//===--- TypeCheckDecl.cpp - Type Checking for 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 semantic analysis for declarations.
//
//===----------------------------------------------------------------------===//

#include "CodeSynthesis.h"
#include "ConstraintSystem.h"
#include "DerivedConformances.h"
#include "TypeChecker.h"
#include "GenericTypeResolver.h"
#include "MiscDiagnostics.h"
#include "swift/AST/AccessScope.h"
#include "swift/AST/ArchetypeBuilder.h"
#include "swift/AST/ASTPrinter.h"
#include "swift/AST/ASTVisitor.h"
#include "swift/AST/ASTWalker.h"
#include "swift/AST/Expr.h"
#include "swift/AST/ForeignErrorConvention.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/NameLookup.h"
#include "swift/AST/PrettyStackTrace.h"
#include "swift/AST/ReferencedNameTracker.h"
#include "swift/AST/TypeWalker.h"
#include "swift/Basic/Statistic.h"
#include "swift/Parse/Lexer.h"
#include "swift/Parse/Parser.h"
#include "swift/Sema/IterativeTypeChecker.h"
#include "swift/Serialization/SerializedModuleLoader.h"
#include "swift/Strings.h"
#include "swift/Basic/Defer.h"
#include "llvm/ADT/APFloat.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/APSInt.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/Twine.h"
using namespace swift;

#define DEBUG_TYPE "TypeCheckDecl"

namespace {

/// Used during enum raw value checking to identify duplicate raw values.
/// Character, string, float, and integer literals are all keyed by value.
/// Float and integer literals are additionally keyed by numeric equivalence.
struct RawValueKey {
  enum class Kind : uint8_t {
    String, Float, Int, Tombstone, Empty
  } kind;
  
  struct IntValueTy {
    uint64_t v0;
    uint64_t v1;

    IntValueTy(const APInt &bits) {
      APInt bits128 = bits.sextOrTrunc(128);
      assert(bits128.getBitWidth() <= 128);
      const uint64_t *data = bits128.getRawData();
      v0 = data[0];
      v1 = data[1];
    }
  };

  struct FloatValueTy {
    uint64_t v0;
    uint64_t v1;
  };

  // FIXME: doesn't accommodate >64-bit or signed raw integer or float values.
  union {
    StringRef stringValue;
    uint32_t charValue;
    IntValueTy intValue;
    FloatValueTy floatValue;
  };
  
  explicit RawValueKey(LiteralExpr *expr) {
    switch (expr->getKind()) {
    case ExprKind::IntegerLiteral:
      kind = Kind::Int;
      intValue = IntValueTy(cast<IntegerLiteralExpr>(expr)->getValue());
      return;
    case ExprKind::FloatLiteral: {
      APFloat value = cast<FloatLiteralExpr>(expr)->getValue();
      llvm::APSInt asInt(127, /*isUnsigned=*/false);
      bool isExact = false;
      APFloat::opStatus status =
          value.convertToInteger(asInt, APFloat::rmTowardZero, &isExact);
      if (asInt.getBitWidth() <= 128 && status == APFloat::opOK && isExact) {
        kind = Kind::Int;
        intValue = IntValueTy(asInt);
        return;
      }
      APInt bits = value.bitcastToAPInt();
      const uint64_t *data = bits.getRawData();
      if (bits.getBitWidth() == 80) {
        kind = Kind::Float;
        floatValue = FloatValueTy{ data[0], data[1] };
      } else {
        assert(bits.getBitWidth() == 64);
        kind = Kind::Float;
        floatValue = FloatValueTy{ data[0], 0 };
      }
      return;
    }
    case ExprKind::StringLiteral:
      kind = Kind::String;
      stringValue = cast<StringLiteralExpr>(expr)->getValue();
      return;
    default:
      llvm_unreachable("not a valid literal expr for raw value");
    }
  }
  
  explicit RawValueKey(Kind k) : kind(k) {
    assert((k == Kind::Tombstone || k == Kind::Empty)
           && "this ctor is only for creating DenseMap special values");
  }
};
  
/// Used during enum raw value checking to identify the source of a raw value,
/// which may have been derived by auto-incrementing, for diagnostic purposes.
struct RawValueSource {
  /// The decl that has the raw value.
  EnumElementDecl *sourceElt;
  /// If the sourceDecl didn't explicitly name a raw value, this is the most
  /// recent preceding decl with an explicit raw value. This is used to
  /// diagnose 'autoincrementing from' messages.
  EnumElementDecl *lastExplicitValueElt;
};

} // end anonymous namespace

namespace llvm {

template<>
class DenseMapInfo<RawValueKey> {
public:
  static RawValueKey getEmptyKey() {
    return RawValueKey(RawValueKey::Kind::Empty);
  }
  static RawValueKey getTombstoneKey() {
    return RawValueKey(RawValueKey::Kind::Tombstone);
  }
  static unsigned getHashValue(RawValueKey k) {
    switch (k.kind) {
    case RawValueKey::Kind::Float:
      // Hash as bits. We want to treat distinct but IEEE-equal values as not
      // equal.
      return DenseMapInfo<uint64_t>::getHashValue(k.floatValue.v0) ^
             DenseMapInfo<uint64_t>::getHashValue(k.floatValue.v1);
    case RawValueKey::Kind::Int:
      return DenseMapInfo<uint64_t>::getHashValue(k.intValue.v0) &
             DenseMapInfo<uint64_t>::getHashValue(k.intValue.v1);
    case RawValueKey::Kind::String:
      return llvm::HashString(k.stringValue);
    case RawValueKey::Kind::Empty:
    case RawValueKey::Kind::Tombstone:
      return 0;
    }

    llvm_unreachable("Unhandled RawValueKey in switch.");
  }
  static bool isEqual(RawValueKey a, RawValueKey b) {
    if (a.kind != b.kind)
      return false;
    switch (a.kind) {
    case RawValueKey::Kind::Float:
      // Hash as bits. We want to treat distinct but IEEE-equal values as not
      // equal.
      return a.floatValue.v0 == b.floatValue.v0 &&
             a.floatValue.v1 == b.floatValue.v1;
    case RawValueKey::Kind::Int:
      return a.intValue.v0 == b.intValue.v0 &&
             a.intValue.v1 == b.intValue.v1;
    case RawValueKey::Kind::String:
      return a.stringValue.equals(b.stringValue);
    case RawValueKey::Kind::Empty:
    case RawValueKey::Kind::Tombstone:
      return true;
    }

    llvm_unreachable("Unhandled RawValueKey in switch.");
  }
};
  
} // namespace llvm

/// Determine whether the given declaration can inherit a class.
static bool canInheritClass(Decl *decl) {
  // Classes can inherit from a class.
  if (isa<ClassDecl>(decl))
    return true;

  // Generic type parameters can inherit a class.
  if (isa<GenericTypeParamDecl>(decl))
    return true;

  // Associated types can inherit a class.
  if (isa<AssociatedTypeDecl>(decl))
    return true;

  return false;
}

// Add implicit conformances to the given declaration.
static void addImplicitConformances(
              TypeChecker &tc, Decl *decl,
              llvm::SmallSetVector<ProtocolDecl *, 4> &allProtocols) {
  if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
    SmallVector<ProtocolDecl *, 2> protocols;
    nominal->getImplicitProtocols(protocols);
    allProtocols.insert(protocols.begin(), protocols.end());
  }
}

/// Check that the declaration attributes are ok.
static void validateAttributes(TypeChecker &TC, Decl *D);

void TypeChecker::resolveSuperclass(ClassDecl *classDecl) {
  IterativeTypeChecker ITC(*this);
  ITC.satisfy(requestTypeCheckSuperclass(classDecl));
}

void TypeChecker::resolveRawType(EnumDecl *enumDecl) {
  IterativeTypeChecker ITC(*this);
  ITC.satisfy(requestTypeCheckRawType(enumDecl));
}

void TypeChecker::resolveInheritedProtocols(ProtocolDecl *protocol) {
  IterativeTypeChecker ITC(*this);
  ITC.satisfy(requestInheritedProtocols(protocol));
}

void TypeChecker::resolveInheritanceClause(
       llvm::PointerUnion<TypeDecl *, ExtensionDecl *> decl) {
  IterativeTypeChecker ITC(*this);
  unsigned numInherited;
  if (auto ext = decl.dyn_cast<ExtensionDecl *>()) {
    numInherited = ext->getInherited().size();
  } else {
    numInherited = decl.get<TypeDecl *>()->getInherited().size();
  }

  for (unsigned i = 0; i != numInherited; ++i) {
    ITC.satisfy(requestResolveInheritedClauseEntry({ decl, i }));
  }
}

/// check the inheritance clause of a type declaration or extension thereof.
///
/// This routine validates all of the types in the parsed inheritance clause,
/// recording the superclass (if any and if allowed) as well as the protocols
/// to which this type declaration conforms.
void TypeChecker::checkInheritanceClause(Decl *decl,
                                         GenericTypeResolver *resolver) {
  TypeResolutionOptions options;
  DeclContext *DC;
  if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
    DC = nominal;
    options |= (TR_GenericSignature |
                TR_InheritanceClause |
                TR_AllowUnavailableProtocol);
  } else if (auto ext = dyn_cast<ExtensionDecl>(decl)) {
    DC = ext;
    options |= (TR_GenericSignature |
                TR_InheritanceClause |
                TR_AllowUnavailableProtocol);
  } else if (isa<GenericTypeParamDecl>(decl)) {
    // For generic parameters, we want name lookup to look at just the
    // signature of the enclosing entity.
    DC = decl->getDeclContext();
    if (auto nominal = dyn_cast<NominalTypeDecl>(DC)) {
      DC = nominal;
      options |= TR_GenericSignature;
    } else if (auto ext = dyn_cast<ExtensionDecl>(DC)) {
      DC = ext;
      options |= TR_GenericSignature;
    } else if (auto func = dyn_cast<AbstractFunctionDecl>(DC)) {
      DC = func;
      options |= TR_GenericSignature;
    } else if (!DC->isModuleScopeContext()) {
      // Skip the generic parameter's context entirely.
      DC = DC->getParent();
    }
  } else {
    DC = decl->getDeclContext();
  }

