(const clang::EnumConstantDecl *decl,
                                         const clang::EnumDecl *clangEnum,
                                         NominalTypeDecl *theStruct) {
  Optional<ImportedName> correctSwiftName;
  ImportedName nameInfo = importFullName(decl, correctSwiftName);
  Identifier name = nameInfo.getDeclName().getBaseIdentifier();
  if (name.empty())
    return nullptr;

  // Create the constant.
  auto convertKind = ConstantConvertKind::Construction;
  if (isa<EnumDecl>(theStruct))
    convertKind = ConstantConvertKind::ConstructionWithUnwrap;
  Decl *CD = Impl.createConstant(
      name, theStruct, theStruct->getDeclaredTypeInContext(),
      clang::APValue(decl->getInitVal()), convertKind, /*isStatic*/ true, decl);
  Impl.importAttributes(decl, CD);

  // NS_OPTIONS members that have a value of 0 (typically named "None") do
  // not operate as a set-like member.  Mark them unavailable with a message
  // that says that they should be used as [].
  if (decl->getInitVal() == 0 && !nameInfo.hasCustomName() &&
      !CD->getAttrs().isUnavailable(Impl.SwiftContext)) {
    /// Create an AvailableAttr that indicates specific availability
    /// for all platforms.
    auto attr = AvailableAttr::createPlatformAgnostic(
        Impl.SwiftContext, "use [] to construct an empty option set");
    CD->getAttrs().add(attr);
  }

  // If this is a compatibility stub, mark it as such.
  if (correctSwiftName)
    markAsVariant(CD, *correctSwiftName);

  return CD;
}

Decl *SwiftDeclConverter::importEnumCaseAlias(
    Identifier name, const clang::EnumConstantDecl *alias, ValueDecl *original,
    const clang::EnumDecl *clangEnum, NominalTypeDecl *importedEnum,
    DeclContext *importIntoDC) {
  if (name.empty())
    return nullptr;

  // Default the DeclContext to the enum type.
  if (!importIntoDC)
    importIntoDC = importedEnum;

  // Construct the original constant. Enum constants without payloads look
  // like simple values, but actually have type 'MyEnum.Type -> MyEnum'.
  auto constantRef =
      new (Impl.SwiftContext) DeclRefExpr(original, DeclNameLoc(),
                                          /*implicit*/ true);
  Type importedEnumTy = importedEnum->getDeclaredTypeInContext();
  auto typeRef = TypeExpr::createImplicit(importedEnumTy, Impl.SwiftContext);
  auto instantiate = new (Impl.SwiftContext)
      DotSyntaxCallExpr(constantRef, SourceLoc(), typeRef);
  instantiate->setType(importedEnumTy);

  Decl *CD = Impl.createConstant(name, importIntoDC, importedEnumTy,
                                 instantiate, ConstantConvertKind::None,
                                 /*isStatic*/ true, alias);
  Impl.importAttributes(alias, CD);
  return CD;
}

NominalTypeDecl *
SwiftDeclConverter::importAsOptionSetType(DeclContext *dc, Identifier name,
                                          const clang::EnumDecl *decl) {
  ASTContext &ctx = Impl.SwiftContext;

  // Compute the underlying type.
  auto underlyingType = Impl.importTypeIgnoreIUO(
      decl->getIntegerType(), ImportTypeKind::Enum, isInSystemModule(dc),
      Bridgeability::None);
  if (!underlyingType)
    return nullptr;

  auto Loc = Impl.importSourceLoc(decl->getLocation());

  // Create a struct with the underlying type as a field.
  auto structDecl = Impl.createDeclWithClangNode<StructDecl>(
      decl, AccessLevel::Public, Loc, name, Loc, None, nullptr, dc);
  structDecl->computeType();
  structDecl->setAddedImplicitInitializers();

  makeStructRawValued(Impl, structDecl, underlyingType,
                      {KnownProtocolKind::OptionSet});
  auto selfType = structDecl->getDeclaredInterfaceType();
  addSynthesizedTypealias(structDecl, ctx.Id_Element, selfType);
  addSynthesizedTypealias(structDecl, ctx.Id_ArrayLiteralElement, selfType);
  return structDecl;
}

Decl *SwiftDeclConverter::importGlobalAsInitializer(
    const clang::FunctionDecl *decl,
    DeclName name,
    DeclContext *dc,
    CtorInitializerKind initKind,
    Optional<ImportedName> correctSwiftName) {
  // TODO: Should this be an error? How can this come up?
  assert(dc->isTypeContext() && "cannot import as member onto non-type");

  // Check for some invalid imports
  if (dc->getAsProtocolOrProtocolExtensionContext()) {
    // FIXME: clang source location
    Impl.SwiftContext.Diags.diagnose({}, diag::swift_name_protocol_static,
                                     /*isInit=*/true);
    Impl.SwiftContext.Diags.diagnose({}, diag::note_while_importing,
                                     decl->getName());
    return nullptr;
  }

  bool allowNSUIntegerAsInt =
      Impl.shouldAllowNSUIntegerAsInt(isInSystemModule(dc), decl);

  ArrayRef<Identifier> argNames = name.getArgumentNames();

  ParameterList *parameterList = nullptr;
  if (argNames.size() == 1 && decl->getNumParams() == 0) {
    // Special case: We need to create an empty first parameter for our
    // argument label
    auto *paramDecl =
        new (Impl.SwiftContext) ParamDecl(
            VarDecl::Specifier::Default, SourceLoc(), SourceLoc(), argNames.front(),
            SourceLoc(), argNames.front(), Impl.SwiftContext.TheEmptyTupleType,
            dc);
    paramDecl->setInterfaceType(Impl.SwiftContext.TheEmptyTupleType);
    paramDecl->setValidationToChecked();

    parameterList = ParameterList::createWithoutLoc(paramDecl);
  } else {
    parameterList = Impl.importFunctionParameterList(
        dc, decl, {decl->param_begin(), decl->param_end()}, decl->isVariadic(),
        allowNSUIntegerAsInt, argNames);
  }
  if (!parameterList)
    return nullptr;

  bool selfIsInOut = !dc->getDeclaredInterfaceType()->hasReferenceSemantics();
  auto selfParam = ParamDecl::createSelf(SourceLoc(), dc, /*isStatic=*/false,
                                         /*isInOut=*/selfIsInOut);

  auto importedType =
      Impl.importFunctionReturnType(dc, decl, allowNSUIntegerAsInt);

  // Update the failability appropriately based on the imported method type.
  OptionalTypeKind initOptionality = OTK_None;
  if (importedType.isImplicitlyUnwrapped()) {
    assert(importedType.getType()->getOptionalObjectType());
    initOptionality = OTK_ImplicitlyUnwrappedOptional;
  } else if (importedType.getType()->getOptionalObjectType()) {
    initOptionality = OTK_Optional;
  }

  auto result = Impl.createDeclWithClangNode<ConstructorDecl>(
      decl, AccessLevel::Public, name, /*NameLoc=*/SourceLoc(),
      initOptionality, /*FailabilityLoc=*/SourceLoc(),
      /*Throws=*/false, /*ThrowsLoc=*/SourceLoc(), selfParam, parameterList,
      /*GenericParams=*/nullptr, dc);
  result->setInitKind(initKind);
  result->setImportAsStaticMember();

  // Set the constructor's type.
  result->computeType();
  Impl.recordImplicitUnwrapForDecl(result,
                                   importedType.isImplicitlyUnwrapped());
  result->setOverriddenDecls({ });
  result->setIsObjC(false);

  finishFuncDecl(decl, result);
  if (correctSwiftName)
    markAsVariant(result, *correctSwiftName);
  return result;
}

Decl *SwiftDeclConverter::importGlobalAsMethod(
    const clang::FunctionDecl *decl,
    DeclName name,
    DeclContext *dc,
    Optional<unsigned> selfIdx,
    Optional<ImportedName> correctSwiftName,
    Optional<AccessorInfo> accessorInfo) {
  if (dc->getAsProtocolOrProtocolExtensionContext() && !selfIdx) {
    // FIXME: source location...
    Impl.SwiftContext.Diags.diagnose({}, diag::swift_name_protocol_static,
                                     /*isInit=*/false);
    Impl.SwiftContext.Diags.diagnose({}, diag::note_while_importing,
                                     decl->getName());
    return nullptr;
  }

  if (!decl->hasPrototype()) {
    // FIXME: source location...
    Impl.SwiftContext.Diags.diagnose({}, diag::swift_name_no_prototype);
    Impl.SwiftContext.Diags.diagnose({}, diag::note_while_importing,
                                     decl->getName());
    return nullptr;
  }

  bool allowNSUIntegerAsInt =
      Impl.shouldAllowNSUIntegerAsInt(isInSystemModule(dc), decl);

  auto &C = Impl.SwiftContext;
  // There is an inout 'self' when the parameter is a pointer to a non-const
  // instance of the type we're importing onto. Importing this as a method means
  // that the method should be treated as mutating in this situation.
  bool selfIsInOut = false;
  if (selfIdx && !dc->getDeclaredInterfaceType()->hasReferenceSemantics()) {
    auto selfParam = decl->getParamDecl(*selfIdx);
    auto selfParamTy = selfParam->getType();
    if ((selfParamTy->isPointerType() || selfParamTy->isReferenceType()) &&
        !selfParamTy->getPointeeType().isConstQualified()) {
      selfIsInOut = true;

      // If there's a swift_newtype, check the levels of indirection: self is
      // only inout if this is a pointer to the typedef type (which itself is a
      // pointer).
      if (auto nominalTypeDecl =
              dc->getAsNominalTypeOrNominalTypeExtensionContext()) {
        if (auto clangDCTy = dyn_cast_or_null<clang::TypedefNameDecl>(
                nominalTypeDecl->getClangDecl()))
          if (getSwiftNewtypeAttr(clangDCTy, getVersion()))
            if (clangDCTy->getUnderlyingType().getCanonicalType() !=
                selfParamTy->getPointeeType().getCanonicalType())
              selfIsInOut = false;
      }
    }
  }

  auto *selfDecl = ParamDecl::createSelf(
      SourceLoc(), dc, !selfIdx.hasValue(), selfIsInOut);
  auto *bodyParams = getNonSelfParamList(
      dc, decl, selfIdx, name.getArgumentNames(), allowNSUIntegerAsInt, !name);

  auto importedType =
      Impl.importFunctionReturnType(dc, decl, allowNSUIntegerAsInt);
  Type swiftResultTy = importedType.getType();

  auto loc = Impl.importSourceLoc(decl->getLocation());
  auto nameLoc = Impl.importSourceLoc(decl->getLocation());
  auto result =
    createFuncOrAccessor(C, loc, accessorInfo, name, nameLoc,
                         selfDecl, bodyParams, swiftResultTy,
                         /*throws*/ false, dc, decl);

  result->setGenericEnvironment(dc->getGenericEnvironmentOfContext());

  result->setAccess(AccessLevel::Public);
  if (selfIsInOut)
    result->setSelfAccessKind(SelfAccessKind::Mutating);
  if (selfIdx) {
    result->setSelfIndex(selfIdx.getValue());
  } else {
    result->setStatic();
    result->setImportAsStaticMember();
  }

  result->computeType();
  result->setValidationToChecked();

  Impl.recordImplicitUnwrapForDecl(result,
                                   importedType.isImplicitlyUnwrapped());

  assert(selfIdx ? result->getSelfIndex() == *selfIdx
                 : result->isImportAsStaticMember());

  if (dc->getAsClassOrClassExtensionContext())
    // FIXME: only if the class itself is not marked final
    result->getAttrs().add(new (C) FinalAttr(/*IsImplicit=*/true));

  finishFuncDecl(decl, result);
  if (correctSwiftName)
    markAsVariant(result, *correctSwiftName);
  return result;
}

/// Create an implicit property given the imported name of one of
/// the accessors.
VarDecl *
SwiftDeclConverter::getImplicitProperty(ImportedName importedName,
                                         const clang::FunctionDecl *accessor) {
  // Check whether we already know about the property.
  auto knownProperty = Impl.FunctionsAsProperties.find(accessor);
  if (knownProperty != Impl.FunctionsAsProperties.end())
    return knownProperty->second;

  // Determine whether we have the getter or setter.
  const clang::FunctionDecl *getter = nullptr;
  ImportedName getterName;
  Optional<ImportedName> swift3GetterName;
  const clang::FunctionDecl *setter = nullptr;
  ImportedName setterName;
  Optional<ImportedName> swift3SetterName;
  switch (importedName.getAccessorKind()) {
  case ImportedAccessorKind::None:
  case ImportedAccessorKind::SubscriptGetter:
  case ImportedAccessorKind::SubscriptSetter:
    llvm_unreachable("Not a property accessor");

  case ImportedAccessorKind::PropertyGetter:
    getter = accessor;
    getterName = importedName;
    break;

  case ImportedAccessorKind::PropertySetter:
    setter = accessor;
    setterName = importedName;
    break;
  }

  // Find the other accessor, if it exists.
  auto propertyName = importedName.getDeclName().getBaseIdentifier();
  auto lookupTable =
      Impl.findLookupTable(*getClangSubmoduleForDecl(accessor));
  assert(lookupTable && "No lookup table?");
  bool foundAccessor = false;
  for (auto entry : lookupTable->lookup(SerializedSwiftName(propertyName),
                                        importedName.getEffectiveContext())) {
    auto decl = entry.dyn_cast<clang::NamedDecl *>();
    if (!decl)
      continue;

    auto function = dyn_cast<clang::FunctionDecl>(decl);
    if (!function)
      continue;

    if (function->getCanonicalDecl() == accessor->getCanonicalDecl()) {
      foundAccessor = true;
      continue;
    }

    if (!getter) {
      // Find the self index for the getter.
      getterName = importFullName(function, swift3GetterName);
      if (!getterName)
        continue;

      getter = function;
      continue;
    }

    if (!setter) {
      // Find the self index for the setter.
      setterName = importFullName(function, swift3SetterName);
      if (!setterName)
        continue;

      setter = function;
      continue;
    }

    // We already have both a getter and a setter; something is
    // amiss, so bail out.
    return nullptr;
  }

  assert(foundAccessor && "Didn't find the original accessor? "
                          "Try clearing your module cache");

  // If there is no getter, there's nothing we can do.
  if (!getter)
    return nullptr;

  // Retrieve the type of the property that is implied by the getter.
  auto propertyType =
      getAccessorPropertyType(getter, false, getterName.getSelfIndex());
  if (propertyType.isNull())
    return nullptr;

  // If there is a setter, check that the property it implies
  // matches that of the getter.
  if (setter) {
    auto setterPropertyType =
        getAccessorPropertyType(setter, true, setterName.getSelfIndex());
    if (setterPropertyType.isNull())
      return nullptr;