  // Establish a default generic type resolver.
  GenericTypeToArchetypeResolver defaultResolver(decl->getInnermostDeclContext());
  if (!resolver)
    resolver = &defaultResolver;

  MutableArrayRef<TypeLoc> inheritedClause;

  // If we already checked the inheritance clause, don't do so again.
  if (auto type = dyn_cast<TypeDecl>(decl)) {
    if (type->checkedInheritanceClause())
      return;

    // This breaks infinite recursion, which will be diagnosed separately.
    type->setCheckedInheritanceClause();
    inheritedClause = type->getInherited();
  } else {
    auto ext = cast<ExtensionDecl>(decl);

    validateExtension(ext);

    if (ext->isInvalid() ||
        ext->checkedInheritanceClause())
      return;

    // This breaks infinite recursion, which will be diagnosed separately.
    ext->setCheckedInheritanceClause();
    inheritedClause = ext->getInherited();

    // Protocol extensions cannot have inheritance clauses.
    if (ext->getExtendedType()->is<ProtocolType>()) {
      if (!inheritedClause.empty()) {
        diagnose(ext->getLoc(), diag::extension_protocol_inheritance,
                 ext->getExtendedType())
          .highlight(SourceRange(inheritedClause.front().getSourceRange().Start,
                                 inheritedClause.back().getSourceRange().End));
        ext->setInherited({ });
        return;
      }
    }

    // Constrained extensions cannot have inheritance clauses.
    if (!inheritedClause.empty() &&
        ext->getGenericParams() &&
        ext->getGenericParams()->hasTrailingWhereClause()) {
      diagnose(ext->getLoc(), diag::extension_constrained_inheritance,
               ext->getExtendedType())
      .highlight(SourceRange(inheritedClause.front().getSourceRange().Start,
                             inheritedClause.back().getSourceRange().End));
      ext->setInherited({ });
    }
  }

  // Check all of the types listed in the inheritance clause.
  Type superclassTy;
  SourceRange superclassRange;
  llvm::SmallSetVector<ProtocolDecl *, 4> allProtocols;
  llvm::SmallDenseMap<CanType, SourceRange> inheritedTypes;
  addImplicitConformances(*this, decl, allProtocols);
  for (unsigned i = 0, n = inheritedClause.size(); i != n; ++i) {
    auto &inherited = inheritedClause[i];

    // Validate the type.
    if (validateType(inherited, DC, options, resolver)) {
      inherited.setInvalidType(Context);
      continue;
    }

    auto inheritedTy = inherited.getType();

    // If this is an error type, ignore it.
    if (inheritedTy->hasError())
      continue;

    // Retrieve the interface type for this inherited type.
    //
    // If we have a generic parameter, mapTypeOutOfContext() might not
    // work yet, if we're calling this while building the generic
    // signature. However, we're also not storing inheritedTy back
    // anywhere, so it's OK to leave it as an archetype.
    //
    // FIXME: Ideally, we wouldn't have code paths that take a mix
    // of archetypes and interface types. Other than generic parameters,
    // the only time we get an interface type here is with invalid
    // circular cases. That should be diagnosed elsewhere.
    if (inheritedTy->hasArchetype() && !isa<GenericTypeParamDecl>(decl))
      inheritedTy = DC->mapTypeOutOfContext(inheritedTy);

    // Check whether we inherited from the same type twice.
    CanType inheritedCanTy = inheritedTy->getCanonicalType();
    auto knownType = inheritedTypes.find(inheritedCanTy);
    if (knownType != inheritedTypes.end()) {
      SourceLoc afterPriorLoc
        = Lexer::getLocForEndOfToken(Context.SourceMgr,
                                     inheritedClause[i-1].getSourceRange().End);
      SourceLoc afterMyEndLoc
        = Lexer::getLocForEndOfToken(Context.SourceMgr,
                                     inherited.getSourceRange().Start);

      diagnose(inherited.getSourceRange().Start,
               diag::duplicate_inheritance, inheritedTy)
        .fixItRemoveChars(afterPriorLoc, afterMyEndLoc)
        .highlight(knownType->second);
      inherited.setInvalidType(Context);
      continue;
    }
    inheritedTypes[inheritedCanTy] = inherited.getSourceRange();

    // If this is a protocol or protocol composition type, record the
    // protocols.
    if (inheritedTy->isExistentialType()) {
      SmallVector<ProtocolDecl *, 4> protocols;
      inheritedTy->isExistentialType(protocols);

      allProtocols.insert(protocols.begin(), protocols.end());
      continue;
    }
    
    // If this is an enum inheritance clause, check for a raw type.
    if (isa<EnumDecl>(decl)) {
      // Check if we already had a raw type.
      if (superclassTy) {
        diagnose(inherited.getSourceRange().Start,
                 diag::multiple_enum_raw_types, superclassTy, inheritedTy)
          .highlight(superclassRange);
        inherited.setInvalidType(Context);
        continue;
      }
      
      // If this is not the first entry in the inheritance clause, complain.
      if (i > 0) {
        SourceLoc afterPriorLoc
          = Lexer::getLocForEndOfToken(
              Context.SourceMgr,
              inheritedClause[i-1].getSourceRange().End);
        SourceLoc afterMyEndLoc
          = Lexer::getLocForEndOfToken(Context.SourceMgr,
                                       inherited.getSourceRange().End);

        diagnose(inherited.getSourceRange().Start,
                 diag::raw_type_not_first, inheritedTy)
          .fixItRemoveChars(afterPriorLoc, afterMyEndLoc)
          .fixItInsert(inheritedClause[0].getSourceRange().Start,
                       inheritedTy.getString() + ", ");

        // Fall through to record the raw type.
      }

      // Record the raw type.
      superclassTy = inheritedTy;
      superclassRange = inherited.getSourceRange();
      
      // Add the RawRepresentable conformance implied by the raw type.
      allProtocols.insert(getProtocol(decl->getLoc(),
                                      KnownProtocolKind::RawRepresentable));
      continue;
    }

    // If this is a class type, it may be the superclass.
    if (inheritedTy->getClassOrBoundGenericClass()) {
      // First, check if we already had a superclass.
      if (superclassTy) {
        // FIXME: Check for shadowed protocol names, i.e., NSObject?

        // Complain about multiple inheritance.
        // Don't emit a Fix-It here. The user has to think harder about this.
        diagnose(inherited.getSourceRange().Start,
                 diag::multiple_inheritance, superclassTy, inheritedTy)
          .highlight(superclassRange);
        inherited.setInvalidType(Context);
        continue;
      }

      // If the declaration we're looking at doesn't allow a superclass,
      // complain.
      if (!canInheritClass(decl)) {
        diagnose(decl->getLoc(),
                 isa<ExtensionDecl>(decl)
                   ? diag::extension_class_inheritance
                   : diag::non_class_inheritance,
                 isa<ExtensionDecl>(decl)
                   ? cast<ExtensionDecl>(decl)->getDeclaredInterfaceType()
                   : cast<TypeDecl>(decl)->getDeclaredInterfaceType(),
                 inheritedTy)
          .highlight(inherited.getSourceRange());
        inherited.setInvalidType(Context);
        continue;
      }

      // If this is not the first entry in the inheritance clause, complain.
      if (i > 0) {
        SourceLoc afterPriorLoc
          = Lexer::getLocForEndOfToken(
              Context.SourceMgr,
              inheritedClause[i-1].getSourceRange().End);
        SourceLoc afterMyEndLoc
          = Lexer::getLocForEndOfToken(Context.SourceMgr,
                                       inherited.getSourceRange().End);

        diagnose(inherited.getSourceRange().Start,
                 diag::superclass_not_first, inheritedTy)
          .fixItRemoveChars(afterPriorLoc, afterMyEndLoc)
          .fixItInsert(inheritedClause[0].getSourceRange().Start,
                       inheritedTy.getString() + ", ");

        // Fall through to record the superclass.
      }

      // Record the superclass.
      superclassTy = inheritedTy;
      superclassRange = inherited.getSourceRange();
      continue;
    }

    // We can't inherit from a non-class, non-protocol type.
    diagnose(decl->getLoc(),
             canInheritClass(decl)
               ? diag::inheritance_from_non_protocol_or_class
               : diag::inheritance_from_non_protocol,
             inheritedTy);
    // FIXME: Note pointing to the declaration 'inheritedTy' references?
    inherited.setInvalidType(Context);
  }

  if (auto proto = dyn_cast<ProtocolDecl>(decl)) {
    // FIXME: If we already set the inherited protocols, bail out. We'd rather
    // not have to check this.
    if (proto->isInheritedProtocolsValid())
      return;

    // Check for circular inheritance.
    // FIXME: The diagnostics here should be improved.
    bool diagnosedCircularity = false;
    for (unsigned i = 0, n = allProtocols.size(); i != n; /*in loop*/) {
      if (allProtocols[i] == proto || allProtocols[i]->inheritsFrom(proto)) {
        if (!diagnosedCircularity) {
          diagnose(proto, diag::circular_protocol_def, proto->getName().str());
          diagnosedCircularity = true;
        }

        allProtocols.remove(allProtocols[i]);
        --n;
        continue;
      }

      ++i;
    }

    proto->setInheritedProtocols(Context.AllocateCopy(allProtocols));
  }
  // Set the superclass.
  else if (auto classDecl = dyn_cast<ClassDecl>(decl)) {
    classDecl->setSuperclass(superclassTy);
  } else if (auto enumDecl = dyn_cast<EnumDecl>(decl)) {
    enumDecl->setRawType(superclassTy);
  } else {
    assert(!superclassTy || isa<AbstractTypeParamDecl>(decl));
  }
}

/// Retrieve the set of protocols the given protocol inherits.
static ArrayRef<ProtocolDecl *>
getInheritedForCycleCheck(TypeChecker &tc,
                          ProtocolDecl *proto,
                          ProtocolDecl **scratch) {
  return tc.getDirectConformsTo(proto);
}