    // If the inferred property types don't match up, we can't
    // form a property.
    if (!getter->getASTContext().hasSameType(propertyType, setterPropertyType))
      return nullptr;
  }

  // Import the property's context.
  auto dc = Impl.importDeclContextOf(getter, getterName.getEffectiveContext());
  if (!dc)
    return nullptr;

  // Is this a static property?
  bool isStatic = false;
  if (dc->isTypeContext() && !getterName.getSelfIndex())
    isStatic = true;

  // Compute the property type.
  bool isFromSystemModule = isInSystemModule(dc);
  auto importedType = Impl.importType(
      propertyType, ImportTypeKind::Property,
      Impl.shouldAllowNSUIntegerAsInt(isFromSystemModule, getter),
      Bridgeability::Full, OTK_ImplicitlyUnwrappedOptional);
  if (!importedType)
    return nullptr;

  Type swiftPropertyType = importedType.getType();

  auto property = Impl.createDeclWithClangNode<VarDecl>(
      getter, AccessLevel::Public, /*IsStatic*/isStatic,
      VarDecl::Specifier::Var, /*IsCaptureList*/false, SourceLoc(),
      propertyName, dc->mapTypeIntoContext(swiftPropertyType), dc);
  property->setInterfaceType(swiftPropertyType);
  property->setIsObjC(false);
  Impl.recordImplicitUnwrapForDecl(property,
                                   importedType.isImplicitlyUnwrapped());

  // Note that we've formed this property.
  Impl.FunctionsAsProperties[getter] = property;
  if (setter)
    Impl.FunctionsAsProperties[setter] = property;

  // If this property is in a class or class extension context,
  // add "final".
  if (dc->getAsClassOrClassExtensionContext())
    property->getAttrs().add(new (Impl.SwiftContext)
                                 FinalAttr(/*IsImplicit=*/true));

  // Import the getter.
  auto *swiftGetter = dyn_cast_or_null<AccessorDecl>(
      importFunctionDecl(getter, getterName, None,
                         AccessorInfo{property, AccessorKind::Get}));
  if (!swiftGetter)
    return nullptr;

  Impl.importAttributes(getter, swiftGetter);
  Impl.ImportedDecls[{getter, getVersion()}] = swiftGetter;
  if (swift3GetterName)
    markAsVariant(swiftGetter, *swift3GetterName);

  // Import the setter.
  AccessorDecl *swiftSetter = nullptr;
  if (setter) {
    swiftSetter = dyn_cast_or_null<AccessorDecl>(
        importFunctionDecl(setter, setterName, None,
                           AccessorInfo{property, AccessorKind::Set}));
    if (!swiftSetter)
      return nullptr;

    Impl.importAttributes(setter, swiftSetter);
    Impl.ImportedDecls[{setter, getVersion()}] = swiftSetter;
    if (swift3SetterName)
      markAsVariant(swiftSetter, *swift3SetterName);
  }

  if (swiftGetter) property->setIsGetterMutating(swiftGetter->isMutating());
  if (swiftSetter) property->setIsSetterMutating(swiftSetter->isMutating());

  // Make this a computed property.
  makeComputed(property, swiftGetter, swiftSetter);

  // Make the property the alternate declaration for the getter.
  Impl.addAlternateDecl(swiftGetter, property);

  return property;
}

ConstructorDecl *SwiftDeclConverter::importConstructor(
    const clang::ObjCMethodDecl *objcMethod, DeclContext *dc, bool implicit,
    Optional<CtorInitializerKind> kind, bool required) {
  // Only methods in the 'init' family can become constructors.
  assert(isInitMethod(objcMethod) && "Not a real init method");

  // Check whether we've already created the constructor.
  auto known =
      Impl.Constructors.find(std::make_tuple(objcMethod, dc, getVersion()));
  if (known != Impl.Constructors.end())
    return known->second;

  // Check whether there is already a method with this selector.
  auto selector = Impl.importSelector(objcMethod->getSelector());
  if (isActiveSwiftVersion() &&
      isMethodAlreadyImported(selector, /*isInstance=*/true, dc,
                              [](AbstractFunctionDecl *fn) {
        return true;
      }))
    return nullptr;

  // Map the name and complete the import.
  ArrayRef<const clang::ParmVarDecl *> params{objcMethod->param_begin(),
                                              objcMethod->param_end()};

  bool variadic = objcMethod->isVariadic();
  Optional<ImportedName> correctSwiftName;
  auto importedName = importFullName(objcMethod, correctSwiftName);
  if (!importedName)
    return nullptr;

  // If we dropped the variadic, handle it now.
  if (importedName.droppedVariadic()) {
    selector = ObjCSelector(Impl.SwiftContext, selector.getNumArgs() - 1,
                            selector.getSelectorPieces().drop_back());
    params = params.drop_back(1);
    variadic = false;
  }

  bool redundant;
  auto result =
      importConstructor(objcMethod, dc, implicit, kind, required, selector,
                        importedName, params, variadic, redundant);

  // If this is a compatibility stub, mark it as such.
  if (result && correctSwiftName)
    markAsVariant(result, *correctSwiftName);

  return result;
}

/// Returns the latest "introduced" version on the current platform for
/// \p D.
clang::VersionTuple
SwiftDeclConverter::findLatestIntroduction(const clang::Decl *D) {
  clang::VersionTuple result;

  for (auto *attr : D->specific_attrs<clang::AvailabilityAttr>()) {
    if (attr->getPlatform()->getName() == "swift") {
      clang::VersionTuple maxVersion{~0U, ~0U, ~0U};
      return maxVersion;
    }

    // Does this availability attribute map to the platform we are
    // currently targeting?
    if (!Impl.platformAvailability.filter ||
        !Impl.platformAvailability.filter(attr->getPlatform()->getName()))
      continue;

    // Take advantage of the empty version being 0.0.0.0.
    result = std::max(result, attr->getIntroduced());
  }

  return result;
}

/// Returns true if importing \p objcMethod will produce a "better"
/// initializer than \p existingCtor.
bool SwiftDeclConverter::existingConstructorIsWorse(
    const ConstructorDecl *existingCtor,
    const clang::ObjCMethodDecl *objcMethod, CtorInitializerKind kind) {
  CtorInitializerKind existingKind = existingCtor->getInitKind();

  // If one constructor is unavailable in Swift and the other is
  // not, keep the available one.
  bool existingIsUnavailable =
      existingCtor->getAttrs().isUnavailable(Impl.SwiftContext);
  bool newIsUnavailable = Impl.isUnavailableInSwift(objcMethod);
  if (existingIsUnavailable != newIsUnavailable)
    return existingIsUnavailable;

  // If the new kind is the same as the existing kind, stick with
  // the existing constructor.
  if (existingKind == kind)
    return false;

  // Check for cases that are obviously better or obviously worse.
  if (kind == CtorInitializerKind::Designated ||
      existingKind == CtorInitializerKind::Factory)
    return true;

  if (kind == CtorInitializerKind::Factory ||
      existingKind == CtorInitializerKind::Designated)
    return false;

  assert(kind == CtorInitializerKind::Convenience ||
         kind == CtorInitializerKind::ConvenienceFactory);
  assert(existingKind == CtorInitializerKind::Convenience ||
         existingKind == CtorInitializerKind::ConvenienceFactory);

  // Between different kinds of convenience initializers, keep the one that
  // was introduced first.
  // FIXME: But if one of them is now deprecated, should we prefer the
  // other?
  clang::VersionTuple introduced = findLatestIntroduction(objcMethod);
  AvailabilityContext existingAvailability =
      AvailabilityInference::availableRange(existingCtor, Impl.SwiftContext);
  assert(!existingAvailability.isKnownUnreachable());

  if (existingAvailability.isAlwaysAvailable()) {
    if (!introduced.empty())
      return false;
  } else {
    VersionRange existingIntroduced = existingAvailability.getOSVersion();
    if (introduced != existingIntroduced.getLowerEndpoint()) {
      return introduced < existingIntroduced.getLowerEndpoint();
    }
  }

  // The "introduced" versions are the same. Prefer Convenience over
  // ConvenienceFactory, but otherwise prefer leaving things as they are.
  if (kind == CtorInitializerKind::Convenience &&
      existingKind == CtorInitializerKind::ConvenienceFactory)
    return true;

  return false;
}

/// \brief Given an imported method, try to import it as a constructor.
///
/// Objective-C methods in the 'init' family are imported as
/// constructors in Swift, enabling object construction syntax, e.g.,
///
/// \code
/// // in objc: [[NSArray alloc] initWithCapacity:1024]
/// NSArray(capacity: 1024)
/// \endcode
///
/// This variant of the function is responsible for actually binding the
/// constructor declaration appropriately.
ConstructorDecl *SwiftDeclConverter::importConstructor(
    const clang::ObjCMethodDecl *objcMethod, DeclContext *dc, bool implicit,
    Optional<CtorInitializerKind> kindIn, bool required, ObjCSelector selector,
    ImportedName importedName, ArrayRef<const clang::ParmVarDecl *> args,
    bool variadic, bool &redundant) {
  redundant = false;

  // Figure out the type of the container.
  auto ownerNominal = dc->getAsNominalTypeOrNominalTypeExtensionContext();
  assert(ownerNominal && "Method in non-type context?");

  // Find the interface, if we can.
  const clang::ObjCInterfaceDecl *interface = nullptr;
  if (auto classDecl = dyn_cast<ClassDecl>(ownerNominal)) {
    interface =
        dyn_cast_or_null<clang::ObjCInterfaceDecl>(classDecl->getClangDecl());
  }

  // If we weren't told what kind of initializer this should be,
  // figure it out now.
  CtorInitializerKind kind;

  if (kindIn) {
    kind = *kindIn;

    // If we know this is a designated initializer, mark it as such.
    if (interface && hasDesignatedInitializers(interface) &&
        isDesignatedInitializer(interface, objcMethod))
      kind = CtorInitializerKind::Designated;
  } else {
    // If the owning Objective-C class has designated initializers and this
    // is not one of them, treat it as a convenience initializer.
    if (interface && hasDesignatedInitializers(interface) &&
        !isDesignatedInitializer(interface, objcMethod)) {
      kind = CtorInitializerKind::Convenience;
    } else {
      kind = CtorInitializerKind::Designated;
    }
  }

  // Import the type that this method will have.
  Optional<ForeignErrorConvention> errorConvention;
  ParameterList *bodyParams;
  auto importedType = Impl.importMethodType(
      dc, objcMethod, args, variadic, isInSystemModule(dc), &bodyParams,
      importedName, errorConvention, SpecialMethodKind::Constructor);
  if (!importedType)
    return nullptr;

  // Determine the failability of this initializer.
  auto oldFnType = importedType.getType()->castTo<AnyFunctionType>();
  bool resultIsOptional = (bool) oldFnType->getResult()->getOptionalObjectType();

  // Update the failability appropriately based on the imported method type.
  assert(resultIsOptional || !importedType.isImplicitlyUnwrapped());
  OptionalTypeKind failability = OTK_None;
  if (resultIsOptional) {
    failability = OTK_Optional;
    if (importedType.isImplicitlyUnwrapped())
      failability = OTK_ImplicitlyUnwrappedOptional;
  }

  // Rebuild the function type with the appropriate result type;
  Type resultTy = dc->getSelfInterfaceType();
  if (resultIsOptional)
    resultTy = OptionalType::get(resultTy);

  // Look for other imported constructors that occur in this context with
  // the same name.
  Type allocParamType = oldFnType->getInput();
  bool ignoreNewExtensions = isa<ClassDecl>(dc);
  for (auto other : ownerNominal->lookupDirect(importedName.getDeclName(),
                                               ignoreNewExtensions)) {
    auto ctor = dyn_cast<ConstructorDecl>(other);
    if (!ctor || ctor->isInvalid() ||
        ctor->getAttrs().isUnavailable(Impl.SwiftContext) ||
        !ctor->getClangDecl())
      continue;

    // Resolve the type of the constructor.
    if (!ctor->hasInterfaceType())
      Impl.getTypeResolver()->resolveDeclSignature(ctor);

    // If the types don't match, this is a different constructor with
    // the same selector. This can happen when an overlay overloads an
    // existing selector with a Swift-only signature.
    Type ctorParamType = ctor->getInterfaceType()
                             ->castTo<AnyFunctionType>()
                             ->getResult()
                             ->castTo<AnyFunctionType>()
                             ->getInput();
    if (!ctorParamType->isEqual(allocParamType)) {
      continue;
    }

    // If the existing constructor has a less-desirable kind, mark
    // the existing constructor unavailable.
    if (existingConstructorIsWorse(ctor, objcMethod, kind)) {
      // Show exactly where this constructor came from.
      llvm::SmallString<32> errorStr;
      errorStr += "superseded by import of ";
      if (objcMethod->isClassMethod())
        errorStr += "+[";
      else
        errorStr += "-[";

      auto objcDC = objcMethod->getDeclContext();
      if (auto objcClass = dyn_cast<clang::ObjCInterfaceDecl>(objcDC)) {
        errorStr += objcClass->getName();
        errorStr += ' ';
      } else if (auto objcCat = dyn_cast<clang::ObjCCategoryDecl>(objcDC)) {
        errorStr += objcCat->getClassInterface()->getName();
        auto catName = objcCat->getName();
        if (!catName.empty()) {
          errorStr += '(';
          errorStr += catName;
          errorStr += ')';
        }
        errorStr += ' ';
      } else if (auto objcProto = dyn_cast<clang::ObjCProtocolDecl>(objcDC)) {
        errorStr += objcProto->getName();
        errorStr += ' ';
      }

      errorStr += objcMethod->getSelector().getAsString();
      errorStr += ']';

      auto attr = AvailableAttr::createPlatformAgnostic(
          Impl.SwiftContext, Impl.SwiftContext.AllocateCopy(errorStr.str()));
      ctor->getAttrs().add(attr);
      continue;
    }

    // Otherwise, we shouldn't create a new constructor, because
    // it will be no better than the existing one.
    redundant = true;
    return nullptr;
  }

  // Check whether we've already created the constructor.
  auto known =
      Impl.Constructors.find(std::make_tuple(objcMethod, dc, getVersion()));
  if (known != Impl.Constructors.end())
    return known->second;

  auto *selfVar = ParamDecl::createSelf(SourceLoc(), dc);

  // Create the actual constructor.
  auto result = Impl.createDeclWithClangNode<ConstructorDecl>(
      objcMethod, AccessLevel::Public, importedName.getDeclName(),
      /*NameLoc=*/SourceLoc(), failability, /*FailabilityLoc=*/SourceLoc(),
      /*Throws=*/importedName.getErrorInfo().hasValue(),
      /*ThrowsLoc=*/SourceLoc(), selfVar, bodyParams,
      /*GenericParams=*/nullptr, dc);