/// Retrieve the superclass of the given class.
static ArrayRef<ClassDecl *> getInheritedForCycleCheck(TypeChecker &tc,
                                                       ClassDecl *classDecl,
                                                       ClassDecl **scratch) {
  tc.checkInheritanceClause(classDecl);

  if (classDecl->hasSuperclass()) {
    *scratch = classDecl->getSuperclass()->getClassOrBoundGenericClass();
    return *scratch;
  }
  return { };
}

/// Retrieve the raw type of the given enum.
static ArrayRef<EnumDecl *> getInheritedForCycleCheck(TypeChecker &tc,
                                                      EnumDecl *enumDecl,
                                                      EnumDecl **scratch) {
  tc.checkInheritanceClause(enumDecl);
  
  if (enumDecl->hasRawType()) {
    *scratch = enumDecl->getRawType()->getEnumOrBoundGenericEnum();
    return *scratch ? ArrayRef<EnumDecl*>(*scratch) : ArrayRef<EnumDecl*>{};
  }
  return { };
}

// Break the inheritance cycle for a protocol by removing all inherited
// protocols.
//
// FIXME: Just remove the problematic inheritance?
static void breakInheritanceCycle(ProtocolDecl *proto) {
  proto->clearInheritedProtocols();
}

/// Break the inheritance cycle for a class by removing its superclass.
static void breakInheritanceCycle(ClassDecl *classDecl) {
  classDecl->setSuperclass(Type());
}

/// Break the inheritance cycle for an enum by removing its raw type.
static void breakInheritanceCycle(EnumDecl *enumDecl) {
  enumDecl->setRawType(Type());
}

/// Check for circular inheritance.
template<typename T>
static void checkCircularity(TypeChecker &tc, T *decl,
                             Diag<StringRef> circularDiag,
                             Diag<Identifier> declHereDiag,
                             SmallVectorImpl<T *> &path) {
  switch (decl->getCircularityCheck()) {
  case CircularityCheck::Checked:
    return;

  case CircularityCheck::Checking: {
    // We're already checking this type, which means we have a cycle.

    // The beginning of the path might not be part of the cycle, so find
    // where the cycle starts.
    assert(!path.empty());

    auto cycleStart = path.end() - 1;
    while (*cycleStart != decl) {
      assert(cycleStart != path.begin() && "Missing cycle start?");
      --cycleStart;
    }

    // If the path length is 1 the type directly references itself.
    if (path.end() - cycleStart == 1) {
      tc.diagnose(path.back()->getLoc(),
                  circularDiag,
                  path.back()->getName().str());

      decl->setInvalid();
      decl->setInterfaceType(ErrorType::get(tc.Context));
      breakInheritanceCycle(decl);
      break;
    }

    // Form the textual path illustrating the cycle.
    llvm::SmallString<128> pathStr;
    for (auto i = cycleStart, iEnd = path.end(); i != iEnd; ++i) {
      if (!pathStr.empty())
        pathStr += " -> ";
      pathStr += ("'" + (*i)->getName().str() + "'").str();
    }
    pathStr += (" -> '" + decl->getName().str() + "'").str();

    // Diagnose the cycle.
    tc.diagnose(decl->getLoc(), circularDiag, pathStr);
    for (auto i = cycleStart + 1, iEnd = path.end(); i != iEnd; ++i) {
      tc.diagnose(*i, declHereDiag, (*i)->getName());
    }

    // Set this declaration as invalid, then break the cycle somehow.
    decl->setInvalid();
    decl->setInterfaceType(ErrorType::get(tc.Context));
    breakInheritanceCycle(decl);
    break;
  }

  case CircularityCheck::Unchecked: {
    // Walk to the inherited class or protocols.
    path.push_back(decl);
    decl->setCircularityCheck(CircularityCheck::Checking);
    T *scratch = nullptr;
    for (auto inherited : getInheritedForCycleCheck(tc, decl, &scratch)) {
      checkCircularity(tc, inherited, circularDiag, declHereDiag, path);
    }
    decl->setCircularityCheck(CircularityCheck::Checked);
    path.pop_back();
    break;
  }
  }
}

/// Set each bound variable in the pattern to have an error type.
static void setBoundVarsTypeError(Pattern *pattern, ASTContext &ctx) {
  pattern->forEachVariable([&](VarDecl *var) {
    // Don't change the type of a variable that we've been able to
    // compute a type for.
    if (var->hasType() && !var->getType()->hasError())
      return;

    var->markInvalid();
  });
}

/// Create a fresh archetype builder.
ArchetypeBuilder TypeChecker::createArchetypeBuilder(ModuleDecl *mod) {
  return ArchetypeBuilder(Context, LookUpConformanceInModule(mod));
}

/// Expose TypeChecker's handling of GenericParamList to SIL parsing.
GenericEnvironment *
TypeChecker::handleSILGenericParams(GenericParamList *genericParams,
                                    DeclContext *DC) {

  SmallVector<GenericParamList *, 2> nestedList;
  for (; genericParams; genericParams = genericParams->getOuterParameters()) {
    nestedList.push_back(genericParams);
  }

  // Since the innermost GenericParamList is in the beginning of the vector,
  // we process in reverse order to handle the outermost list first.
  GenericSignature *parentSig = nullptr;
  GenericEnvironment *parentEnv = nullptr;

  for (unsigned i = 0, e = nestedList.size(); i < e; i++) {
    auto genericParams = nestedList.rbegin()[i];
    prepareGenericParamList(genericParams, DC);

    parentEnv = checkGenericEnvironment(genericParams, DC, parentSig,
                                        /*allowConcreteGenericParams=*/true);
    parentSig = parentEnv->getGenericSignature();

    // Compute the final set of archetypes.
    revertGenericParamList(genericParams);
    GenericTypeToArchetypeResolver archetypeResolver(parentEnv);
    checkGenericParamList(nullptr, genericParams, parentSig,
                          &archetypeResolver);
  }

  return parentEnv;
}

/// Check whether the given type representation will be
/// default-initializable.
static bool isDefaultInitializable(TypeRepr *typeRepr) {
  // Look through most attributes.
  if (auto attributed = dyn_cast<AttributedTypeRepr>(typeRepr)) {
    // Weak ownership implies optionality.
    if (attributed->getAttrs().getOwnership() == Ownership::Weak)
      return true;
    
    return isDefaultInitializable(attributed->getTypeRepr());
  }

  // Optional types are default-initializable.
  if (isa<OptionalTypeRepr>(typeRepr) ||
      isa<ImplicitlyUnwrappedOptionalTypeRepr>(typeRepr))
    return true;

  // Tuple types are default-initializable if all of their element
  // types are.
  if (auto tuple = dyn_cast<TupleTypeRepr>(typeRepr)) {
    // ... but not variadic ones.
    if (tuple->hasEllipsis())
      return false;

    for (auto elt : tuple->getElements()) {
      if (!isDefaultInitializable(elt))
        return false;
    }

    return true;
  }

  // Not default initializable.
  return false;
}

// @NSManaged properties never get default initialized, nor do debugger
// variables and immutable properties.
static bool isNeverDefaultInitializable(Pattern *p) {
  bool result = false;

  p->forEachVariable([&](VarDecl *var) {
    if (var->getAttrs().hasAttribute<NSManagedAttr>())
      return;

    if (var->isDebuggerVar() ||
        var->isLet())
      result = true;
  });

  return result;
}

/// Determine whether the given pattern binding declaration either has
/// an initializer expression, or is default initialized, without performing
/// any type checking on it.
static bool isDefaultInitializable(PatternBindingDecl *pbd) {
  assert(pbd->hasStorage());

  for (auto entry : pbd->getPatternList()) {
    // If it has an initializer expression, this is trivially true.
    if (entry.getInit())
      continue;

    if (isNeverDefaultInitializable(entry.getPattern()))
      return false;

    // If the pattern is typed as optional (or tuples thereof), it is
    // default initializable.
    if (auto typedPattern = dyn_cast<TypedPattern>(entry.getPattern())) {
      if (auto typeRepr = typedPattern->getTypeLoc().getTypeRepr())
        if (isDefaultInitializable(typeRepr))
          continue;
    }

    // Otherwise, we can't default initialize this binding.
    return false;
  }
  
  return true;
}

/// Build a default initializer for the given type.
static Expr *buildDefaultInitializer(TypeChecker &tc, Type type) {
  // Default-initialize optional types and weak values to 'nil'.
  if (type->getReferenceStorageReferent()->getAnyOptionalObjectType())
    return new (tc.Context) NilLiteralExpr(SourceLoc(), /*Implicit=*/true);

  // Build tuple literals for tuple types.
  if (auto tupleType = type->getAs<TupleType>()) {
    SmallVector<Expr *, 2> inits;
    for (const auto &elt : tupleType->getElements()) {
      if (elt.isVararg())
        return nullptr;

      auto eltInit = buildDefaultInitializer(tc, elt.getType());
      if (!eltInit)
        return nullptr;

      inits.push_back(eltInit);
    }

    return TupleExpr::createImplicit(tc.Context, inits, { });
  }

  // We don't default-initialize anything else.
  return nullptr;
}

/// Check whether \c current is a redeclaration.
static void checkRedeclaration(TypeChecker &tc, ValueDecl *current) {
  // If we've already checked this declaration, don't do it again.
  if (current->alreadyCheckedRedeclaration())
    return;

  // If there's no type yet, come back to it later.
  if (!current->hasInterfaceType())
    return;
  
  // Make sure we don't do this checking again.
  current->setCheckedRedeclaration(true);

  // Ignore invalid and anonymous declarations.
  if (current->isInvalid() || !current->hasName())
    return;

  // If this declaration isn't from a source file, don't check it.
  // FIXME: Should restrict this to the source file we care about.
  DeclContext *currentDC = current->getDeclContext();
  SourceFile *currentFile = currentDC->getParentSourceFile();
  if (!currentFile || currentDC->isLocalContext())
    return;