  // Make the constructor declaration immediately visible in its
  // class or protocol type.
  ownerNominal->makeMemberVisible(result);

  addObjCAttribute(result, selector);

  // Calculate the function type of the result.
  result->setGenericEnvironment(dc->getGenericEnvironmentOfContext());
  result->computeType();

  Impl.recordImplicitUnwrapForDecl(result,
                                   importedType.isImplicitlyUnwrapped());

  if (implicit)
    result->setImplicit();

  // Set the kind of initializer.
  result->setInitKind(kind);

  // Consult API notes to determine whether this initializer is required.
  if (!required && isRequiredInitializer(objcMethod))
    required = true;

  // Check whether this initializer satisfies a requirement in a protocol.
  if (!required && !isa<ProtocolDecl>(dc) && objcMethod->isInstanceMethod()) {
    auto objcParent =
        cast<clang::ObjCContainerDecl>(objcMethod->getDeclContext());

    if (isa<clang::ObjCProtocolDecl>(objcParent)) {
      // An initializer declared in a protocol is required.
      required = true;
    } else {
      // If the class in which this initializer was declared conforms to a
      // protocol that requires this initializer, then this initializer is
      // required.
      SmallPtrSet<clang::ObjCProtocolDecl *, 8> objcProtocols;
      objcParent->getASTContext().CollectInheritedProtocols(objcParent,
                                                            objcProtocols);
      for (auto objcProto : objcProtocols) {
        for (auto decl : objcProto->lookup(objcMethod->getSelector())) {
          if (cast<clang::ObjCMethodDecl>(decl)->isInstanceMethod()) {
            required = true;
            break;
          }
        }

        if (required)
          break;
      }
    }
  }

  // If this initializer is required, add the appropriate attribute.
  if (required) {
    result->getAttrs().add(new (Impl.SwiftContext)
                               RequiredAttr(/*IsImplicit=*/true));
  }

  // Record the error convention.
  if (errorConvention) {
    result->setForeignErrorConvention(*errorConvention);
  }

  // Record the constructor for future re-use.
  Impl.Constructors[std::make_tuple(objcMethod, dc, getVersion())] = result;

  // If this constructor overrides another constructor, mark it as such.
  recordObjCOverride(result);

  // Inform the context that we have external definitions.
  Impl.registerExternalDecl(result);

  return result;
}

void SwiftDeclConverter::recordObjCOverride(AbstractFunctionDecl *decl) {
  // Make sure that we always set the overriden declarations.
  SWIFT_DEFER {
    if (!decl->overriddenDeclsComputed())
      decl->setOverriddenDecls({ });
  };

  // Figure out the class in which this method occurs.
  auto classDecl = decl->getDeclContext()->getAsClassOrClassExtensionContext();
  if (!classDecl)
    return;
  auto superTy = classDecl->getSuperclass();
  if (!superTy)
    return;
  // Dig out the Objective-C superclass.
  auto superDecl = superTy->getAnyNominal();
  SmallVector<ValueDecl *, 4> results;
  superDecl->lookupQualified(superTy, decl->getFullName(),
                             NL_QualifiedDefault | NL_KnownNoDependency,
                             Impl.getTypeResolver(), results);
  for (auto member : results) {
    if (member->getKind() != decl->getKind() ||
        member->isInstanceMember() != decl->isInstanceMember() ||
        member->isObjC() != decl->isObjC())
      continue;
    // Set function override.
    if (auto func = dyn_cast<FuncDecl>(decl)) {
      auto foundFunc = cast<FuncDecl>(member);
      // Require a selector match.
      if (foundFunc->isObjC() &&
          func->getObjCSelector() != foundFunc->getObjCSelector())
        continue;
      func->setOverriddenDecl(foundFunc);
      func->getAttrs().add(new (func->getASTContext()) OverrideAttr(true));
      return;
    }
    // Set constructor override.
    auto ctor = cast<ConstructorDecl>(decl);
    auto memberCtor = cast<ConstructorDecl>(member);
    // Require a selector match.
    if (ctor->isObjC() &&
        ctor->getObjCSelector() != memberCtor->getObjCSelector())
      continue;
    ctor->setOverriddenDecl(memberCtor);
    ctor->getAttrs().add(new (ctor->getASTContext()) OverrideAttr(true));

    // Propagate 'required' to subclass initializers.
    if (memberCtor->isRequired() &&
        !ctor->getAttrs().hasAttribute<RequiredAttr>()) {
      ctor->getAttrs().add(new (Impl.SwiftContext)
                               RequiredAttr(/*IsImplicit=*/true));
    }
  }
}

void SwiftDeclConverter::recordObjCOverride(SubscriptDecl *subscript) {
  // Figure out the class in which this subscript occurs.
  auto classTy =
      subscript->getDeclContext()->getAsClassOrClassExtensionContext();
  if (!classTy)
    return;

  auto superTy = classTy->getSuperclass();
  if (!superTy)
    return;

  // Determine whether this subscript operation overrides another subscript
  // operation.
  SmallVector<ValueDecl *, 2> lookup;
  subscript->getModuleContext()->lookupQualified(
      superTy, subscript->getFullName(),
      NL_QualifiedDefault | NL_KnownNoDependency, Impl.getTypeResolver(),
      lookup);
  Type unlabeledIndices;
  for (auto result : lookup) {
    auto parentSub = dyn_cast<SubscriptDecl>(result);
    if (!parentSub)
      continue;

    // Compute the type of indices for our own subscript operation, lazily.
    if (!unlabeledIndices) {
      unlabeledIndices = subscript->getIndices()
                             ->getInterfaceType(Impl.SwiftContext)
                             ->getUnlabeledType(Impl.SwiftContext);
    }

    // Compute the type of indices for the subscript we found.
    auto parentUnlabeledIndices = parentSub->getIndices()
                                      ->getInterfaceType(Impl.SwiftContext)
                                      ->getUnlabeledType(Impl.SwiftContext);
    if (!unlabeledIndices->isEqual(parentUnlabeledIndices))
      continue;

    // The index types match. This is an override, so mark it as such.
    subscript->setOverriddenDecl(parentSub);
    auto getterThunk = subscript->getGetter();
    getterThunk->setOverriddenDecl(parentSub->getGetter());
    if (auto parentSetter = parentSub->getSetter()) {
      if (auto setterThunk = subscript->getSetter())
        setterThunk->setOverriddenDecl(parentSetter);
    }

    // FIXME: Eventually, deal with multiple overrides.
    break;
  }
}

/// \brief Given either the getter or setter for a subscript operation,
/// create the Swift subscript declaration.
SubscriptDecl *
SwiftDeclConverter::importSubscript(Decl *decl,
                                    const clang::ObjCMethodDecl *objcMethod) {
  assert(objcMethod->isInstanceMethod() && "Caller must filter");

  // If the method we're attempting to import has the
  // swift_private attribute, don't import as a subscript.
  if (objcMethod->hasAttr<clang::SwiftPrivateAttr>())
    return nullptr;

  // Figure out where to look for the counterpart.
  const clang::ObjCInterfaceDecl *interface = nullptr;
  const clang::ObjCProtocolDecl *protocol =
      dyn_cast<clang::ObjCProtocolDecl>(objcMethod->getDeclContext());
  if (!protocol)
    interface = objcMethod->getClassInterface();
  auto lookupInstanceMethod = [&](
      clang::Selector Sel) -> const clang::ObjCMethodDecl * {
    if (interface)
      return interface->lookupInstanceMethod(Sel);

    return protocol->lookupInstanceMethod(Sel);
  };

  auto findCounterpart = [&](clang::Selector sel) -> FuncDecl * {
    // If the declaration we're starting from is in a class, first
    // look for a class member with the appropriate selector.
    if (auto classDecl =
            decl->getDeclContext()->getAsClassOrClassExtensionContext()) {
      auto swiftSel = Impl.importSelector(sel);
      for (auto found : classDecl->lookupDirect(swiftSel, true)) {
        if (auto foundFunc = dyn_cast<FuncDecl>(found))
          if (foundFunc->hasClangNode())
            return foundFunc;
      }
    }

    // Find based on selector within the current type.
    auto counterpart = lookupInstanceMethod(sel);
    if (!counterpart)
      return nullptr;

    return cast_or_null<FuncDecl>(
        Impl.importDecl(counterpart, getActiveSwiftVersion()));
  };

  // Determine the selector of the counterpart.
  FuncDecl *getter = nullptr, *setter = nullptr;
  const clang::ObjCMethodDecl *getterObjCMethod = nullptr,
                              *setterObjCMethod = nullptr;
  clang::Selector counterpartSelector;
  if (objcMethod->getSelector() == Impl.objectAtIndexedSubscript) {
    getter = cast<FuncDecl>(decl);
    getterObjCMethod = objcMethod;
    counterpartSelector = Impl.setObjectAtIndexedSubscript;
  } else if (objcMethod->getSelector() == Impl.setObjectAtIndexedSubscript) {
    setter = cast<FuncDecl>(decl);
    setterObjCMethod = objcMethod;
    counterpartSelector = Impl.objectAtIndexedSubscript;
  } else if (objcMethod->getSelector() == Impl.objectForKeyedSubscript) {
    getter = cast<FuncDecl>(decl);
    getterObjCMethod = objcMethod;
    counterpartSelector = Impl.setObjectForKeyedSubscript;
  } else if (objcMethod->getSelector() == Impl.setObjectForKeyedSubscript) {
    setter = cast<FuncDecl>(decl);
    setterObjCMethod = objcMethod;
    counterpartSelector = Impl.objectForKeyedSubscript;
  } else {
    llvm_unreachable("Unknown getter/setter selector");
  }

  // Find the counterpart.
  bool optionalMethods = (objcMethod->getImplementationControl() ==
                          clang::ObjCMethodDecl::Optional);

  if (auto *counterpart = findCounterpart(counterpartSelector)) {
    const clang::ObjCMethodDecl *counterpartMethod = nullptr;

    // If the counterpart to the method we're attempting to import has the
    // swift_private attribute, don't import as a subscript.
    if (auto importedFrom = counterpart->getClangDecl()) {
      if (importedFrom->hasAttr<clang::SwiftPrivateAttr>())
        return nullptr;

      counterpartMethod = cast<clang::ObjCMethodDecl>(importedFrom);
      if (optionalMethods)
        optionalMethods = (counterpartMethod->getImplementationControl() ==
                           clang::ObjCMethodDecl::Optional);
    }

    assert(!counterpart || !counterpart->isStatic());

    if (getter) {
      setter = counterpart;
      setterObjCMethod = counterpartMethod;
    } else {
      getter = counterpart;
      getterObjCMethod = counterpartMethod;
    }
  }

  // Swift doesn't have write-only subscripting.
  if (!getter)
    return nullptr;

  // Check whether we've already created a subscript operation for
  // this getter/setter pair.
  if (auto subscript = Impl.Subscripts[{getter, setter}]) {
    return subscript->getDeclContext() == decl->getDeclContext() ? subscript
                                                                 : nullptr;
  }

  // Find the getter indices and make sure they match.
  ParamDecl *getterIndex;
  {
    auto params = getter->getParameters();
    if (params->size() != 1)
      return nullptr;
    getterIndex = params->get(0);
  }

  // Compute the element type based on the getter, looking through
  // the implicit 'self' parameter and the normal function
  // parameters.
  auto elementTy = getter->getResultInterfaceType();

  // Local function to mark the setter unavailable.
  auto makeSetterUnavailable = [&] {
    if (setter && !setter->getAttrs().isUnavailable(Impl.SwiftContext))
      Impl.markUnavailable(setter, "use subscripting");
  };

  // If we have a setter, rectify it with the getter.
  ParamDecl *setterIndex;
  bool getterAndSetterInSameType = false;
  bool isIUO =
      getter->getAttrs().hasAttribute<ImplicitlyUnwrappedOptionalAttr>();
  if (setter) {
    // Whether there is an existing read-only subscript for which
    // we have now found a setter.
    SubscriptDecl *existingSubscript = Impl.Subscripts[{getter, nullptr}];

    // Are the getter and the setter in the same type.
    getterAndSetterInSameType =
        (getter->getDeclContext()
             ->getAsNominalTypeOrNominalTypeExtensionContext() ==
         setter->getDeclContext()
             ->getAsNominalTypeOrNominalTypeExtensionContext());

    // Whether we can update the types involved in the subscript
    // operation.
    bool canUpdateSubscriptType =
        !existingSubscript && getterAndSetterInSameType;

    // Determine the setter's element type and indices.
    Type setterElementTy;
    std::tie(setterElementTy, setterIndex) = decomposeSubscriptSetter(setter);

    // Rectify the setter element type with the getter's element type,
    // and determine if the result is an implicitly unwrapped optional
    // type.
    auto importedType = rectifySubscriptTypes(elementTy, isIUO, setterElementTy,
                                              canUpdateSubscriptType);
    if (!importedType)
      return decl == getter ? existingSubscript : nullptr;

    isIUO = importedType.isImplicitlyUnwrapped();

    // Update the element type.
    elementTy = importedType.getType();

    // Make sure that the index types are equivalent.
    // FIXME: Rectify these the same way we do for element types.
    if (!setterIndex->getType()->isEqual(getterIndex->getType())) {
      // If there is an existing subscript operation, we're done.
      if (existingSubscript)
        return decl == getter ? existingSubscript : nullptr;

      // Otherwise, just forget we had a setter.
      // FIXME: This feels very, very wrong.
      setter = nullptr;
      setterObjCMethod = nullptr;
      setterIndex = nullptr;
    }

    // If there is an existing subscript within this context, we
    // cannot create a new subscript. Update it if possible.
    if (setter && existingSubscript && getterAndSetterInSameType) {
      // Can we update the subscript by adding the setter?
      if (existingSubscript->hasClangNode() &&
          !existingSubscript->isSettable()) {
        // Create the setter thunk.
        auto setterThunk = buildSubscriptSetterDecl(
            Impl, existingSubscript, setter, elementTy,
            setter->getDeclContext(), setterIndex);

        // Set the computed setter.
        existingSubscript->setComputedSetter(setterThunk);

        // Mark the setter as unavailable; one should use
        // subscripting when it is present.
        makeSetterUnavailable();
      }

      return decl == getter ? existingSubscript : nullptr;
    }
  }

  // The context into which the subscript should go. We prefer wherever the
  // getter is declared unless the two accessors are in different types and the
  // one we started with is the setter. This happens when:
  // - A read-only subscript is made read/write is a subclass.
  // - A setter is redeclared in a subclass, but not the getter.
  // And not when:
  // - A getter is redeclared in a subclass, but not the setter.
  // - The getter and setter are part of the same type.
  // - There is no setter.
  bool associateWithSetter = !getterAndSetterInSameType && setter == decl;
  DeclContext *dc =
      associateWithSetter ? setter->getDeclContext() : getter->getDeclContext();

  // Build the subscript declaration.
  auto &C = Impl.SwiftContext;
  auto bodyParams = ParameterList::create(C, getterIndex);
  DeclName name(C, DeclBaseName::createSubscript(), {Identifier()});
  auto subscript = Impl.createDeclWithClangNode<SubscriptDecl>(
      getter->getClangNode(), getOverridableAccessLevel(dc), name,
      decl->getLoc(), bodyParams, decl->getLoc(),
      TypeLoc::withoutLoc(elementTy), dc,
      /*GenericParams=*/nullptr);

  // Build the thunks.
  AccessorDecl *getterThunk =
      buildSubscriptGetterDecl(Impl, subscript, getter, elementTy,
                               dc, getterIndex);

  AccessorDecl *setterThunk = nullptr;
  if (setter)
    setterThunk =
        buildSubscriptSetterDecl(Impl, subscript, setter, elementTy,
                                 dc, setterIndex);

  // Record the subscript as an alternative declaration.
  Impl.addAlternateDecl(associateWithSetter ? setter : getter, subscript);

  // Import attributes for the accessors if there is a pair.
  Impl.importAttributes(getterObjCMethod, getterThunk);
  if (setterObjCMethod)
    Impl.importAttributes(setterObjCMethod, setterThunk);

  subscript->setGenericEnvironment(dc->getGenericEnvironmentOfContext());

  subscript->setIsSetterMutating(false);
  makeComputed(subscript, getterThunk, setterThunk);
  subscript->computeType();

  Impl.recordImplicitUnwrapForDecl(subscript, isIUO);

  addObjCAttribute(subscript, None);

  // Optional subscripts in protocols.
  if (optionalMethods && isa<ProtocolDecl>(dc))
    subscript->getAttrs().add(new (Impl.SwiftContext) OptionalAttr(true));

  // Note that we've created this subscript.
  Impl.Subscripts[{getter, setter}] = subscript;
  if (setter && !Impl.Subscripts[{getter, nullptr}])
    Impl.Subscripts[{getter, nullptr}] = subscript;

  // Make the getter/setter methods unavailable.
  if (!getter->getAttrs().isUnavailable(Impl.SwiftContext))
    Impl.markUnavailable(getter, "use subscripting");
  makeSetterUnavailable();

  // Wire up overrides.
  recordObjCOverride(subscript);

  return subscript;
}

AccessorDecl *
SwiftDeclConverter::importAccessor(clang::ObjCMethodDecl *clangAccessor,
                                   AbstractStorageDecl *storage,
                                   AccessorKind accessorKind,
                                   DeclContext *dc) {
  SwiftDeclConverter converter(Impl, getActiveSwiftVersion());
  auto *accessor = cast_or_null<AccessorDecl>(
    converter.importObjCMethodDecl(clangAccessor, dc,
                                   AccessorInfo{storage, accessorKind}));
  if (!accessor) {
    return nullptr;
  }

  Impl.importAttributes(clangAccessor, accessor);

  return accessor;
}

void SwiftDeclConverter::addProtocols(
    ProtocolDecl *protocol, SmallVectorImpl<ProtocolDecl *> &protocols,
    llvm::SmallPtrSetImpl<ProtocolDecl *> &known) {
  if (!known.insert(protocol).second)
    return;

  protocols.push_back(protocol);
  for (auto inherited : protocol->getInheritedProtocols())
    addProtocols(inherited, protocols, known);
}

void SwiftDeclConverter::importObjCProtocols(
    Decl *decl, const clang::ObjCProtocolList &clangProtocols,
    SmallVectorImpl<TypeLoc> &inheritedTypes) {
  SmallVector<ProtocolDecl *, 4> protocols;
  llvm::SmallPtrSet<ProtocolDecl *, 4> knownProtocols;
  if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
    nominal->getImplicitProtocols(protocols);
    knownProtocols.insert(protocols.begin(), protocols.end());
  }

  for (auto cp = clangProtocols.begin(), cpEnd = clangProtocols.end();
       cp != cpEnd; ++cp) {
    if (auto proto = castIgnoringCompatibilityAlias<ProtocolDecl>(
            Impl.importDecl(*cp, getActiveSwiftVersion()))) {
      addProtocols(proto, protocols, knownProtocols);
      inheritedTypes.push_back(TypeLoc::withoutLoc(proto->getDeclaredType()));
    }
  }

  addObjCProtocolConformances(decl, protocols);
}

void SwiftDeclConverter::addObjCProtocolConformances(
    Decl *decl, ArrayRef<ProtocolDecl *> protocols) {
  // Nothing to do for protocols.
  if (isa<ProtocolDecl>(decl)) return;

  Impl.recordImportedProtocols(decl, protocols);

  // Synthesize trivial conformances for each of the protocols.
  SmallVector<ProtocolConformance *, 4> conformances;

  auto dc = decl->getInnermostDeclContext();
  auto &ctx = Impl.SwiftContext;
  for (unsigned i = 0, n = protocols.size(); i != n; ++i) {
    // FIXME: Build a superclass conformance if the superclass
    // conforms.
    auto conformance = ctx.getConformance(dc->getDeclaredInterfaceType(),
                                          protocols[i], SourceLoc(), dc,
                                          ProtocolConformanceState::Incomplete);
    conformance->setLazyLoader(&Impl, /*context*/0);
    conformance->setState(ProtocolConformanceState::Complete);
    conformances.push_back(conformance);
  }

  // Set the conformances.
  // FIXME: This could be lazier.
  unsigned id = Impl.allocateDelayedConformance(std::move(conformances));
  if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
    nominal->setConformanceLoader(&Impl, id);
  } else {
    auto ext = cast<ExtensionDecl>(decl);
    ext->setConformanceLoader(&Impl, id);
  }
}

Optional<GenericParamList *> SwiftDeclConverter::importObjCGenericParams(
    const clang::ObjCInterfaceDecl *decl, DeclContext *dc) {
  auto typeParamList = decl->getTypeParamList();
  if (!typeParamList) {
    return nullptr;
  }
  if (shouldSuppressGenericParamsImport(Impl.SwiftContext.LangOpts, decl)) {
    return nullptr;
  }
  assert(typeParamList->size() > 0);
  SmallVector<GenericTypeParamDecl *, 4> genericParams;
  for (auto *objcGenericParam : *typeParamList) {
    auto genericParamDecl = Impl.createDeclWithClangNode<GenericTypeParamDecl>(
        objcGenericParam, AccessLevel::Public, dc,
        Impl.SwiftContext.getIdentifier(objcGenericParam->getName()),
        Impl.importSourceLoc(objcGenericParam->getLocation()),
        /*depth*/ 0, /*index*/ genericParams.size());
    // NOTE: depth is always 0 for ObjC generic type arguments, since only
    // classes may have generic types in ObjC, and ObjC classes cannot be
    // nested.

    // Import parameter constraints.
    SmallVector<TypeLoc, 1> inherited;
    if (objcGenericParam->hasExplicitBound()) {
      assert(!objcGenericParam->getUnderlyingType().isNull());
      auto clangBound = objcGenericParam->getUnderlyingType()
                            ->castAs<clang::ObjCObjectPointerType>();
      if (clangBound->getInterfaceDecl()) {
        auto unqualifiedClangBound =
            clangBound->stripObjCKindOfTypeAndQuals(Impl.getClangASTContext());
        Type superclassType = Impl.importTypeIgnoreIUO(
            clang::QualType(unqualifiedClangBound, 0), ImportTypeKind::Abstract,
            false, Bridgeability::None);
        if (!superclassType) {
          return None;
        }
        inherited.push_back(TypeLoc::withoutLoc(superclassType));
      }
      for (clang::ObjCProtocolDecl *clangProto : clangBound->quals()) {
        ProtocolDecl *proto = castIgnoringCompatibilityAlias<ProtocolDecl>(
            Impl.importDecl(clangProto, getActiveSwiftVersion()));
        if (!proto) {
          return None;
        }
        inherited.push_back(TypeLoc::withoutLoc(proto->getDeclaredType()));
      }
    }
    if (inherited.empty()) {
      inherited.push_back(
        TypeLoc::withoutLoc(Impl.SwiftContext.getAnyObjectType()));
    }
    genericParamDecl->setInherited(Impl.SwiftContext.AllocateCopy(inherited));

    genericParams.push_back(genericParamDecl);
  }
  return GenericParamList::create(
      Impl.SwiftContext, Impl.importSourceLoc(typeParamList->getLAngleLoc()),
      genericParams, Impl.importSourceLoc(typeParamList->getRAngleLoc()));
}

void SwiftDeclConverter::importMirroredProtocolMembers(
    const clang::ObjCContainerDecl *decl, DeclContext *dc,
    ArrayRef<ProtocolDecl *> protocols, SmallVectorImpl<Decl *> &members,
    ASTContext &Ctx) {
  assert(dc);
  const clang::ObjCInterfaceDecl *interfaceDecl = nullptr;
  const ClangModuleUnit *declModule;
  const ClangModuleUnit *interfaceModule;

  // 'protocols' is, for some reason, the full recursive expansion of
  // the protocol hierarchy, so there's no need to recursively descend
  // into inherited protocols.

  // Try to import only the most specific methods with a particular name.
  // We use a MapVector to get deterministic iteration order later.
  llvm::MapVector<clang::Selector, std::vector<MirroredMethodEntry>>
    methodsByName;

  for (auto proto : protocols) {
    auto clangProto =
        cast_or_null<clang::ObjCProtocolDecl>(proto->getClangDecl());
    if (!clangProto)
      continue;

    if (!interfaceDecl) {
      declModule = Impl.getClangModuleForDecl(decl);
      if ((interfaceDecl = dyn_cast<clang::ObjCInterfaceDecl>(decl))) {
        interfaceModule = declModule;
      } else {
        auto category = cast<clang::ObjCCategoryDecl>(decl);
        interfaceDecl = category->getClassInterface();
        interfaceModule = Impl.getClangModuleForDecl(interfaceDecl);
      }
    }

    // Don't import a protocol's members if the superclass already adopts
    // the protocol, or (for categories) if the class itself adopts it
    // in its main @interface.
    if (decl != interfaceDecl)
      if (classImplementsProtocol(interfaceDecl, clangProto, false))
        continue;
    if (auto superInterface = interfaceDecl->getSuperClass())
      if (classImplementsProtocol(superInterface, clangProto, true))
        continue;

    const auto &languageVersion =
        Impl.SwiftContext.LangOpts.EffectiveLanguageVersion;
    for (auto member : proto->getMembers()) {
      // Skip compatibility stubs; there's no reason to mirror them.
      if (member->getAttrs().isUnavailableInSwiftVersion(languageVersion))
        continue;

      if (auto prop = dyn_cast<VarDecl>(member)) {
        auto objcProp =
            dyn_cast_or_null<clang::ObjCPropertyDecl>(prop->getClangDecl());
        if (!objcProp)
          continue;

        // We can't import a property if there's already a method with this
        // name. (This also covers other properties with that same name.)
        // FIXME: We should still mirror the setter as a method if it's
        // not already there.
        clang::Selector sel = objcProp->getGetterName();
        if (interfaceDecl->getInstanceMethod(sel))
          continue;

        bool inNearbyCategory =
            std::any_of(interfaceDecl->visible_categories_begin(),
                        interfaceDecl->visible_categories_end(),
                        [=](const clang::ObjCCategoryDecl *category) -> bool {
                          if (category != decl) {
                            auto *categoryModule =
                                Impl.getClangModuleForDecl(category);
                            if (categoryModule != declModule &&
                                categoryModule != interfaceModule) {
                              return false;
                            }
                          }
                          return category->getInstanceMethod(sel);
                        });
        if (inNearbyCategory)
          continue;

        if (auto imported =
                Impl.importMirroredDecl(objcProp, dc, getVersion(), proto)) {
          members.push_back(imported);
          // FIXME: We should mirror properties of the root class onto the
          // metatype.
        }

        continue;
      }

      auto afd = dyn_cast<AbstractFunctionDecl>(member);
      if (!afd)
        continue;

      if (isa<AccessorDecl>(afd))
        continue;

      auto objcMethod =
          dyn_cast_or_null<clang::ObjCMethodDecl>(member->getClangDecl());
      if (!objcMethod)
        continue;

      // For now, just remember that we saw this method.
      methodsByName[objcMethod->getSelector()]
        .push_back(MirroredMethodEntry{objcMethod, proto});
    }
  }

  // Process all the methods, now that we've arranged them by selector.
  for (auto &mapEntry : methodsByName) {
    importNonOverriddenMirroredMethods(dc, mapEntry.second, members);
  }
}

enum MirrorImportComparison {
  // There's no suppression relationship between the methods.
  NoSuppression,

  // The first method suppresses the second.
  Suppresses,

  // The second method suppresses the first.
  IsSuppressed,
};

/// Should the mirror import of the first method be suppressed in favor
/// of the second method?  The methods are known to have the same selector
/// and (because this is mirror-import) to be declared on protocols.
///
/// The algorithm that uses this assumes that it is transitive.
static bool isMirrorImportSuppressedBy(ClangImporter::Implementation &importer,
                                       const clang::ObjCMethodDecl *first,
                                       const clang::ObjCMethodDecl *second) {
  if (first->isInstanceMethod() != second->isInstanceMethod())
    return false;

  auto firstProto = cast<clang::ObjCProtocolDecl>(first->getDeclContext());
  auto secondProto = cast<clang::ObjCProtocolDecl>(second->getDeclContext());

  // If the first method's protocol is a super-protocol of the second's,
  // then the second method overrides the first and we should suppress.
  // Clang provides a function to check that, phrased in terms of whether
  // a value of one protocol (the RHS) can be assigned to an l-value of
  // the other (the LHS).
  auto &ctx = importer.getClangASTContext();
  return ctx.ProtocolCompatibleWithProtocol(
                          const_cast<clang::ObjCProtocolDecl*>(firstProto),
                          const_cast<clang::ObjCProtocolDecl*>(secondProto));
}

/// Compare two methods for mirror-import purposes.
static MirrorImportComparison
compareMethodsForMirrorImport(ClangImporter::Implementation &importer,
                              const clang::ObjCMethodDecl *first,
                              const clang::ObjCMethodDecl *second) {
  if (isMirrorImportSuppressedBy(importer, first, second))
    return IsSuppressed;
  if (isMirrorImportSuppressedBy(importer, second, first))
    return Suppresses;
  return NoSuppression;
}

/// Mark any methods in the given array that are overridden by this method
/// as suppressed by nulling their entries out.
/// Return true if this method is overridden by any methods in the array.
static bool suppressOverriddenMethods(ClangImporter::Implementation &importer,
                                      const clang::ObjCMethodDecl *method,
                               MutableArrayRef<MirroredMethodEntry> entries) {
  assert(method && "method was already suppressed");

  for (auto &entry: entries) {
    auto otherMethod = entry.first;
    if (!otherMethod) continue;

    assert(method != otherMethod && "found same method twice?");
    switch (compareMethodsForMirrorImport(importer, method, otherMethod)) {
    // If the second method is suppressed, null it out.
    case Suppresses:
      entry.first = nullptr;
      continue;

    // If the first method is suppressed, return immediately.  We should
    // be able to suppress any following methods.
    case IsSuppressed:
      return true;

    case NoSuppression:
      continue;
    }
    llvm_unreachable("bad comparison result");
  }

  return false;
}

/// Given a set of methods with the same selector, each taken from a
/// different protocol in the protocol hierarchy of a class into which
/// we want to introduce mirror imports, import only the methods which
/// are not overridden by another method in the set.
///
/// It's possible that we'll end up selecting multiple methods to import
/// here, in the cases where there's no hierarchical relationship between
/// two methods.  The importer already has code to handle this case.
void SwiftDeclConverter::importNonOverriddenMirroredMethods(DeclContext *dc,
                               MutableArrayRef<MirroredMethodEntry> entries,
                                           SmallVectorImpl<Decl *> &members) {
  for (size_t i = 0, e = entries.size(); i != e; ++i) {
    auto objcMethod = entries[i].first;

    // If the method was suppressed by a previous method, ignore it.
    if (!objcMethod)
      continue;

    // Compare this method to all the following methods, suppressing any
    // that it overrides.  If it is overridden by any of them, suppress it
    // instead; but there's no need to mark that in the array, just continue
    // on to the next method.
    if (suppressOverriddenMethods(Impl, objcMethod, entries.slice(i + 1)))
      continue;

    // Okay, the method wasn't suppressed, import it.