  ReferencedNameTracker *tracker = currentFile->getReferencedNameTracker();
  bool isCascading = true;
  if (current->hasAccessibility())
    isCascading = (current->getFormalAccess() > Accessibility::FilePrivate);

  // Find other potential definitions.
  SmallVector<ValueDecl *, 4> otherDefinitionsVec;
  ArrayRef<ValueDecl *> otherDefinitions;
  if (currentDC->isTypeContext()) {
    // Look within a type context.
    if (auto nominal = currentDC->getAsNominalTypeOrNominalTypeExtensionContext()) {
      otherDefinitions = nominal->lookupDirect(current->getBaseName());
      if (tracker)
        tracker->addUsedMember({nominal, current->getName()}, isCascading);
    }
  } else {
    // Look within a module context.
    currentFile->getParentModule()->lookupValue({ }, current->getBaseName(),
                                                NLKind::QualifiedLookup,
                                                otherDefinitionsVec);
    otherDefinitions = otherDefinitionsVec;
    if (tracker)
      tracker->addTopLevelName(current->getName(), isCascading);
  }

  // Compare this signature against the signature of other
  // declarations with the same name.
  OverloadSignature currentSig = current->getOverloadSignature();
  ModuleDecl *currentModule = current->getModuleContext();
  for (auto other : otherDefinitions) {
    // Skip invalid declarations and ourselves.
    if (current == other || other->isInvalid())
      continue;

    // Skip declarations in other modules.
    if (currentModule != other->getModuleContext())
      continue;

    // Don't compare methods vs. non-methods (which only happens with
    // operators).
    if (currentDC->isTypeContext() != other->getDeclContext()->isTypeContext())
      continue;

    // Validate the declaration.
    tc.validateDecl(other);
    if (other->isInvalid() || !other->hasInterfaceType())
      continue;

    // Skip declarations in other files.
    // In practice, this means we will warn on a private declaration that
    // shadows a non-private one, but only in the file where the shadowing
    // happens. We will warn on conflicting non-private declarations in both
    // files.
    if (!other->isAccessibleFrom(currentDC))
      continue;

    const auto markInvalid = [&current, &tc]() {
      current->setInvalid();
      if (auto *varDecl = dyn_cast<VarDecl>(current))
        if (varDecl->hasType())
          varDecl->setType(ErrorType::get(tc.Context));
      if (current->hasInterfaceType())
        current->setInterfaceType(ErrorType::get(tc.Context));
    };

    // Thwart attempts to override the same declaration more than once.
    const auto *currentOverride = current->getOverriddenDecl();
    const auto *otherOverride = other->getOverriddenDecl();
    if (currentOverride && currentOverride == otherOverride) {
      tc.diagnose(current, diag::multiple_override, current->getFullName());
      tc.diagnose(other, diag::multiple_override_prev, other->getFullName());
      markInvalid();
      break;
    }

    // If there is another conflict, complain.
    if (conflicting(currentSig, other->getOverloadSignature())) {
      // If the two declarations occur in the same source file, make sure
      // we get the diagnostic ordering to be sensible.
      if (auto otherFile = other->getDeclContext()->getParentSourceFile()) {
        if (currentFile == otherFile &&
            current->getLoc().isValid() &&
            other->getLoc().isValid() &&
            tc.Context.SourceMgr.isBeforeInBuffer(current->getLoc(),
                                                  other->getLoc())) {
          std::swap(current, other);
        }
      }

      // If we're currently looking at a .sil and the conflicting declaration
      // comes from a .sib, don't error since we won't be considering the sil
      // from the .sib. So it's fine for the .sil to shadow it, since that's the
      // one we want.
      if (currentFile->Kind == SourceFileKind::SIL) {
        auto *otherFile = dyn_cast<SerializedASTFile>(
            other->getDeclContext()->getModuleScopeContext());
        if (otherFile && otherFile->isSIB())
          continue;
      }

      tc.diagnose(current, diag::invalid_redecl, current->getFullName());
      tc.diagnose(other, diag::invalid_redecl_prev, other->getFullName());
      markInvalid();
      break;
    }
  }
}

/// Does the context allow pattern bindings that don't bind any variables?
static bool contextAllowsPatternBindingWithoutVariables(DeclContext *dc) {
  
  // Property decls in type context must bind variables.
  if (dc->isTypeContext())
    return false;
  
  // Global variable decls must bind variables, except in scripts.
  if (dc->isModuleScopeContext()) {
    if (dc->getParentSourceFile()
        && dc->getParentSourceFile()->isScriptMode())
      return true;
    
    return false;
  }
  
  return true;
}

/// Validate the given pattern binding declaration.
static void validatePatternBindingDecl(TypeChecker &tc,
                                       PatternBindingDecl *binding,
                                       unsigned entryNumber) {
  // If the pattern already has a type, we're done.
  if (binding->getPattern(entryNumber)->hasType() ||
      binding->isBeingValidated())
    return;

  binding->setIsBeingValidated();

  // On any path out of this function, make sure to mark the binding as done
  // being type checked.
  SWIFT_DEFER { binding->setIsBeingValidated(false); };

  // Resolve the pattern.
  auto *pattern = tc.resolvePattern(binding->getPattern(entryNumber),
                                    binding->getDeclContext(),
                                    /*isStmtCondition*/true);
  if (!pattern) {
    binding->setInvalid();
    binding->getPattern(entryNumber)->setType(ErrorType::get(tc.Context));
    return;
  }

  binding->setPattern(entryNumber, pattern,
                      binding->getPatternList()[entryNumber].getInitContext());

  // Validate 'static'/'class' on properties in nominal type decls.
  auto StaticSpelling = binding->getStaticSpelling();
  if (StaticSpelling != StaticSpellingKind::None &&
      binding->getDeclContext()->isExtensionContext()) {
    if (auto *NTD = binding->getDeclContext()
        ->getAsNominalTypeOrNominalTypeExtensionContext()) {
      if (!isa<ClassDecl>(NTD)) {
        if (StaticSpelling == StaticSpellingKind::KeywordClass) {
          tc.diagnose(binding, diag::class_var_not_in_class)
            .fixItReplace(binding->getStaticLoc(), "static");
          tc.diagnose(NTD, diag::extended_type_declared_here);
        }
      }
    }
  }

  // Check the pattern. We treat type-checking a PatternBindingDecl like
  // type-checking an expression because that's how the initial binding is
  // checked, and they have the same effect on the file's dependencies.
  //
  // In particular, it's /not/ correct to check the PBD's DeclContext because
  // top-level variables in a script file are accessible from other files,
  // even though the PBD is inside a TopLevelCodeDecl.
  TypeResolutionOptions options = TR_InExpression;
  if (binding->getInit(entryNumber)) {
    // If we have an initializer, we can also have unknown types.
    options |= TR_AllowUnspecifiedTypes;
    options |= TR_AllowUnboundGenerics;
  }
  if (tc.typeCheckPattern(pattern, binding->getDeclContext(), options)) {
    setBoundVarsTypeError(pattern, tc.Context);
    binding->setInvalid();
    pattern->setType(ErrorType::get(tc.Context));
    return;
  }

  // If the pattern didn't get a type or if it contains an unbound generic type,
  // we'll need to check the initializer.
  if (!pattern->hasType() || pattern->getType()->hasUnboundGenericType()) {
    bool skipClosures = false;
    if (auto var = binding->getSingleVar())
      skipClosures = var->getAttrs().hasAttribute<LazyAttr>();

    if (tc.typeCheckPatternBinding(binding, entryNumber, skipClosures))
      return;
  }

  // If the pattern binding appears in a type or library file context, then
  // it must bind at least one variable.
  if (!contextAllowsPatternBindingWithoutVariables(binding->getDeclContext())) {
    llvm::SmallVector<VarDecl*, 2> vars;
    binding->getPattern(entryNumber)->collectVariables(vars);
    if (vars.empty()) {
      // Selector for error message.
      enum : unsigned {
        Property,
        GlobalVariable,
      };
      tc.diagnose(binding->getPattern(entryNumber)->getLoc(),
                  diag::pattern_binds_no_variables,
                  binding->getDeclContext()->isTypeContext()
                                                   ? Property : GlobalVariable);
    }
  }

  // If we have any type-adjusting attributes, apply them here.
  if (binding->getPattern(entryNumber)->hasType())
    if (auto var = binding->getSingleVar())
      tc.checkTypeModifyingDeclAttributes(var);
}

void swift::makeFinal(ASTContext &ctx, ValueDecl *D) {
  if (D && !D->isFinal()) {
    D->getAttrs().add(new (ctx) FinalAttr(/*IsImplicit=*/true));
  }
}

void swift::makeDynamic(ASTContext &ctx, ValueDecl *D) {
  if (D && !D->isDynamic()) {
    D->getAttrs().add(new (ctx) DynamicAttr(/*IsImplicit=*/true));
  }
}

/// Configure the implicit 'self' parameter of a function, setting its type,
/// pattern, etc.
///
/// \param func The function whose 'self' is being configured.
static void configureImplicitSelf(TypeChecker &tc,
                                  AbstractFunctionDecl *func) {
  auto selfDecl = func->getImplicitSelfDecl();

  // Compute the type of self.
  Type selfIfaceTy = func->computeInterfaceSelfType(/*isInitializingCtor*/true);
  assert(selfDecl && selfIfaceTy && "Not a method");

  // 'self' is 'let' for reference types (i.e., classes) or when 'self' is
  // neither inout.
  selfDecl->setLet(!selfIfaceTy->is<InOutType>());

  selfDecl->setInterfaceType(selfIfaceTy);
}

/// Record the context type of 'self' after the generic environment of
/// the function has been determined.
static void recordSelfContextType(AbstractFunctionDecl *func) {
  auto selfDecl = func->getImplicitSelfDecl();
  Type selfTy = func->computeInterfaceSelfType(/*isInitializingCtor*/true,
                                               /*wantDynamicSelf*/true);
  selfTy = func->mapTypeIntoContext(selfTy);
  selfDecl->setType(selfTy);
}

namespace {

class AccessScopeChecker {
  const SourceFile *File;
  TypeChecker::TypeAccessScopeCacheMap &Cache;

protected:
  ASTContext &Context;
  Optional<AccessScope> Scope = AccessScope::getPublic();