    // When mirroring an initializer, make it designated and required.
    if (isInitMethod(objcMethod)) {
      // Import the constructor.
      if (auto imported = importConstructor(objcMethod, dc, /*implicit=*/true,
                                            CtorInitializerKind::Designated,
                                            /*required=*/true)) {
        members.push_back(imported);
      }
      continue;
    }

    // Import the method.
    auto proto = entries[i].second;
    if (auto imported =
            Impl.importMirroredDecl(objcMethod, dc, getVersion(), proto)) {
      members.push_back(imported);

      for (auto alternate : Impl.getAlternateDecls(imported))
        if (imported->getDeclContext() == alternate->getDeclContext())
          members.push_back(alternate);
    }
  }
}

void SwiftDeclConverter::importInheritedConstructors(
    ClassDecl *classDecl, SmallVectorImpl<Decl *> &newMembers) {
  if (!classDecl->hasSuperclass())
    return;

  auto curObjCClass = cast<clang::ObjCInterfaceDecl>(classDecl->getClangDecl());

  auto inheritConstructors = [&](ArrayRef<ValueDecl *> members,
                                 Optional<CtorInitializerKind> kind) {
    const auto &languageVersion =
        Impl.SwiftContext.LangOpts.EffectiveLanguageVersion;

    for (auto member : members) {
      auto ctor = dyn_cast<ConstructorDecl>(member);
      if (!ctor)
        continue;

      // Don't inherit compatibility stubs.
      if (ctor->getAttrs().isUnavailableInSwiftVersion(languageVersion))
        continue;

      // Don't inherit (non-convenience) factory initializers.
      // Note that convenience factories return instancetype and can be
      // inherited.
      switch (ctor->getInitKind()) {
      case CtorInitializerKind::Factory:
        continue;
      case CtorInitializerKind::ConvenienceFactory:
      case CtorInitializerKind::Convenience:
      case CtorInitializerKind::Designated:
        break;
      }

      auto objcMethod =
          dyn_cast_or_null<clang::ObjCMethodDecl>(ctor->getClangDecl());
      if (!objcMethod)
        continue;

      auto &clangSourceMgr = Impl.getClangASTContext().getSourceManager();
      clang::PrettyStackTraceDecl trace(objcMethod, clang::SourceLocation(),
                                        clangSourceMgr,
                                        "importing (inherited)");

      // If this initializer came from a factory method, inherit
      // it as an initializer.
      if (objcMethod->isClassMethod()) {
        assert(ctor->getInitKind() == CtorInitializerKind::ConvenienceFactory);

        Optional<ImportedName> correctSwiftName;
        ImportedName importedName =
            importFullName(objcMethod, correctSwiftName);
        assert(
            !correctSwiftName &&
            "Import inherited initializers never references correctSwiftName");
        importedName.setHasCustomName();
        bool redundant;
        if (auto newCtor =
                importConstructor(objcMethod, classDecl,
                                  /*implicit=*/true, ctor->getInitKind(),
                                  /*required=*/false, ctor->getObjCSelector(),
                                  importedName, objcMethod->parameters(),
                                  objcMethod->isVariadic(), redundant)) {
          // If this is a compatibility stub, mark it as such.
          if (correctSwiftName)
            markAsVariant(newCtor, *correctSwiftName);

          Impl.importAttributes(objcMethod, newCtor, curObjCClass);
          newMembers.push_back(newCtor);
        }
        continue;
      }

      // Figure out what kind of constructor this will be.
      CtorInitializerKind myKind;
      bool isRequired = false;
      if (ctor->isRequired()) {
        // Required initializers are always considered designated.
        isRequired = true;
        myKind = CtorInitializerKind::Designated;
      } else if (kind) {
        myKind = *kind;
      } else {
        myKind = ctor->getInitKind();
      }

      // Import the constructor into this context.
      if (auto newCtor =
              importConstructor(objcMethod, classDecl,
                                /*implicit=*/true, myKind, isRequired)) {
        Impl.importAttributes(objcMethod, newCtor, curObjCClass);
        newMembers.push_back(newCtor);
      }
    }
  };

  // The kind of initializer to import. If this class has designated
  // initializers, everything it imports is a convenience initializer.
  Optional<CtorInitializerKind> kind;
  if (hasDesignatedInitializers(curObjCClass))
    kind = CtorInitializerKind::Convenience;

  auto superclass =
      cast<ClassDecl>(classDecl->getSuperclass()->getAnyNominal());

  // If we have a superclass, import from it.
  if (auto superclassClangDecl = superclass->getClangDecl()) {
    if (isa<clang::ObjCInterfaceDecl>(superclassClangDecl)) {
      inheritConstructors(superclass->lookupDirect(DeclBaseName::createConstructor()),
                          kind);
    }
  }
}

Decl *ClangImporter::Implementation::importDeclCached(
    const clang::NamedDecl *ClangDecl,
    ImportNameVersion version) {
  auto Known = ImportedDecls.find({ClangDecl->getCanonicalDecl(), version});
  if (Known != ImportedDecls.end())
    return Known->second;

  return nullptr;
}

/// Checks if we don't need to import the typedef itself.  If the typedef
/// should be skipped, returns the underlying declaration that the typedef
/// refers to -- this declaration should be imported instead.
static const clang::TagDecl *
canSkipOverTypedef(ClangImporter::Implementation &Impl,
                   const clang::NamedDecl *D,
                   bool &TypedefIsSuperfluous) {
  // If we have a typedef that refers to a tag type of the same name,
  // skip the typedef and import the tag type directly.

  TypedefIsSuperfluous = false;

  auto *ClangTypedef = dyn_cast<clang::TypedefNameDecl>(D);
  if (!ClangTypedef)
    return nullptr;

  const clang::DeclContext *RedeclContext =
      ClangTypedef->getDeclContext()->getRedeclContext();
  if (!RedeclContext->isTranslationUnit())
    return nullptr;

  clang::QualType UnderlyingType = ClangTypedef->getUnderlyingType();

  // A typedef to a typedef should get imported as a typealias.
  auto *TypedefT = UnderlyingType->getAs<clang::TypedefType>();
  if (TypedefT)
    return nullptr;

  auto *TT = UnderlyingType->getAs<clang::TagType>();
  if (!TT)
    return nullptr;

  clang::TagDecl *UnderlyingDecl = TT->getDecl();
  if (UnderlyingDecl->getDeclContext()->getRedeclContext() != RedeclContext)
    return nullptr;

  if (UnderlyingDecl->getDeclName().isEmpty())
    return UnderlyingDecl;

  auto TypedefName = ClangTypedef->getDeclName();
  auto TagDeclName = UnderlyingDecl->getDeclName();
  if (TypedefName != TagDeclName)
    return nullptr;

  TypedefIsSuperfluous = true;
  return UnderlyingDecl;
}

StringRef ClangImporter::Implementation::
getSwiftNameFromClangName(StringRef replacement) {
  auto &clangSema = getClangSema();

  clang::IdentifierInfo *identifier =
      &clangSema.getASTContext().Idents.get(replacement);
  clang::LookupResult lookupResult(clangSema, identifier,
                                   clang::SourceLocation(),
                                   clang::Sema::LookupOrdinaryName);
  if (!clangSema.LookupName(lookupResult, nullptr))
    return "";

  auto clangDecl = lookupResult.getAsSingle<clang::NamedDecl>();
  if (!clangDecl)
    return "";

  auto importedName = importFullName(clangDecl, CurrentVersion);
  if (!importedName)
    return "";

  llvm::SmallString<64> renamed;
  {
    // Render a swift_name string.
    llvm::raw_svector_ostream os(renamed);
    printSwiftName(importedName, CurrentVersion, /*fullyQualified=*/true, os);
  }

  return SwiftContext.AllocateCopy(StringRef(renamed));
}

bool importer::isSpecialUIKitStructZeroProperty(const clang::NamedDecl *decl) {
  // FIXME: Once UIKit removes the "nonswift" availability in their versioned
  // API notes, this workaround can go away.
  auto *constant = dyn_cast<clang::VarDecl>(decl);
  if (!constant)
    return false;

  clang::DeclarationName name = constant->getDeclName();
  const clang::IdentifierInfo *ident = name.getAsIdentifierInfo();
  if (!ident)
    return false;

  return ident->isStr("UIEdgeInsetsZero") || ident->isStr("UIOffsetZero");
}

/// Import Clang attributes as Swift attributes.
void ClangImporter::Implementation::importAttributes(
    const clang::NamedDecl *ClangDecl,
    Decl *MappedDecl,
    const clang::ObjCContainerDecl *NewContext)
{
  // Subscripts are special-cased since there isn't a 1:1 mapping
  // from its accessor(s) to the subscript declaration.
  if (isa<SubscriptDecl>(MappedDecl))
    return;

  ASTContext &C = SwiftContext;

  if (auto maybeDefinition = getDefinitionForClangTypeDecl(ClangDecl))
    if (maybeDefinition.getValue())
      ClangDecl = cast<clang::NamedDecl>(maybeDefinition.getValue());

  // Scan through Clang attributes and map them onto Swift
  // equivalents.
  bool AnyUnavailable = MappedDecl->getAttrs().isUnavailable(C);
  for (clang::NamedDecl::attr_iterator AI = ClangDecl->attr_begin(),
       AE = ClangDecl->attr_end(); AI != AE; ++AI) {
    //
    // __attribute__((unavailable))
    //
    // Mapping: @available(*,unavailable)
    //
    if (auto unavailable = dyn_cast<clang::UnavailableAttr>(*AI)) {
      auto Message = unavailable->getMessage();
      auto attr = AvailableAttr::createPlatformAgnostic(C, Message);
      MappedDecl->getAttrs().add(attr);
      AnyUnavailable = true;
      continue;
    }

    //
    // __attribute__((annotate(swift1_unavailable)))
    //
    // Mapping: @available(*, unavailable)
    //
    if (auto unavailable_annot = dyn_cast<clang::AnnotateAttr>(*AI))
      if (unavailable_annot->getAnnotation() == "swift1_unavailable") {
        auto attr = AvailableAttr::createPlatformAgnostic(
            C, "", "", PlatformAgnosticAvailabilityKind::UnavailableInSwift);
        MappedDecl->getAttrs().add(attr);
        AnyUnavailable = true;
        continue;
      }

    //
    // __attribute__((deprecated))
    //
    // Mapping: @available(*,deprecated)
    //
    if (auto deprecated = dyn_cast<clang::DeprecatedAttr>(*AI)) {
      auto Message = deprecated->getMessage();
      auto attr = AvailableAttr::createPlatformAgnostic(C, Message, "",
                    PlatformAgnosticAvailabilityKind::Deprecated);
      MappedDecl->getAttrs().add(attr);
      continue;
    }

    // __attribute__((availability))
    //
    if (auto avail = dyn_cast<clang::AvailabilityAttr>(*AI)) {
      StringRef Platform = avail->getPlatform()->getName();

      // Is this our special "availability(swift, unavailable)" attribute?
      if (Platform == "swift") {
        // FIXME: Until Apple gets a chance to update UIKit's API notes, ignore
        // the Swift-unavailability for certain properties.
        if (isSpecialUIKitStructZeroProperty(ClangDecl))
          continue;

        auto replacement = avail->getReplacement();
        StringRef swiftReplacement = "";
        if (!replacement.empty())
          swiftReplacement = getSwiftNameFromClangName(replacement);

        auto attr = AvailableAttr::createPlatformAgnostic(
            C, avail->getMessage(), swiftReplacement,
            PlatformAgnosticAvailabilityKind::UnavailableInSwift);
        MappedDecl->getAttrs().add(attr);
        AnyUnavailable = true;
        continue;
      }

      // Does this availability attribute map to the platform we are
      // currently targeting?
      if (!platformAvailability.filter ||
          !platformAvailability.filter(Platform))
        continue;

      auto platformK =
        llvm::StringSwitch<Optional<PlatformKind>>(Platform)
          .Case("ios", PlatformKind::iOS)
          .Case("macos", PlatformKind::OSX)
          .Case("tvos", PlatformKind::tvOS)
          .Case("watchos", PlatformKind::watchOS)
          .Case("ios_app_extension", PlatformKind::iOSApplicationExtension)
          .Case("macos_app_extension",
                PlatformKind::OSXApplicationExtension)
          .Case("tvos_app_extension",
                PlatformKind::tvOSApplicationExtension)
          .Case("watchos_app_extension",
                PlatformKind::watchOSApplicationExtension)
          .Default(None);
      if (!platformK)
        continue;

      // Is this declaration marked platform-agnostically unavailable?
      auto PlatformAgnostic = PlatformAgnosticAvailabilityKind::None;
      if (avail->getUnavailable()) {
        PlatformAgnostic = PlatformAgnosticAvailabilityKind::Unavailable;
        AnyUnavailable = true;
      }

      StringRef message = avail->getMessage();

      clang::VersionTuple deprecated = avail->getDeprecated();

      if (!deprecated.empty()) {
        if (platformAvailability.deprecatedAsUnavailableFilter &&
            platformAvailability.deprecatedAsUnavailableFilter(
                deprecated.getMajor(), deprecated.getMinor())) {
          AnyUnavailable = true;
          PlatformAgnostic = PlatformAgnosticAvailabilityKind::Unavailable;
          if (message.empty())
            message = platformAvailability.deprecatedAsUnavailableMessage;
        }
      }

      clang::VersionTuple obsoleted = avail->getObsoleted();
      clang::VersionTuple introduced = avail->getIntroduced();

      // Swift only allows "." separators.
      obsoleted.UseDotAsSeparator();
      introduced.UseDotAsSeparator();
      deprecated.UseDotAsSeparator();

      const auto &replacement = avail->getReplacement();

      StringRef swiftReplacement = "";
      if (!replacement.empty())
        swiftReplacement = getSwiftNameFromClangName(replacement);

      auto AvAttr = new (C) AvailableAttr(SourceLoc(), SourceRange(),
                                          platformK.getValue(),
                                          message, swiftReplacement,
                                          introduced,
                                          /*IntroducedRange=*/SourceRange(),
                                          deprecated,
                                          /*DeprecatedRange=*/SourceRange(),
                                          obsoleted,
                                          /*ObsoletedRange=*/SourceRange(),
                                          PlatformAgnostic, /*Implicit=*/false);

      MappedDecl->getAttrs().add(AvAttr);
    }
  }

  // If the declaration is unavailable, we're done.
  if (AnyUnavailable)
    return;

  if (auto ID = dyn_cast<clang::ObjCInterfaceDecl>(ClangDecl)) {
    // Ban NSInvocation.
    if (ID->getName() == "NSInvocation") {
      auto attr = AvailableAttr::createPlatformAgnostic(C, "");
      MappedDecl->getAttrs().add(attr);
      return;
    }

    // Map Clang's swift_objc_members attribute to @objcMembers. Also handle
    // inheritance of @objcMembers by looking at the superclass.
    if (ID->hasAttr<clang::SwiftObjCMembersAttr>() ||
        (isa<ClassDecl>(MappedDecl) &&
         cast<ClassDecl>(MappedDecl)->hasSuperclass() &&
         cast<ClassDecl>(MappedDecl)->getSuperclassDecl()
           ->getAttrs().hasAttribute<ObjCMembersAttr>())) {
      if (!MappedDecl->getAttrs().hasAttribute<ObjCMembersAttr>()) {
        auto attr = new (C) ObjCMembersAttr(/*IsImplicit=*/true);
        MappedDecl->getAttrs().add(attr);
      }
    }

    // Infer @objcMembers on XCTestCase.
    if (ID->getName() == "XCTestCase") {
      if (!MappedDecl->getAttrs().hasAttribute<ObjCMembersAttr>()) {
        auto attr = new (C) ObjCMembersAttr(/*IsImplicit=*/true);
        MappedDecl->getAttrs().add(attr);
      }
    }
  }

  // Ban CFRelease|CFRetain|CFAutorelease(CFTypeRef) as well as custom ones
  // such as CGColorRelease(CGColorRef).
  if (auto FD = dyn_cast<clang::FunctionDecl>(ClangDecl)) {
    if (FD->getNumParams() == 1 &&
         (FD->getName().endswith("Release") ||
          FD->getName().endswith("Retain") ||
          FD->getName().endswith("Autorelease")) &&
        !FD->getAttr<clang::SwiftNameAttr>()) {
      if (auto t = FD->getParamDecl(0)->getType()->getAs<clang::TypedefType>()){
        if (isCFTypeDecl(t->getDecl())) {
          auto attr = AvailableAttr::createPlatformAgnostic(C,
            "Core Foundation objects are automatically memory managed");
          MappedDecl->getAttrs().add(attr);
          return;
        }
      }
    }
  }

  // Hack: mark any method named "print" with less than two parameters as
  // warn_unqualified_access.
  if (auto MD = dyn_cast<FuncDecl>(MappedDecl)) {
    if (isPrintLikeMethod(MD->getFullName(), MD->getDeclContext())) {
      // Use a non-implicit attribute so it shows up in the generated
      // interface.
      MD->getAttrs().add(new (C) WarnUnqualifiedAccessAttr(/*implicit*/false));
    }
  }

  // Map __attribute__((warn_unused_result)).
  if (!ClangDecl->hasAttr<clang::WarnUnusedResultAttr>()) {
    if (auto MD = dyn_cast<FuncDecl>(MappedDecl)) {
      if (!MD->getResultInterfaceType()->isVoid()) {
        MD->getAttrs().add(new (C) DiscardableResultAttr(/*implicit*/true));
      }
    }
  }
  // Map __attribute__((const)).
  if (ClangDecl->hasAttr<clang::ConstAttr>()) {
    MappedDecl->getAttrs().add(new (C) EffectsAttr(EffectsKind::ReadNone));
  }
  // Map __attribute__((pure)).
  if (ClangDecl->hasAttr<clang::PureAttr>()) {
    MappedDecl->getAttrs().add(new (C) EffectsAttr(EffectsKind::ReadOnly));
  }
}

Decl *
ClangImporter::Implementation::importDeclImpl(const clang::NamedDecl *ClangDecl,
                                              ImportNameVersion version,
                                              bool &TypedefIsSuperfluous,
                                              bool &HadForwardDeclaration) {
  assert(ClangDecl);

  bool SkippedOverTypedef = false;
  Decl *Result = nullptr;
  if (auto *UnderlyingDecl = canSkipOverTypedef(*this, ClangDecl,
                                                TypedefIsSuperfluous)) {
    Result = importDecl(UnderlyingDecl, version);
    SkippedOverTypedef = true;
  }

  if (!Result) {
    SwiftDeclConverter converter(*this, version);
    Result = converter.Visit(ClangDecl);
    HadForwardDeclaration = converter.hadForwardDeclaration();
  }
  if (!Result && version == CurrentVersion) {
    // If we couldn't import this Objective-C entity, determine
    // whether it was a required member of a protocol, or a designated
    // initializer of a class.
    bool hasMissingRequiredMember = false;
    if (auto clangProto
          = dyn_cast<clang::ObjCProtocolDecl>(ClangDecl->getDeclContext())) {
      if (auto method = dyn_cast<clang::ObjCMethodDecl>(ClangDecl)) {
        if (method->getImplementationControl()
              == clang::ObjCMethodDecl::Required)
          hasMissingRequiredMember = true;
      } else if (auto prop = dyn_cast<clang::ObjCPropertyDecl>(ClangDecl)) {
        if (prop->getPropertyImplementation()
              == clang::ObjCPropertyDecl::Required)
          hasMissingRequiredMember = true;
      }

      if (hasMissingRequiredMember) {
        // Mark the protocol as having missing requirements.
        if (auto proto = castIgnoringCompatibilityAlias<ProtocolDecl>(
                importDecl(clangProto, CurrentVersion))) {
          proto->setHasMissingRequirements(true);
        }
      }
    }
    if (auto method = dyn_cast<clang::ObjCMethodDecl>(ClangDecl)) {
      if (method->isDesignatedInitializerForTheInterface()) {
        const clang::ObjCInterfaceDecl *theClass = method->getClassInterface();
        assert(theClass && "cannot be a protocol method here");
        // Only allow this to affect declarations in the same top-level module
        // as the original class.
        if (getClangModuleForDecl(theClass) == getClangModuleForDecl(method)) {
          if (auto swiftClass = castIgnoringCompatibilityAlias<ClassDecl>(
                  importDecl(theClass, CurrentVersion))) {
            swiftClass->setHasMissingDesignatedInitializers();
          }
        }
      }
    }

    return nullptr;
  }

  // Finalize the imported declaration.
  auto finalizeDecl = [&](Decl *result) {
    importAttributes(ClangDecl, result);

    // Hack to deal with Objective-C protocols without availability annotation.
    // If the protocol comes from clang and is not annotated and the protocol
    // requirement itself is not annotated, then infer availability of the
    // requirement based on its types. This makes it possible for a type to
    // conform to an Objective-C protocol that is missing annotations but whose
    // requirements use types that are less available than the conforming type.
    auto dc = result->getDeclContext();
    auto *proto = dyn_cast<ProtocolDecl>(dc);
    if (!proto || proto->getAttrs().hasAttribute<AvailableAttr>())
      return;

    inferProtocolMemberAvailability(*this, dc, result);
  };

  if (Result) {
    finalizeDecl(Result);

    for (auto alternate : getAlternateDecls(Result))
      finalizeDecl(alternate);
  }

#ifndef NDEBUG
  auto Canon = cast<clang::NamedDecl>(ClangDecl->getCanonicalDecl());

  // Note that the decl was imported from Clang.  Don't mark Swift decls as
  // imported.
  if (Result &&
      (!Result->getDeclContext()->isModuleScopeContext() ||
       isa<ClangModuleUnit>(Result->getDeclContext()))) {
    // Either the Swift declaration was from stdlib,
    // or we imported the underlying decl of the typedef,
    // or we imported the decl itself.
    bool ImportedCorrectly =
        !Result->getClangDecl() || SkippedOverTypedef ||
        Result->getClangDecl()->getCanonicalDecl() == Canon;

    // Or the other type is a typedef,
    if (!ImportedCorrectly &&
        isa<clang::TypedefNameDecl>(Result->getClangDecl())) {
      // both types are ValueDecls:
      if (isa<clang::ValueDecl>(Result->getClangDecl())) {
        ImportedCorrectly =
            getClangASTContext().hasSameType(
                cast<clang::ValueDecl>(Result->getClangDecl())->getType(),
                cast<clang::ValueDecl>(Canon)->getType());
      } else if (isa<clang::TypeDecl>(Result->getClangDecl())) {
        // both types are TypeDecls:
        ImportedCorrectly =
            getClangASTContext().hasSameUnqualifiedType(
                getClangASTContext().getTypeDeclType(
                    cast<clang::TypeDecl>(Result->getClangDecl())),
                getClangASTContext().getTypeDeclType(
                    cast<clang::TypeDecl>(Canon)));
      }
      assert(ImportedCorrectly);
    }
    assert(Result->hasClangNode());
  }
#else
  (void)SkippedOverTypedef;
#endif

  return Result;
}

void ClangImporter::Implementation::startedImportingEntity() {
  ++NumTotalImportedEntities;
  // FIXME: (transitional) increment the redundant "always-on" counter.
  if (SwiftContext.Stats)
    SwiftContext.Stats->getFrontendCounters().NumTotalClangImportedEntities++;
}

/// Look up associated type requirements in the conforming type.
static void finishTypeWitnesses(
    NormalProtocolConformance *conformance) {
  auto *dc = conformance->getDeclContext();
  auto *module = dc->getParentModule();
  auto &ctx = module->getASTContext();

  auto *proto = conformance->getProtocol();
  auto selfType = conformance->getType();

  for (auto *req : proto->getMembers()) {
    if (auto *assocType = dyn_cast<AssociatedTypeDecl>(req)) {
      // FIXME: This should not happen?
      if (conformance->hasTypeWitness(assocType)) continue;

      bool satisfied = false;

      SmallVector<ValueDecl *, 4> lookupResults;
      NLOptions options = (NL_QualifiedDefault |
                           NL_OnlyTypes |
                           NL_ProtocolMembers);

      dc->lookupQualified(selfType, assocType->getFullName(), options,
                          ctx.getLazyResolver(), lookupResults);
      for (auto member : lookupResults) {
        auto typeDecl = cast<TypeDecl>(member);
        if (isa<AssociatedTypeDecl>(typeDecl)) continue;

        auto memberType = typeDecl->getDeclaredInterfaceType();
        auto subMap = selfType->getContextSubstitutionMap(
            module, typeDecl->getDeclContext());
        memberType = memberType.subst(subMap);
        conformance->setTypeWitness(assocType, memberType, typeDecl);
        satisfied = true;
        break;
      }

      if (!satisfied) {
        llvm::errs() << ("Cannot look up associated type for "
                         "imported conformance:\n");
        conformance->getType().dump(llvm::errs());
        assocType->dump(llvm::errs());
        abort();
      }
    }
  }
}

/// Make sure any inherited conformances also get completed, if necessary.
static void finishInheritedConformances(
    NormalProtocolConformance *conformance) {
  auto *proto = conformance->getProtocol();

  SmallVector<ProtocolDecl *, 2> inheritedProtos;
  for (auto *inherited : proto->getInheritedProtocols())
    inheritedProtos.push_back(inherited);

  // Sort for deterministic import.
  ProtocolType::canonicalizeProtocols(inheritedProtos);

  // Schedule any that aren't complete.
  for (auto *inherited : inheritedProtos) {
    ModuleDecl *M = conformance->getDeclContext()->getParentModule();
    auto inheritedConformance = M->lookupConformance(conformance->getType(),
                                                     inherited);
    assert(inheritedConformance && inheritedConformance->isConcrete() &&
           "inherited conformance not found");
  }
}

/// A stripped-down version of Type::subst that only works on non-generic
/// associated types.
///
/// This is used to finish a conformance for a concrete imported type that may
/// rely on default associated types defined in protocol extensions...without
/// having to do all the work of gathering conformances from scratch.
static Type
recursivelySubstituteBaseType(const NormalProtocolConformance *conformance,
                              DependentMemberType *depMemTy) {
  Type origBase = depMemTy->getBase();
  if (auto *depBase = origBase->getAs<DependentMemberType>()) {
    Type substBase = recursivelySubstituteBaseType(conformance, depBase);
    ModuleDecl *module = conformance->getDeclContext()->getParentModule();
    return depMemTy->substBaseType(module, substBase);
  }

  const ProtocolDecl *proto = conformance->getProtocol();
  assert(origBase->isEqual(proto->getSelfInterfaceType()));
  (void)proto;
  return conformance->getTypeWitness(depMemTy->getAssocType(),
                                     /*resolver=*/nullptr);
}

/// Collect conformances for the requirement signature.
static void finishSignatureConformances(
    NormalProtocolConformance *conformance) {
  auto *proto = conformance->getProtocol();

  SmallVector<ProtocolConformanceRef, 4> reqConformances;
  for (const auto &req : proto->getRequirementSignature()) {
    if (req.getKind() != RequirementKind::Conformance)
      continue;

    Type substTy;
    auto origTy = req.getFirstType();
    if (origTy->isEqual(proto->getSelfInterfaceType())) {
      substTy = conformance->getType();
    } else {
      auto *depMemTy = origTy->castTo<DependentMemberType>();
      substTy = recursivelySubstituteBaseType(conformance, depMemTy);
    }
    auto reqProto = req.getSecondType()->castTo<ProtocolType>()->getDecl();