  AccessScopeChecker(const DeclContext *useDC,
                     decltype(TypeChecker::TypeAccessScopeCache) &caches)
      : File(useDC->getParentSourceFile()),
        Cache(caches[File]),
        Context(File->getASTContext()) {}

  bool visitDecl(ValueDecl *VD) {
    if (!VD || isa<GenericTypeParamDecl>(VD))
      return true;

    // FIXME: Figure out why AssociatedTypeDecls don't always have
    // accessibility here.
    if (!VD->hasAccessibility()) {
      if (isa<AssociatedTypeDecl>(VD))
        return true;
    }

    auto cached = Cache.find(VD);
    if (cached != Cache.end()) {
      Scope = Scope->intersectWith(cached->second);
      return Scope.hasValue();
    }

    auto AS = VD->getFormalAccessScope(File);
    auto result = Cache.insert(std::make_pair(VD, AS));
    assert(result.second);
    (void) result;

    Scope = Scope->intersectWith(AS);
    return Scope.hasValue();
  }
};

class TypeReprAccessScopeChecker : private ASTWalker, AccessScopeChecker {
  TypeReprAccessScopeChecker(const DeclContext *useDC,
                             decltype(TypeChecker::TypeAccessScopeCache) &caches)
      : AccessScopeChecker(useDC, caches) {
  }

  bool walkToTypeReprPre(TypeRepr *TR) override {
    if (auto CITR = dyn_cast<ComponentIdentTypeRepr>(TR))
      return visitDecl(CITR->getBoundDecl());
    return true;
  }

  bool walkToTypeReprPost(TypeRepr *TR) override {
    return Scope.hasValue();
  }

public:
  static Optional<AccessScope>
  getAccessScope(TypeRepr *TR, const DeclContext *useDC,
                 decltype(TypeChecker::TypeAccessScopeCache) &caches) {
    TypeReprAccessScopeChecker checker(useDC, caches);
    TR->walk(checker);
    return checker.Scope;
  }
};

class TypeAccessScopeChecker : private TypeWalker, AccessScopeChecker {
  bool CanonicalizeParentTypes;

  TypeAccessScopeChecker(const DeclContext *useDC,
                         decltype(TypeChecker::TypeAccessScopeCache) &caches,
                         bool canonicalizeParentTypes)
      : AccessScopeChecker(useDC, caches),
        CanonicalizeParentTypes(canonicalizeParentTypes) {}

  Action walkToTypePre(Type T) override {
    ValueDecl *VD;
    if (auto *TAD = dyn_cast<NameAliasType>(T.getPointer()))
      VD = TAD->getDecl();
    else if (auto *NTD = T->getAnyNominal())
      VD = NTD;
    else
      VD = nullptr;

    if (!visitDecl(VD))
      return Action::Stop;

    if (!CanonicalizeParentTypes)
      return Action::Continue;

    Type nominalParentTy;
    if (auto nominalTy = dyn_cast<NominalType>(T.getPointer())) {
      nominalParentTy = nominalTy->getParent();
    } else if (auto genericTy = dyn_cast<BoundGenericType>(T.getPointer())) {
      nominalParentTy = genericTy->getParent();
      for (auto genericArg : genericTy->getGenericArgs())
        genericArg.walk(*this);
    } else {
      return Action::Continue;
    }

    if (nominalParentTy)
      nominalParentTy->getCanonicalType().walk(*this);
    return Action::SkipChildren;
  }

public:
  static Optional<AccessScope>
  getAccessScope(Type T, const DeclContext *useDC,
                 decltype(TypeChecker::TypeAccessScopeCache) &caches,
                 bool canonicalizeParentTypes = false) {
    TypeAccessScopeChecker checker(useDC, caches, canonicalizeParentTypes);
    T.walk(checker);
    return checker.Scope;
  }
};

} // end anonymous namespace


void TypeChecker::computeDefaultAccessibility(ExtensionDecl *ED) {
  if (ED->hasDefaultAccessibility())
    return;

  validateExtension(ED);

  if (ED->hasDefaultAccessibility())
    return;

  Accessibility maxAccess = Accessibility::Public;

  if (!ED->getExtendedType().isNull() &&
      !ED->getExtendedType()->hasError()) {
    if (NominalTypeDecl *nominal = ED->getExtendedType()->getAnyNominal()) {
      validateDecl(nominal);
      maxAccess = std::max(nominal->getFormalAccess(),
                           Accessibility::FilePrivate);
    }
  }

  if (const GenericParamList *genericParams = ED->getGenericParams()) {
    auto getTypeAccess = [this, ED](const TypeLoc &TL) -> Accessibility {
      if (!TL.getType())
        return Accessibility::Public;
      auto accessScope =
          TypeReprAccessScopeChecker::getAccessScope(TL.getTypeRepr(),
                                                     ED->getDeclContext(),
                                                     TypeAccessScopeCache);
      // This is an error case and will be diagnosed elsewhere.
      if (!accessScope.hasValue())
        return Accessibility::Public;

      if (accessScope->isPublic())
        return Accessibility::Public;
      if (isa<ModuleDecl>(accessScope->getDeclContext()))
        return Accessibility::Internal;
      // Because extensions are always at top-level, they should never
      // reference declarations not at the top level. (And any such references
      // should be diagnosed elsewhere.) This code should not crash if that
      // occurs, though.
      return Accessibility::FilePrivate;
    };

    // Only check the trailing 'where' requirements. Other requirements come
    // from the extended type and have already been checked.
    for (const RequirementRepr &req : genericParams->getTrailingRequirements()){
      switch (req.getKind()) {
      case RequirementReprKind::TypeConstraint:
        maxAccess = std::min(getTypeAccess(req.getSubjectLoc()), maxAccess);
        maxAccess = std::min(getTypeAccess(req.getConstraintLoc()), maxAccess);
        break;
      case RequirementReprKind::LayoutConstraint:
        maxAccess = std::min(getTypeAccess(req.getSubjectLoc()), maxAccess);
        break;
      case RequirementReprKind::SameType:
        maxAccess = std::min(getTypeAccess(req.getFirstTypeLoc()), maxAccess);
        maxAccess = std::min(getTypeAccess(req.getSecondTypeLoc()), maxAccess);
        break;
      }
    }
  }

  Accessibility defaultAccess;
  if (auto *AA = ED->getAttrs().getAttribute<AccessibilityAttr>())
    defaultAccess = std::max(AA->getAccess(), Accessibility::FilePrivate);
  else
    defaultAccess = Accessibility::Internal;

  // Don't set the max or default accessibility to 'open'.  This should
  // be diagnosed as invalid anyway.
  defaultAccess = std::min(defaultAccess, Accessibility::Public);
  maxAccess = std::min(maxAccess, Accessibility::Public);

  // Normally putting a public member in an internal extension is harmless,
  // because that member can never be used elsewhere. But if some of the types
  // in the signature are public, it could actually end up getting picked in
  // overload resolution. Therefore, we only enforce the maximum access if the
  // extension has a 'where' clause.
  if (ED->getTrailingWhereClause())
    defaultAccess = std::min(defaultAccess, maxAccess);
  else
    maxAccess = Accessibility::Public;

  ED->setDefaultAndMaxAccessibility(defaultAccess, maxAccess);
}

void TypeChecker::computeAccessibility(ValueDecl *D) {
  if (D->hasAccessibility())
    return;

  // Check if the decl has an explicit accessibility attribute.
  if (auto *AA = D->getAttrs().getAttribute<AccessibilityAttr>()) {
    D->setAccessibility(AA->getAccess());

  } else if (auto fn = dyn_cast<FuncDecl>(D)) {
    // Special case for accessors, which inherit the access of their storage.
    // decl. A setter attribute can also override this.
    if (AbstractStorageDecl *storage = fn->getAccessorStorageDecl()) {
      if (storage->hasAccessibility()) {
        if (fn->getAccessorKind() == AccessorKind::IsSetter ||
            fn->getAccessorKind() == AccessorKind::IsMaterializeForSet)
          fn->setAccessibility(storage->getSetterAccessibility());
        else
          fn->setAccessibility(storage->getFormalAccess());
      } else {
        computeAccessibility(storage);
      }
    }
  }

  if (!D->hasAccessibility()) {
    DeclContext *DC = D->getDeclContext();
    switch (DC->getContextKind()) {
    case DeclContextKind::TopLevelCodeDecl:
      // Variables declared in a top-level 'guard' statement can be accessed in
      // later top-level code.
      D->setAccessibility(Accessibility::FilePrivate);
      break;
    case DeclContextKind::AbstractClosureExpr:
      if (isa<ParamDecl>(D)) {
        // Closure parameters may need to be accessible to the enclosing
        // context, for single-expression closures.
        D->setAccessibility(Accessibility::FilePrivate);
      } else {
        D->setAccessibility(Accessibility::Private);
      }
      break;
    case DeclContextKind::SerializedLocal:
    case DeclContextKind::Initializer:
    case DeclContextKind::AbstractFunctionDecl:
    case DeclContextKind::SubscriptDecl:
      D->setAccessibility(Accessibility::Private);
      break;
    case DeclContextKind::Module:
    case DeclContextKind::FileUnit:
      D->setAccessibility(Accessibility::Internal);
      break;
    case DeclContextKind::GenericTypeDecl: {
      auto generic = cast<GenericTypeDecl>(DC);
      validateAccessibility(generic);
      Accessibility access = Accessibility::Internal;
      if (isa<ProtocolDecl>(generic))
        access = std::max(Accessibility::FilePrivate,
                          generic->getFormalAccess());
      D->setAccessibility(access);
      break;
    }
    case DeclContextKind::ExtensionDecl: {
      auto extension = cast<ExtensionDecl>(DC);
      computeDefaultAccessibility(extension);
      if (!D->hasAccessibility()) {
        auto access = extension->getDefaultAccessibility();
        D->setAccessibility(access);
      }
    }
    }
  }

  if (auto ASD = dyn_cast<AbstractStorageDecl>(D)) {
    if (auto *AA = D->getAttrs().getAttribute<SetterAccessibilityAttr>())
      ASD->setSetterAccessibility(AA->getAccess());
    else
      ASD->setSetterAccessibility(ASD->getFormalAccess());

    if (auto getter = ASD->getGetter())
      computeAccessibility(getter);
    if (auto setter = ASD->getSetter())
      computeAccessibility(setter);
  }
}

namespace {

class TypeAccessScopeDiagnoser : private ASTWalker {
  AccessScope accessScope;
  const DeclContext *useDC;
  const ComponentIdentTypeRepr *offendingType = nullptr;

  bool walkToTypeReprPre(TypeRepr *TR) override {
    // Exit early if we've already found a problem type.
    if (offendingType)
      return false;

    auto CITR = dyn_cast<ComponentIdentTypeRepr>(TR);
    if (!CITR)
      return true;

    const ValueDecl *VD = CITR->getBoundDecl();
    if (!VD)
      return true;

    if (VD->getFormalAccessScope(useDC) != accessScope)
      return true;

    offendingType = CITR;
    return false;
  }

  bool walkToTypeReprPost(TypeRepr *T) override {
    // Exit early if we've already found a problem type.
    return offendingType != nullptr;
  }

  explicit TypeAccessScopeDiagnoser(AccessScope accessScope,
                                    const DeclContext *useDC)
    : accessScope(accessScope), useDC(useDC) {}

public:
  static const TypeRepr *findTypeWithScope(TypeRepr *TR,
                                           AccessScope accessScope,
                                           const DeclContext *useDC) {
    assert(!accessScope.isPublic() &&
           "why would we need to find a public access scope?");
    if (TR == nullptr)
      return nullptr;
    TypeAccessScopeDiagnoser diagnoser(accessScope, useDC);
    TR->walk(diagnoser);
    return diagnoser.offendingType;
  }
};

/// A uniquely-typed boolean to reduce the chances of accidentally inverting
/// a check.
///
/// \see checkTypeAccessibility
enum class DowngradeToWarning: bool {
  No,
  Yes
};

/// \see checkTypeAccessibility
using CheckTypeAccessCallback =
    void(AccessScope, const TypeRepr *, DowngradeToWarning);

} // end anonymous namespace

/// Checks if the access scope of the type described by \p TL contains
/// \p contextAccessScope. If it isn't, calls \p diagnose with a TypeRepr
/// representing the offending part of \p TL.
///
/// If \p contextAccessScope is null, checks that \p TL is only made up of
/// public types.
///
/// The TypeRepr passed to \p diagnose may be null, in which case a particular
/// part of the type that caused the problem could not be found. The DeclContext
/// is never null.
static void checkTypeAccessibilityImpl(
    TypeChecker &TC, TypeLoc TL, AccessScope contextAccessScope,
    const DeclContext *useDC,
    llvm::function_ref<CheckTypeAccessCallback> diagnose) {
  if (!TC.getLangOpts().EnableAccessControl)
    return;
  if (!TL.getType())
    return;
  // Don't spend time checking local declarations; this is always valid by the
  // time we get to this point.
  if (!contextAccessScope.isPublic() &&
      contextAccessScope.getDeclContext()->isLocalContext())
    return;

  // TypeRepr checking is more accurate, but we must also look at TypeLocs
  // without a TypeRepr, for example for 'var' declarations with an inferred
  // type.
  auto typeAccessScope =
    (TL.getTypeRepr()
     ? TypeReprAccessScopeChecker::getAccessScope(TL.getTypeRepr(), useDC,
                                                  TC.TypeAccessScopeCache)
     : TypeAccessScopeChecker::getAccessScope(TL.getType(), useDC,
                                              TC.TypeAccessScopeCache));

  // Note: This means that the type itself is invalid for this particular
  // context, because it references declarations from two incompatible scopes.
  // In this case we should have diagnosed the bad reference already.
  if (!typeAccessScope.hasValue())
    return;

  auto shouldComplainAboutAccessScope =
      [contextAccessScope](AccessScope scope) -> bool {
    if (scope.isPublic())
      return false;
    if (scope.hasEqualDeclContextWith(contextAccessScope))
      return false;
    if (contextAccessScope.isChildOf(scope))
      return false;
    return true;
  };

  if (!shouldComplainAboutAccessScope(typeAccessScope.getValue()))
    return;

  // Swift 3.0 wasn't nearly as strict as checking types because it didn't
  // look at the TypeRepr at all except to highlight a particular part of the
  // type in diagnostics, and looked through typealiases in other cases.
  // Approximate this behavior by running our non-TypeRepr-based check again
  // and downgrading to a warning when the checks disagree.
  auto downgradeToWarning = DowngradeToWarning::No;
  if (TC.getLangOpts().isSwiftVersion3()) {
    auto typeOnlyAccessScope =
       TypeAccessScopeChecker::getAccessScope(TL.getType(), useDC,
                                              TC.TypeAccessScopeCache,
                                              /*canonicalizeParents*/true);
    if (typeOnlyAccessScope.hasValue()) {
      // If Swift 4 would have complained about a private type, but Swift 4
      // would only diagnose an internal type, complain about the Swift 3
      // offense first to avoid confusing users.
      if (shouldComplainAboutAccessScope(typeOnlyAccessScope.getValue()))
        typeAccessScope = typeOnlyAccessScope;
      else
        downgradeToWarning = DowngradeToWarning::Yes;
    }
  }

  const TypeRepr *complainRepr =
        TypeAccessScopeDiagnoser::findTypeWithScope(
            TL.getTypeRepr(),
            *typeAccessScope,
            useDC);
  diagnose(*typeAccessScope, complainRepr, downgradeToWarning);
}

/// Checks if the access scope of the type described by \p TL is valid for the
/// type to be the type of \p context. If it isn't, calls \p diagnose with a
/// TypeRepr representing the offending part of \p TL.
///
/// The TypeRepr passed to \p diagnose may be null, in which case a particular
/// part of the type that caused the problem could not be found. The DeclContext
/// is never null. The DowngradeToWarning parameter is a hack to deal with
/// early versions of Swift 3 not diagnosing certain access violations.
static void checkTypeAccessibility(
    TypeChecker &TC, TypeLoc TL, const ValueDecl *context,
    llvm::function_ref<CheckTypeAccessCallback> diagnose) {
  const DeclContext *DC = context->getDeclContext();
  if (isa<ParamDecl>(context)) {
    context = dyn_cast<AbstractFunctionDecl>(DC);
    if (!context)
      context = cast<SubscriptDecl>(DC);
    DC = context->getDeclContext();
  }

  AccessScope contextAccessScope = context->getFormalAccessScope();
  checkTypeAccessibilityImpl(TC, TL, contextAccessScope, DC,
                             [=](AccessScope requiredAccessScope,
                                 const TypeRepr *offendingTR,
                                 DowngradeToWarning downgradeToWarning) {
    if (!contextAccessScope.isPublic() &&
        !isa<ModuleDecl>(contextAccessScope.getDeclContext())) {
      // Swift 3.0.0 mistakenly didn't diagnose any issues when the context
      // access scope represented a private or fileprivate level.
      downgradeToWarning = DowngradeToWarning::Yes;
    }
    diagnose(requiredAccessScope, offendingTR, downgradeToWarning);
  });
}

/// Highlights the given TypeRepr, and adds a note pointing to the type's
/// declaration if possible.
///
/// Just flushes \p diag as is if \p complainRepr is null.
static void highlightOffendingType(TypeChecker &TC, InFlightDiagnostic &diag,
                                   const TypeRepr *complainRepr) {
  if (!complainRepr) {
    diag.flush();
    return;
  }

  diag.highlight(complainRepr->getSourceRange());
  diag.flush();

  if (auto CITR = dyn_cast<ComponentIdentTypeRepr>(complainRepr)) {
    const ValueDecl *VD = CITR->getBoundDecl();
    TC.diagnose(VD, diag::type_declared_here);
  }
}

static void checkGenericParamAccessibility(TypeChecker &TC,
                                           const GenericParamList *params,
                                           const Decl *owner,
                                           AccessScope accessScope,
                                           Accessibility contextAccess) {
  if (!params)
    return;

  // This must stay in sync with diag::generic_param_access.
  enum {
    AEK_Parameter = 0,
    AEK_Requirement
  } accessibilityErrorKind;
  auto minAccessScope = AccessScope::getPublic();
  const TypeRepr *complainRepr = nullptr;
  auto downgradeToWarning = DowngradeToWarning::Yes;

  for (auto param : *params) {
    if (param->getInherited().empty())
      continue;
    assert(param->getInherited().size() == 1);
    checkTypeAccessibilityImpl(TC, param->getInherited().front(), accessScope,
                               owner->getDeclContext(),
                               [&](AccessScope typeAccessScope,
                                   const TypeRepr *thisComplainRepr,
                                   DowngradeToWarning thisDowngrade) {
      if (typeAccessScope.isChildOf(minAccessScope) ||
          (thisDowngrade == DowngradeToWarning::No &&
           downgradeToWarning == DowngradeToWarning::Yes) ||
          (!complainRepr &&
           typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
        minAccessScope = typeAccessScope;
        complainRepr = thisComplainRepr;
        accessibilityErrorKind = AEK_Parameter;
        downgradeToWarning = thisDowngrade;
      }
    });
  }

  for (auto &requirement : params->getRequirements()) {
    auto callback = [&](AccessScope typeAccessScope,
                        const TypeRepr *thisComplainRepr,
                        DowngradeToWarning thisDowngrade) {
      if (typeAccessScope.isChildOf(minAccessScope) ||
          (thisDowngrade == DowngradeToWarning::No &&
           downgradeToWarning == DowngradeToWarning::Yes) ||
          (!complainRepr &&
           typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
        minAccessScope = typeAccessScope;
        complainRepr = thisComplainRepr;
        accessibilityErrorKind = AEK_Requirement;
        downgradeToWarning = thisDowngrade;
      }
    };
    switch (requirement.getKind()) {
    case RequirementReprKind::TypeConstraint:
      checkTypeAccessibilityImpl(TC, requirement.getSubjectLoc(),
                                 accessScope, owner->getDeclContext(),
                                 callback);
      checkTypeAccessibilityImpl(TC, requirement.getConstraintLoc(),
                                 accessScope, owner->getDeclContext(),
                                 callback);
      break;
    case RequirementReprKind::LayoutConstraint:
      checkTypeAccessibilityImpl(TC, requirement.getSubjectLoc(),
                                 accessScope, owner->getDeclContext(),
                                 callback);
      break;
    case RequirementReprKind::SameType:
      checkTypeAccessibilityImpl(TC, requirement.getFirstTypeLoc(),
                                 accessScope, owner->getDeclContext(),
                                 callback);
      checkTypeAccessibilityImpl(TC, requirement.getSecondTypeLoc(),
                                 accessScope, owner->getDeclContext(),
                                 callback);
      break;
    }
  }

  if (minAccessScope.isPublic())
    return;