    ModuleDecl *M = conformance->getDeclContext()->getParentModule();
    auto reqConformance = M->lookupConformance(substTy, reqProto);
    assert(reqConformance && reqConformance->isConcrete() &&
           "required conformance not found");
    reqConformances.push_back(*reqConformance);
  }
  conformance->setSignatureConformances(reqConformances);
}

/// Create witnesses for requirements not already met.
static void finishMissingOptionalWitnesses(
    NormalProtocolConformance *conformance) {
  auto *proto = conformance->getProtocol();

  for (auto req : proto->getMembers()) {
    auto valueReq = dyn_cast<ValueDecl>(req);
    if (!valueReq)
      continue;

    if (!conformance->hasWitness(valueReq)) {
      if (auto func = dyn_cast<AbstractFunctionDecl>(valueReq)){
        // For an optional requirement, record an empty witness:
        // we'll end up querying this at runtime.
        auto Attrs = func->getAttrs();
        if (Attrs.hasAttribute<OptionalAttr>()) {
          conformance->setWitness(valueReq, Witness());
          continue;
        }
      }

      conformance->setWitness(valueReq, valueReq);
    } else {
      // An initializer that conforms to a requirement is required.
      auto witness = conformance->getWitness(valueReq, nullptr).getDecl();
      if (auto ctor = dyn_cast_or_null<ConstructorDecl>(witness)) {
        if (!ctor->getAttrs().hasAttribute<RequiredAttr>()) {
          auto &ctx = proto->getASTContext();
          ctor->getAttrs().add(new (ctx) RequiredAttr(/*IsImplicit=*/true));
        }
      }
    }
  }
}

void ClangImporter::Implementation::finishNormalConformance(
    NormalProtocolConformance *conformance,
    uint64_t unused) {
  (void)unused;

  auto *proto = conformance->getProtocol();
  PrettyStackTraceType trace(SwiftContext, "completing conformance for",
                             conformance->getType());
  PrettyStackTraceDecl traceTo("... to", proto);

  if (!proto->isRequirementSignatureComputed())
    proto->computeRequirementSignature();

  finishTypeWitnesses(conformance);
  finishInheritedConformances(conformance);
  finishSignatureConformances(conformance);

  // Imported conformances to @objc protocols also require additional
  // initialization to complete the requirement to witness mapping.
  if (!proto->isObjC())
    return;

  assert(conformance->isComplete());
  conformance->setState(ProtocolConformanceState::Incomplete);

  finishMissingOptionalWitnesses(conformance);

  conformance->setState(ProtocolConformanceState::Complete);
}

Decl *ClangImporter::Implementation::importDeclAndCacheImpl(
    const clang::NamedDecl *ClangDecl,
    ImportNameVersion version,
    bool SuperfluousTypedefsAreTransparent) {
  if (!ClangDecl)
    return nullptr;

  FrontendStatsTracer StatsTracer(SwiftContext.Stats,
                                  "import-clang-decl", ClangDecl);
  clang::PrettyStackTraceDecl trace(ClangDecl, clang::SourceLocation(),
                                    Instance->getSourceManager(), "importing");

  auto Canon = cast<clang::NamedDecl>(ClangDecl->getCanonicalDecl());

  if (auto Known = importDeclCached(Canon, version)) {
    if (!SuperfluousTypedefsAreTransparent &&
        SuperfluousTypedefs.count(Canon))
      return nullptr;
    return Known;
  }

  bool TypedefIsSuperfluous = false;
  bool HadForwardDeclaration = false;

  startedImportingEntity();
  Decl *Result = importDeclImpl(ClangDecl, version, TypedefIsSuperfluous,
                                HadForwardDeclaration);
  if (!Result)
    return nullptr;

  if (TypedefIsSuperfluous) {
    SuperfluousTypedefs.insert(Canon);
    if (auto tagDecl = dyn_cast_or_null<clang::TagDecl>(Result->getClangDecl()))
      DeclsWithSuperfluousTypedefs.insert(tagDecl);
  }

  if (!HadForwardDeclaration)
    ImportedDecls[{Canon, version}] = Result;

  if (!SuperfluousTypedefsAreTransparent && TypedefIsSuperfluous)
    return nullptr;

  return Result;
}

Decl *
ClangImporter::Implementation::importMirroredDecl(const clang::NamedDecl *decl,
                                                  DeclContext *dc,
                                                  ImportNameVersion version,
                                                  ProtocolDecl *proto) {
  assert(dc);
  if (!decl)
    return nullptr;

  clang::PrettyStackTraceDecl trace(decl, clang::SourceLocation(),
                                    Instance->getSourceManager(),
                                    "importing (mirrored)");

  auto canon = decl->getCanonicalDecl();
  auto known = ImportedProtocolDecls.find(std::make_tuple(canon, dc, version));
  if (known != ImportedProtocolDecls.end())
    return known->second;

  SwiftDeclConverter converter(*this, version);
  Decl *result;
  if (auto method = dyn_cast<clang::ObjCMethodDecl>(decl)) {
    result = converter.importObjCMethodDecl(method, dc, /*accessor*/None);
  } else if (auto prop = dyn_cast<clang::ObjCPropertyDecl>(decl)) {
    result = converter.importObjCPropertyDecl(prop, dc);
  } else {
    llvm_unreachable("unexpected mirrored decl");
  }

  if (result) {
    assert(result->getClangDecl() && result->getClangDecl() == canon);

    auto updateMirroredDecl = [&](Decl *result) {
      result->setImplicit();
    
      // Map the Clang attributes onto Swift attributes.
      importAttributes(decl, result);

      if (proto->getAttrs().hasAttribute<AvailableAttr>()) {
        if (!result->getAttrs().hasAttribute<AvailableAttr>()) {
          AvailabilityContext protoRange =
            AvailabilityInference::availableRange(proto, SwiftContext);
          applyAvailableAttribute(result, protoRange, SwiftContext);
        }
      } else {
        // Infer the same availability for the mirrored declaration as
        // we would for the protocol member it is mirroring.
        inferProtocolMemberAvailability(*this, dc, result);
      }
    };

    updateMirroredDecl(result);

    // Update the alternate declaration as well.
    for (auto alternate : getAlternateDecls(result))
      updateMirroredDecl(alternate);
  }
  if (result || !converter.hadForwardDeclaration())
    ImportedProtocolDecls[std::make_tuple(canon, dc, version)] = result;
  return result;
}

DeclContext *ClangImporter::Implementation::importDeclContextImpl(
    const clang::DeclContext *dc) {
  // Every declaration should come from a module, so we should not see the
  // TranslationUnit DeclContext here.
  assert(!dc->isTranslationUnit());

  auto decl = dyn_cast<clang::NamedDecl>(dc);
  if (!decl)
    return nullptr;

  auto swiftDecl = importDecl(decl, CurrentVersion);
  if (!swiftDecl)
    return nullptr;

  if (auto nominal = dynCastIgnoringCompatibilityAlias<NominalTypeDecl>(swiftDecl))
    return nominal;
  if (auto extension = dyn_cast<ExtensionDecl>(swiftDecl))
    return extension;
  if (auto constructor = dyn_cast<ConstructorDecl>(swiftDecl))
    return constructor;
  if (auto destructor = dyn_cast<DestructorDecl>(swiftDecl))
    return destructor;
  return nullptr;
}

GenericSignature *ClangImporter::Implementation::buildGenericSignature(
    GenericParamList *genericParams, DeclContext *dc) {
  GenericSignatureBuilder builder(SwiftContext);
  SmallVector<GenericTypeParamType *, 4> allGenericParams;
  for (auto param : *genericParams) {
    builder.addGenericParameter(param);
    allGenericParams.push_back(
      param->getDeclaredInterfaceType()->castTo<GenericTypeParamType>());
  }
  for (auto param : *genericParams) {
    bool result = builder.addGenericParameterRequirements(param);
    assert(!result);
    (void) result;
  }

  return std::move(builder).computeGenericSignature(SourceLoc());
}

// Calculate the generic environment from an imported generic param list.
GenericEnvironment *ClangImporter::Implementation::buildGenericEnvironment(
    GenericParamList *genericParams, DeclContext *dc) {
  return buildGenericSignature(genericParams, dc)->createGenericEnvironment();
}

DeclContext *
ClangImporter::Implementation::importDeclContextOf(
  const clang::Decl *decl,
  EffectiveClangContext context)
{
  DeclContext *importedDC = nullptr;
  switch (context.getKind()) {
  case EffectiveClangContext::DeclContext: {
    auto dc = context.getAsDeclContext();
    if (dc->isTranslationUnit()) {
      if (auto *module = getClangModuleForDecl(decl))
        return module;
      else
        return nullptr;
    }

    // Import the DeclContext.
    importedDC = importDeclContextImpl(dc);
    break;
  }

  case EffectiveClangContext::TypedefContext: {
    // Import the typedef-name as a declaration.
    auto importedDecl = importDecl(context.getTypedefName(), CurrentVersion);
    if (!importedDecl) return nullptr;

    // Dig out the imported DeclContext.
    importedDC = dynCastIgnoringCompatibilityAlias<NominalTypeDecl>(importedDecl);
    break;
  }

  case EffectiveClangContext::UnresolvedContext: {
    // FIXME: Resolve through name lookup. This is brittle.
    auto submodule =
      getClangSubmoduleForDecl(decl, /*allowForwardDeclaration=*/false);
    if (!submodule) return nullptr;

    if (auto lookupTable = findLookupTable(*submodule)) {
      if (auto clangDecl
            = lookupTable->resolveContext(context.getUnresolvedName())) {
        // Import the Clang declaration.
        auto decl = importDecl(clangDecl, CurrentVersion);
        if (!decl) return nullptr;

        // Look through typealiases.
        if (auto typealias = dyn_cast<TypeAliasDecl>(decl))
          importedDC = typealias->getDeclaredInterfaceType()->getAnyNominal();
        else // Map to a nominal type declaration.
          importedDC = dyn_cast<NominalTypeDecl>(decl);
        break;
      }
    }
  }
  }

  // If we didn't manage to import the declaration context, we're done.
  if (!importedDC) return nullptr;

  // If the declaration was not global to start with, we're done.
  bool isGlobal =
    decl->getDeclContext()->getRedeclContext()->isTranslationUnit();
  if (!isGlobal) return importedDC;

  // If the resulting declaration context is not a nominal type,
  // we're done.
  auto nominal = dyn_cast<NominalTypeDecl>(importedDC);
  if (!nominal) return importedDC;

  // Look for the extension for the given nominal type within the
  // Clang submodule of the declaration.
  const clang::Module *declSubmodule = *getClangSubmoduleForDecl(decl);
  auto extensionKey = std::make_pair(nominal, declSubmodule);
  auto knownExtension = extensionPoints.find(extensionKey);
  if (knownExtension != extensionPoints.end())
    return knownExtension->second;

  // Create a new extension for this nominal type/Clang submodule pair.
  auto swiftTyLoc = TypeLoc::withoutLoc(nominal->getDeclaredType());
  auto ext = ExtensionDecl::create(SwiftContext, SourceLoc(), swiftTyLoc, {},
                                   getClangModuleForDecl(decl), nullptr);
  ext->setValidationToChecked();
  ext->setMemberLoader(this, reinterpret_cast<uintptr_t>(declSubmodule));

  if (auto protoDecl = ext->getAsProtocolExtensionContext()) {
    ext->setGenericParams(protoDecl->createGenericParams(ext));

    auto *env = buildGenericEnvironment(ext->getGenericParams(), ext);
    ext->setGenericEnvironment(env);
  }

  // Add the extension to the nominal type.
  nominal->addExtension(ext);

  // Record this extension so we can find it later.
  extensionPoints[extensionKey] = ext;
  return ext;
}

ValueDecl *
ClangImporter::Implementation::createConstant(Identifier name, DeclContext *dc,
                                              Type type,
                                              const clang::APValue &value,
                                              ConstantConvertKind convertKind,
                                              bool isStatic,
                                              ClangNode ClangN) {
  auto &context = SwiftContext;

  // Create the integer literal value.
  Expr *expr = nullptr;
  switch (value.getKind()) {
  case clang::APValue::AddrLabelDiff:
  case clang::APValue::Array:
  case clang::APValue::ComplexFloat:
  case clang::APValue::ComplexInt:
  case clang::APValue::LValue:
  case clang::APValue::MemberPointer:
  case clang::APValue::Struct:
  case clang::APValue::Uninitialized:
  case clang::APValue::Union:
  case clang::APValue::Vector:
    llvm_unreachable("Unhandled APValue kind");

  case clang::APValue::Float:
  case clang::APValue::Int: {
    // Print the value.
    llvm::SmallString<16> printedValueBuf;
    if (value.getKind() == clang::APValue::Int) {
      value.getInt().toString(printedValueBuf);
    } else {
      assert(value.getFloat().isFinite() && "can't handle infinities or NaNs");
      value.getFloat().toString(printedValueBuf);
    }
    StringRef printedValue = printedValueBuf.str();

    // If this was a negative number, record that and strip off the '-'.
    bool isNegative = printedValue.front() == '-';
    if (isNegative)
      printedValue = printedValue.drop_front();

    // Create the expression node.
    StringRef printedValueCopy(context.AllocateCopy(printedValue));
    if (value.getKind() == clang::APValue::Int) {
      if (type->getCanonicalType()->isBool()) {
        expr = new (context) BooleanLiteralExpr(value.getInt().getBoolValue(),
                                                SourceLoc(),
                                                /**Implicit=*/true);
      } else {
        expr = new (context) IntegerLiteralExpr(printedValueCopy, SourceLoc(),
                                                /*Implicit=*/true);
      }
    } else {
      expr = new (context) FloatLiteralExpr(printedValueCopy, SourceLoc(),
                                            /*Implicit=*/true);
    }

    if (isNegative)
      cast<NumberLiteralExpr>(expr)->setNegative(SourceLoc());

    break;
  }
  }

  assert(expr);
  return createConstant(name, dc, type, expr, convertKind, isStatic, ClangN);
}


ValueDecl *
ClangImporter::Implementation::createConstant(Identifier name, DeclContext *dc,
                                              Type type, StringRef value,
                                              ConstantConvertKind convertKind,
                                              bool isStatic,
                                              ClangNode ClangN) {
  auto expr = new (SwiftContext) StringLiteralExpr(value, SourceRange());
  return createConstant(name, dc, type, expr, convertKind, isStatic, ClangN);
}


ValueDecl *
ClangImporter::Implementation::createConstant(Identifier name, DeclContext *dc,
                                              Type type, Expr *valueExpr,
                                              ConstantConvertKind convertKind,
                                              bool isStatic,
                                              ClangNode ClangN) {
  auto &C = SwiftContext;