  // Swift 3.0.0 mistakenly didn't diagnose any issues when the context access
  // scope represented a private or fileprivate level.
  // FIXME: Conditionalize this on Swift 3 mode.
  if (downgradeToWarning == DowngradeToWarning::No) {
    if (!accessScope.isPublic() &&
        !isa<ModuleDecl>(accessScope.getDeclContext())) {
      downgradeToWarning = DowngradeToWarning::Yes;
    }
  }

  auto minAccess = minAccessScope.accessibilityForDiagnostics();

  bool isExplicit =
    owner->getAttrs().hasAttribute<AccessibilityAttr>() ||
    owner->getDeclContext()->getAsProtocolOrProtocolExtensionContext();
  auto diagID = diag::generic_param_access;
  if (downgradeToWarning == DowngradeToWarning::Yes)
    diagID = diag::generic_param_access_warn;
  auto diag = TC.diagnose(owner, diagID,
                          owner->getDescriptiveKind(), isExplicit,
                          contextAccess, minAccess,
                          isa<FileUnit>(owner->getDeclContext()),
                          accessibilityErrorKind);
  highlightOffendingType(TC, diag, complainRepr);
}

static void checkGenericParamAccessibility(TypeChecker &TC,
                                           const GenericParamList *params,
                                           const ValueDecl *owner) {
  checkGenericParamAccessibility(TC, params, owner,
                                 owner->getFormalAccessScope(),
                                 owner->getFormalAccess());
}

/// Checks the given declaration's accessibility to make sure it is valid given
/// the way it is defined.
///
/// \p D must be a ValueDecl or a Decl that can appear in a type context.
static void checkAccessibility(TypeChecker &TC, const Decl *D) {
  if (D->isInvalid() || D->isImplicit())
    return;

  switch (D->getKind()) {
  case DeclKind::Import:
  case DeclKind::Extension:
  case DeclKind::TopLevelCode:
  case DeclKind::InfixOperator:
  case DeclKind::PrefixOperator:
  case DeclKind::PostfixOperator:
  case DeclKind::PrecedenceGroup:
  case DeclKind::Module:
    llvm_unreachable("cannot appear in a type context");

  case DeclKind::Param:
  case DeclKind::GenericTypeParam:
    llvm_unreachable("does not have accessibility");

  case DeclKind::IfConfig:
    // Does not have accessibility.
  case DeclKind::EnumCase:
    // Handled at the EnumElement level.
  case DeclKind::Var:
    // Handled at the PatternBindingDecl level.
  case DeclKind::Destructor:
    // Always correct.
    return;

  case DeclKind::PatternBinding: {
    auto PBD = cast<PatternBindingDecl>(D);
    bool isTypeContext = PBD->getDeclContext()->isTypeContext();

    llvm::DenseSet<const VarDecl *> seenVars;
    for (auto entry : PBD->getPatternList())
    entry.getPattern()->forEachNode([&](const Pattern *P) {
      if (auto *NP = dyn_cast<NamedPattern>(P)) {
        // Only check individual variables if we didn't check an enclosing
        // TypedPattern.
        const VarDecl *theVar = NP->getDecl();
        if (seenVars.count(theVar) || theVar->isInvalid())
          return;

        checkTypeAccessibility(TC, TypeLoc::withoutLoc(theVar->getType()),
                               theVar,
                               [&](AccessScope typeAccessScope,
                                   const TypeRepr *complainRepr,
                                   DowngradeToWarning downgradeToWarning) {
          auto typeAccess = typeAccessScope.accessibilityForDiagnostics();
          bool isExplicit =
            theVar->getAttrs().hasAttribute<AccessibilityAttr>();
          auto theVarAccess = isExplicit
            ? theVar->getFormalAccess()
            : typeAccessScope.requiredAccessibilityForDiagnostics();
          auto diagID = diag::pattern_type_access_inferred;
          if (downgradeToWarning == DowngradeToWarning::Yes)
            diagID = diag::pattern_type_access_inferred_warn;
          auto diag = TC.diagnose(P->getLoc(), diagID,
                                  theVar->isLet(),
                                  isTypeContext,
                                  isExplicit,
                                  theVarAccess,
                                  isa<FileUnit>(theVar->getDeclContext()),
                                  typeAccess,
                                  theVar->getType());
        });
        return;
      }

      auto *TP = dyn_cast<TypedPattern>(P);
      if (!TP)
        return;

      // FIXME: We need an accessibility value to check against, so we pull
      // one out of some random VarDecl in the pattern. They're all going to
      // be the same, but still, ick.
      const VarDecl *anyVar = nullptr;
      TP->forEachVariable([&](VarDecl *V) {
        seenVars.insert(V);
        anyVar = V;
      });
      if (!anyVar)
        return;

      checkTypeAccessibility(TC, TP->getTypeLoc(), anyVar,
                             [&](AccessScope typeAccessScope,
                                 const TypeRepr *complainRepr,
                                 DowngradeToWarning downgradeToWarning) {
        auto typeAccess = typeAccessScope.accessibilityForDiagnostics();
        bool isExplicit =
          anyVar->getAttrs().hasAttribute<AccessibilityAttr>() ||
          anyVar->getDeclContext()->getAsProtocolOrProtocolExtensionContext();
        auto diagID = diag::pattern_type_access;
        if (downgradeToWarning == DowngradeToWarning::Yes)
          diagID = diag::pattern_type_access_warn;
        auto anyVarAccess = isExplicit
          ? anyVar->getFormalAccess()
          : typeAccessScope.requiredAccessibilityForDiagnostics();
        auto diag = TC.diagnose(P->getLoc(), diagID,
                                anyVar->isLet(),
                                isTypeContext,
                                isExplicit,
                                anyVarAccess,
                                isa<FileUnit>(anyVar->getDeclContext()),
                                typeAccess);
        highlightOffendingType(TC, diag, complainRepr);
      });
    });
    return;
  }

  case DeclKind::TypeAlias: {
    auto TAD = cast<TypeAliasDecl>(D);

    checkTypeAccessibility(TC, TAD->getUnderlyingTypeLoc(), TAD,
                           [&](AccessScope typeAccessScope,
                               const TypeRepr *complainRepr,
                               DowngradeToWarning downgradeToWarning) {
      auto typeAccess = typeAccessScope.accessibilityForDiagnostics();
      bool isExplicit = TAD->getAttrs().hasAttribute<AccessibilityAttr>();
      auto diagID = diag::type_alias_underlying_type_access;
      if (downgradeToWarning == DowngradeToWarning::Yes)
        diagID = diag::type_alias_underlying_type_access_warn;
      auto diag = TC.diagnose(TAD, diagID,
                              isExplicit, TAD->getFormalAccess(),
                              typeAccess, isa<FileUnit>(TAD->getDeclContext()));
      highlightOffendingType(TC, diag, complainRepr);
    });

    return;
  }

  case DeclKind::AssociatedType: {
    auto assocType = cast<AssociatedTypeDecl>(D);

    // This must stay in sync with diag::associated_type_access.
    enum {
      AEK_DefaultDefinition = 0,
      AEK_Requirement
    } accessibilityErrorKind;
    auto minAccessScope = AccessScope::getPublic();
    const TypeRepr *complainRepr = nullptr;
    auto downgradeToWarning = DowngradeToWarning::No;

    std::for_each(assocType->getInherited().begin(),
                  assocType->getInherited().end(),
                  [&](TypeLoc requirement) {
      checkTypeAccessibility(TC, requirement, assocType,
                             [&](AccessScope typeAccessScope,
                                 const TypeRepr *thisComplainRepr,
                                 DowngradeToWarning downgradeDiag) {
        if (typeAccessScope.isChildOf(minAccessScope) ||
            (!complainRepr &&
             typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
          minAccessScope = typeAccessScope;
          complainRepr = thisComplainRepr;
          accessibilityErrorKind = AEK_Requirement;
          downgradeToWarning = downgradeDiag;
        }
      });
    });
    checkTypeAccessibility(TC, assocType->getDefaultDefinitionLoc(), assocType,
                           [&](AccessScope typeAccessScope,
                               const TypeRepr *thisComplainRepr,
                               DowngradeToWarning downgradeDiag) {
      if (typeAccessScope.isChildOf(minAccessScope) ||
          (!complainRepr &&
           typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
        minAccessScope = typeAccessScope;
        complainRepr = thisComplainRepr;
        accessibilityErrorKind = AEK_DefaultDefinition;
        downgradeToWarning = downgradeDiag;
      }
    });