  VarDecl *var = nullptr;
  if (ClangN) {
    var = createDeclWithClangNode<VarDecl>(ClangN, AccessLevel::Public,
                                           /*IsStatic*/isStatic, VarDecl::Specifier::Var,
                                           /*IsCaptureList*/false, SourceLoc(),
                                           name, dc->mapTypeIntoContext(type), dc);
  } else {
    var = new (SwiftContext)
        VarDecl(/*IsStatic*/isStatic, VarDecl::Specifier::Var, /*IsCaptureList*/false,
                SourceLoc(), name, dc->mapTypeIntoContext(type), dc);
    var->setValidationToChecked();
  }

  var->setInterfaceType(type);
  var->setIsObjC(false);

  // 'self'
  ParamDecl *selfDecl = nullptr;
  auto *params = ParameterList::createEmpty(C);
  if (dc->isTypeContext())
    selfDecl = ParamDecl::createSelf(SourceLoc(), dc, isStatic);

  // Create the getter function declaration.
  auto func = AccessorDecl::create(C,
                     /*FuncLoc=*/SourceLoc(),
                     /*AccessorKeywordLoc=*/SourceLoc(),
                     AccessorKind::Get,
                     AddressorKind::NotAddressor,
                     var,
                     /*StaticLoc=*/SourceLoc(),
                     StaticSpellingKind::None,
                     /*Throws=*/false,
                     /*ThrowsLoc=*/SourceLoc(),
                     /*GenericParams=*/nullptr,
                     selfDecl, params,
                     TypeLoc::withoutLoc(type), dc);
  func->setStatic(isStatic);
  func->computeType();
  func->setAccess(getOverridableAccessLevel(dc));
  func->setValidationToChecked();
  func->setImplicit();
  func->setIsObjC(false);

  // If we're not done type checking, build the getter body.
  if (!hasFinishedTypeChecking()) {
    auto expr = valueExpr;

    // If we need a conversion, add one now.
    switch (convertKind) {
    case ConstantConvertKind::None:
      break;

    case ConstantConvertKind::Construction:
    case ConstantConvertKind::ConstructionWithUnwrap: {
      auto typeRef = TypeExpr::createImplicit(type, C);
      
      expr = CallExpr::createImplicit(C, typeRef, { expr }, { C.Id_rawValue });
      if (convertKind == ConstantConvertKind::ConstructionWithUnwrap)
        expr = new (C) ForceValueExpr(expr, SourceLoc());
      break;
    }

    case ConstantConvertKind::Coerce:
      break;

    case ConstantConvertKind::Downcast: {
      expr = new (C) ForcedCheckedCastExpr(expr, SourceLoc(), SourceLoc(),
                                           TypeLoc::withoutLoc(type));
      expr->setImplicit();
      break;
    }
    }

    // Create the return statement.
    auto ret = new (C) ReturnStmt(SourceLoc(), expr);

    // Finally, set the body.
    func->setBody(BraceStmt::create(C, SourceLoc(),
                                    ASTNode(ret),
                                    SourceLoc()));
  }

  // Mark the function transparent so that we inline it away completely.
  func->getAttrs().add(new (C) TransparentAttr(/*implicit*/ true));
  
  // Set the function up as the getter.
  makeComputed(var, func, nullptr);

  // Register this thunk as an external definition.
  registerExternalDecl(func);

  return var;
}

/// \brief Create a decl with error type and an "unavailable" attribute on it
/// with the specified message.
void ClangImporter::Implementation::
markUnavailable(ValueDecl *decl, StringRef unavailabilityMsgRef) {

  unavailabilityMsgRef = SwiftContext.AllocateCopy(unavailabilityMsgRef);
  auto ua = AvailableAttr::createPlatformAgnostic(SwiftContext,
                                                  unavailabilityMsgRef);
  decl->getAttrs().add(ua);
}

/// \brief Create a decl with error type and an "unavailable" attribute on it
/// with the specified message.
ValueDecl *ClangImporter::Implementation::
createUnavailableDecl(Identifier name, DeclContext *dc, Type type,
                      StringRef UnavailableMessage, bool isStatic,
                      ClangNode ClangN) {

  // Create a new VarDecl with dummy type.
  auto var = createDeclWithClangNode<VarDecl>(ClangN, AccessLevel::Public,
                                              /*IsStatic*/isStatic,
                                              VarDecl::Specifier::Var,
                                              /*IsCaptureList*/false,
                                              SourceLoc(), name, type, dc);
  var->setIsObjC(false);
  var->setInterfaceType(type);
  markUnavailable(var, UnavailableMessage);

  return var;
}


void
ClangImporter::Implementation::loadAllMembers(Decl *D, uint64_t extra) {

  FrontendStatsTracer tracer(D->getASTContext().Stats, "load-all-members", D);
  assert(D);

  // Check whether we're importing an Objective-C container of some sort.
  auto objcContainer =
    dyn_cast_or_null<clang::ObjCContainerDecl>(D->getClangDecl());

  // If not, we're importing globals-as-members into an extension.
  if (objcContainer) {
    loadAllMembersOfObjcContainer(D, objcContainer);
    return;
  }
  loadAllMembersIntoExtension(D, extra);
}

void ClangImporter::Implementation::loadAllMembersIntoExtension(
    Decl *D, uint64_t extra) {
  // We have extension.
  auto ext = cast<ExtensionDecl>(D);
  auto nominal = ext->getExtendedType()->getAnyNominal();

  // The submodule of the extension is encoded in the extra data.
  clang::Module *submodule =
      reinterpret_cast<clang::Module *>(static_cast<uintptr_t>(extra));

  // Find the lookup table.
  auto topLevelModule = submodule;
  if (topLevelModule)
    topLevelModule = topLevelModule->getTopLevelModule();
  auto table = findLookupTable(topLevelModule);
  if (!table)
    return;

  PrettyStackTraceStringAction trace(
      "loading import-as-members from",
      topLevelModule ? topLevelModule->getTopLevelModuleName()
                     : "(bridging header)");
  PrettyStackTraceDecl trace2("...for", nominal);

  // Dig out the effective Clang context for this nominal type.
  auto effectiveClangContext = getEffectiveClangContext(nominal);
  if (!effectiveClangContext)
    return;

  // Get ready to actually load the members.
  startedImportingEntity();

  // Load the members.
  for (auto entry : table->lookupGlobalsAsMembers(effectiveClangContext)) {
    auto decl = entry.get<clang::NamedDecl *>();

    // Only include members in the same submodule as this extension.
    if (getClangSubmoduleForDecl(decl) != submodule)
      continue;

    forEachDistinctName(
        decl, [&](ImportedName newName, ImportNameVersion nameVersion) -> bool {
      return addMemberAndAlternatesToExtension(decl, newName, nameVersion, ext);
    });
  }
}

static Decl *findMemberThatWillLandInAnExtensionContext(Decl *member) {
  Decl *result = member;
  while (!isa<ExtensionDecl>(result->getDeclContext())) {
    auto nominal = dyn_cast<NominalTypeDecl>(result->getDeclContext());
    if (!nominal)
      return nullptr;

    result = nominal;
    if (result->hasClangNode())
      return nullptr;
  }
  return result;
}

bool ClangImporter::Implementation::addMemberAndAlternatesToExtension(
    clang::NamedDecl *decl, ImportedName newName, ImportNameVersion nameVersion,
    ExtensionDecl *ext) {
  // Quickly check the context and bail out if it obviously doesn't
  // belong here.
  if (auto *importDC = newName.getEffectiveContext().getAsDeclContext())
    if (importDC->isTranslationUnit())
      return true;

  // Then try to import the decl under the specified name.
  auto *member = importDecl(decl, nameVersion);
  if (!member)
    return false;

  member = findMemberThatWillLandInAnExtensionContext(member);
  if (!member || member->getDeclContext() != ext)
    return true;
  ext->addMember(member);

  for (auto alternate : getAlternateDecls(member)) {
    if (alternate->getDeclContext() == ext)
      ext->addMember(alternate);
  }
  return true;
}

static ExtensionDecl *
figureOutTheDeclarationContextToImportInto(Decl *D, DeclContext *&DC,
                                           IterableDeclContext *&IDC) {
  if (auto *nominal = dyn_cast<NominalTypeDecl>(D)) {
    DC = nominal;
    IDC = nominal;
    return nullptr;
  }
  ExtensionDecl *ext = cast<ExtensionDecl>(D);
  DC = ext;
  IDC = ext;
  return ext;
}

static void loadMembersOfBaseImportedFromClang(ExtensionDecl *ext) {
  const NominalTypeDecl *base = ext->getExtendedType()->getAnyNominal();
  auto *clangBase = base->getClangDecl();
  if (!clangBase)
    return;
  base->loadAllMembers();

  // Sanity check: make sure we don't jump over to a category /while/
  // loading the original class's members. Right now we only check if this
  // happens on the first member.
  if (auto *clangContainer = dyn_cast<clang::ObjCContainerDecl>(clangBase))
    assert((clangContainer->decls_empty() || !base->getMembers().empty()) &&
           "can't load extension members before base has finished");
}

void ClangImporter::Implementation::loadAllMembersOfObjcContainer(
    Decl *D, const clang::ObjCContainerDecl *objcContainer) {
  clang::PrettyStackTraceDecl trace(objcContainer, clang::SourceLocation(),
                                    Instance->getSourceManager(),
                                    "loading members for");

  DeclContext *DC;
  IterableDeclContext *IDC;
  if (ExtensionDecl *ext =
          figureOutTheDeclarationContextToImportInto(D, DC, IDC)) {
    // If the base is also imported from Clang, load its members first.
    loadMembersOfBaseImportedFromClang(ext);
  }

  startedImportingEntity();

  SmallVector<Decl *, 16> members;
  collectMembersToAdd(objcContainer, D, DC, members);

  for (auto member : members) {
    IDC->addMember(member);
  }
}

void ClangImporter::Implementation::insertMembersAndAlternates(
    const clang::NamedDecl *nd, SmallVectorImpl<Decl *> &members) {
  llvm::SmallPtrSet<Decl *, 4> knownAlternateMembers;
  forEachDistinctName(
      nd, [&](ImportedName name, ImportNameVersion nameVersion) -> bool {
    auto member = importDecl(nd, nameVersion);
    if (!member)
      return false;

    // If there are alternate declarations for this member, add them.
    for (auto alternate : getAlternateDecls(member)) {
      if (alternate->getDeclContext() == member->getDeclContext() &&
          knownAlternateMembers.insert(alternate).second) {
        members.push_back(alternate);
      }
    }

    // If this declaration shouldn't be visible, don't add it to
    // the list.
    if (shouldSuppressDeclImport(nd))
      return true;

    members.push_back(member);
    return true;
  });
}

void ClangImporter::Implementation::collectMembersToAdd(
    const clang::ObjCContainerDecl *objcContainer, Decl *D, DeclContext *DC,
    SmallVectorImpl<Decl *> &members) {
  for (const clang::Decl *m : objcContainer->decls()) {
    auto nd = dyn_cast<clang::NamedDecl>(m);
    if (nd && nd == nd->getCanonicalDecl() &&
        nd->getDeclContext() == objcContainer)
      insertMembersAndAlternates(nd, members);
  }

  SwiftDeclConverter converter(*this, CurrentVersion);

  SmallVector<ProtocolDecl *, 4> protos = takeImportedProtocols(D);
  if (auto clangClass = dyn_cast<clang::ObjCInterfaceDecl>(objcContainer)) {
    auto swiftClass = cast<ClassDecl>(D);
    objcContainer = clangClass = clangClass->getDefinition();

    // Imported inherited initializers.
    if (clangClass->getName() != "Protocol") {
      converter.importInheritedConstructors(const_cast<ClassDecl *>(swiftClass),
                                            members);
    }

  } else if (auto clangProto
               = dyn_cast<clang::ObjCProtocolDecl>(objcContainer)) {
    objcContainer = clangProto->getDefinition();
  }
  // Import mirrored declarations for protocols to which this category
  // or extension conforms.
  // FIXME: This is supposed to be a short-term hack.
  converter.importMirroredProtocolMembers(objcContainer, DC,
                                          protos, members, SwiftContext);
}

void ClangImporter::Implementation::loadAllConformances(
       const Decl *D, uint64_t contextData,
       SmallVectorImpl<ProtocolConformance *> &Conformances) {
  Conformances = takeDelayedConformance(contextData);
}

Optional<MappedTypeNameKind>
ClangImporter::Implementation::getSpecialTypedefKind(clang::TypedefNameDecl *decl) {
  auto iter = SpecialTypedefNames.find(decl->getCanonicalDecl());
  if (iter == SpecialTypedefNames.end())
    return None;
  return iter->second;
}

Identifier
ClangImporter::getEnumConstantName(const clang::EnumConstantDecl *enumConstant){
  return Impl.importFullName(enumConstant, Impl.CurrentVersion)
      .getDeclName()
      .getBaseIdentifier();
}

// See swift/Basic/Statistic.h for declaration: this enables tracing
// clang::Decls, is defined here to avoid too much layering violation / circular
// linkage dependency.

struct ClangDeclTraceFormatter : public UnifiedStatsReporter::TraceFormatter {
  void traceName(const void *Entity, raw_ostream &OS) const {
    if (!Entity)
      return;
    const clang::Decl *CD = static_cast<const clang::Decl *>(Entity);
    if (auto const *ND = dyn_cast<const clang::NamedDecl>(CD)) {
      ND->printName(OS);
    } else {
      OS << "<unnamed-clang-decl>";
    }
  }

  static inline bool printClangShortLoc(raw_ostream &OS,
                                        clang::SourceManager *CSM,
                                        clang::SourceLocation L) {
    if (!L.isValid() || !L.isFileID())
      return false;
    auto PLoc = CSM->getPresumedLoc(L);
    OS << llvm::sys::path::filename(PLoc.getFilename()) << ':' << PLoc.getLine()
       << ':' << PLoc.getColumn();
    return true;
  }

  void traceLoc(const void *Entity, SourceManager *SM,
                clang::SourceManager *CSM, raw_ostream &OS) const {
    if (!Entity)
      return;
    if (CSM) {
      const clang::Decl *CD = static_cast<const clang::Decl *>(Entity);
      auto Range = CD->getSourceRange();
      if (printClangShortLoc(OS, CSM, Range.getBegin()))
        OS << '-';
      printClangShortLoc(OS, CSM, Range.getEnd());
    }
  }
};

static ClangDeclTraceFormatter TF;

template<>
const UnifiedStatsReporter::TraceFormatter*
FrontendStatsTracer::getTraceFormatter<const clang::Decl *>() {
  return &TF;
}