    if (!minAccessScope.isPublic()) {
      auto minAccess = minAccessScope.accessibilityForDiagnostics();
      auto diagID = diag::associated_type_access;
      if (downgradeToWarning == DowngradeToWarning::Yes)
        diagID = diag::associated_type_access_warn;
      auto diag = TC.diagnose(assocType, diagID,
                              assocType->getFormalAccess(),
                              minAccess, accessibilityErrorKind);
      highlightOffendingType(TC, diag, complainRepr);
    }
    return;
  }

  case DeclKind::Enum: {
    auto ED = cast<EnumDecl>(D);

    checkGenericParamAccessibility(TC, ED->getGenericParams(), ED);

    if (ED->hasRawType()) {
      Type rawType = ED->getRawType();
      auto rawTypeLocIter = std::find_if(ED->getInherited().begin(),
                                         ED->getInherited().end(),
                                         [&](TypeLoc inherited) {
        if (!inherited.wasValidated())
          return false;
        return inherited.getType().getPointer() == rawType.getPointer();
      });
      if (rawTypeLocIter == ED->getInherited().end())
        return;
      checkTypeAccessibility(TC, *rawTypeLocIter, ED,
                             [&](AccessScope typeAccessScope,
                                 const TypeRepr *complainRepr,
                                 DowngradeToWarning downgradeToWarning) {
        auto typeAccess = typeAccessScope.accessibilityForDiagnostics();
        bool isExplicit = ED->getAttrs().hasAttribute<AccessibilityAttr>();
        auto diagID = diag::enum_raw_type_access;
        if (downgradeToWarning == DowngradeToWarning::Yes)
          diagID = diag::enum_raw_type_access_warn;
        auto diag = TC.diagnose(ED, diagID, isExplicit,
                                ED->getFormalAccess(), typeAccess,
                                isa<FileUnit>(ED->getDeclContext()));
        highlightOffendingType(TC, diag, complainRepr);
      });
    }

    return;
  }

  case DeclKind::Struct: {
    auto SD = cast<StructDecl>(D);
    checkGenericParamAccessibility(TC, SD->getGenericParams(), SD);
    return;
  }

  case DeclKind::Class: {
    auto CD = cast<ClassDecl>(D);

    checkGenericParamAccessibility(TC, CD->getGenericParams(), CD);

    if (CD->hasSuperclass()) {
      Type superclass = CD->getSuperclass();
      auto superclassLocIter = std::find_if(CD->getInherited().begin(),
                                            CD->getInherited().end(),
                                            [&](TypeLoc inherited) {
        if (!inherited.wasValidated())
          return false;
        return inherited.getType().getPointer() == superclass.getPointer();
      });
      if (superclassLocIter == CD->getInherited().end())
        return;
      checkTypeAccessibility(TC, *superclassLocIter, CD,
                             [&](AccessScope typeAccessScope,
                                 const TypeRepr *complainRepr,
                                 DowngradeToWarning downgradeToWarning) {
        auto typeAccess = typeAccessScope.accessibilityForDiagnostics();
        bool isExplicit = CD->getAttrs().hasAttribute<AccessibilityAttr>();
        auto diagID = diag::class_super_access;
        if (downgradeToWarning == DowngradeToWarning::Yes)
          diagID = diag::class_super_access_warn;
        auto diag = TC.diagnose(CD, diagID, isExplicit, CD->getFormalAccess(),
                                typeAccess,
                                isa<FileUnit>(CD->getDeclContext()));
        highlightOffendingType(TC, diag, complainRepr);
      });
    }

    return;
  }

  case DeclKind::Protocol: {
    auto proto = cast<ProtocolDecl>(D);

    auto minAccessScope = AccessScope::getPublic();
    const TypeRepr *complainRepr = nullptr;
    auto downgradeToWarning = DowngradeToWarning::No;

    std::for_each(proto->getInherited().begin(),
                  proto->getInherited().end(),
                  [&](TypeLoc requirement) {
      checkTypeAccessibility(TC, requirement, proto,
                             [&](AccessScope typeAccessScope,
                                 const TypeRepr *thisComplainRepr,
                                 DowngradeToWarning downgradeDiag) {
        if (typeAccessScope.isChildOf(minAccessScope) ||
            (!complainRepr &&
             typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
          minAccessScope = typeAccessScope;
          complainRepr = thisComplainRepr;
          downgradeToWarning = downgradeDiag;
        }
      });
    });

    if (!minAccessScope.isPublic()) {
      auto minAccess = minAccessScope.accessibilityForDiagnostics();
      bool isExplicit = proto->getAttrs().hasAttribute<AccessibilityAttr>();
      auto diagID = diag::protocol_refine_access;
      if (downgradeToWarning == DowngradeToWarning::Yes)
        diagID = diag::protocol_refine_access_warn;
      auto diag = TC.diagnose(proto, diagID,
                              isExplicit, proto->getFormalAccess(), minAccess,
                              isa<FileUnit>(proto->getDeclContext()));
      highlightOffendingType(TC, diag, complainRepr);
    }
    return;
  }

  case DeclKind::Subscript: {
    auto SD = cast<SubscriptDecl>(D);

    auto minAccessScope = AccessScope::getPublic();
    const TypeRepr *complainRepr = nullptr;
    auto downgradeToWarning = DowngradeToWarning::No;
    bool problemIsElement = false;

    for (auto &P : *SD->getIndices()) {
      checkTypeAccessibility(TC, P->getTypeLoc(), P,
                             [&](AccessScope typeAccessScope,
                                 const TypeRepr *thisComplainRepr,
                                 DowngradeToWarning downgradeDiag) {
        if (typeAccessScope.isChildOf(minAccessScope) ||
            (!complainRepr &&
             typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
          minAccessScope = typeAccessScope;
          complainRepr = thisComplainRepr;
          downgradeToWarning = downgradeDiag;
        }
      });
    }

    checkTypeAccessibility(TC, SD->getElementTypeLoc(), SD,
                           [&](AccessScope typeAccessScope,
                               const TypeRepr *thisComplainRepr,
                               DowngradeToWarning downgradeDiag) {
      if (typeAccessScope.isChildOf(minAccessScope) ||
          (!complainRepr &&
           typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
        minAccessScope = typeAccessScope;
        complainRepr = thisComplainRepr;
        downgradeToWarning = downgradeDiag;
        problemIsElement = true;
      }
    });

    if (!minAccessScope.isPublic()) {
      auto minAccess = minAccessScope.accessibilityForDiagnostics();
      bool isExplicit =
        SD->getAttrs().hasAttribute<AccessibilityAttr>() ||
        SD->getDeclContext()->getAsProtocolOrProtocolExtensionContext();
      auto diagID = diag::subscript_type_access;
      if (downgradeToWarning == DowngradeToWarning::Yes)
        diagID = diag::subscript_type_access_warn;
      auto subscriptDeclAccess = isExplicit
        ? SD->getFormalAccess()
        : minAccessScope.requiredAccessibilityForDiagnostics();
      auto diag = TC.diagnose(SD, diagID,
                              isExplicit,
                              subscriptDeclAccess,
                              minAccess,
                              problemIsElement);
      highlightOffendingType(TC, diag, complainRepr);
    }
    return;
  }

  case DeclKind::Func:
    if (cast<FuncDecl>(D)->isAccessor())
      return;
    SWIFT_FALLTHROUGH;
  case DeclKind::Constructor: {
    auto fn = cast<AbstractFunctionDecl>(D);
    bool isTypeContext = fn->getDeclContext()->isTypeContext();

    checkGenericParamAccessibility(TC, fn->getGenericParams(), fn);

    // This must stay in sync with diag::function_type_access.
    enum {
      FK_Function = 0,
      FK_Method,
      FK_Initializer
    };

    auto minAccessScope = AccessScope::getPublic();
    const TypeRepr *complainRepr = nullptr;
    auto downgradeToWarning = DowngradeToWarning::No;

    for (auto *PL : fn->getParameterLists().slice(isTypeContext)) {
      for (auto &P : *PL) {
        checkTypeAccessibility(TC, P->getTypeLoc(), P,
                               [&](AccessScope typeAccessScope,
                                   const TypeRepr *thisComplainRepr,
                                   DowngradeToWarning downgradeDiag) {
          if (typeAccessScope.isChildOf(minAccessScope) ||
              (!complainRepr &&
               typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
            minAccessScope = typeAccessScope;
            complainRepr = thisComplainRepr;
            downgradeToWarning = downgradeDiag;
          }
        });
      }
    }

    bool problemIsResult = false;
    if (auto FD = dyn_cast<FuncDecl>(fn)) {
      checkTypeAccessibility(TC, FD->getBodyResultTypeLoc(), FD,
                             [&](AccessScope typeAccessScope,
                                 const TypeRepr *thisComplainRepr,
                                 DowngradeToWarning downgradeDiag) {
        if (typeAccessScope.isChildOf(minAccessScope) ||
            (!complainRepr &&
             typeAccessScope.hasEqualDeclContextWith(minAccessScope))) {
          minAccessScope = typeAccessScope;
          complainRepr = thisComplainRepr;
          downgradeToWarning = downgradeDiag;
          problemIsResult = true;
        }
      });
    }

    if (!minAccessScope.isPublic()) {
      auto minAccess = minAccessScope.accessibilityForDiagnostics();
      auto functionKind = isa<ConstructorDecl>(fn)
        ? FK_Initializer
        : isTypeContext ? FK_Method : FK_Function;
      bool isExplicit =
        fn->getAttrs().hasAttribute<AccessibilityAttr>() ||
        fn->getDeclContext()->getAsProtocolOrProtocolExtensionContext();
      auto diagID = diag::function_type_access;
      if (downgradeToWarning == DowngradeToWarning::Yes)
        diagID = diag::function_type_access_warn;
      auto fnAccess = isExplicit
        ? fn->getFormalAccess()
        : minAccessScope.requiredAccessibilityForDiagnostics();
      auto diag = TC.diagnose(fn, diagID,
                              isExplicit,
                              fnAccess,
                              isa<FileUnit>(fn->getDeclContext()),
                              minAccess,
                              functionKind,
                              problemIsResult);
      highlightOffendingType(TC, diag, complainRepr);
    }
    return;
  }

  case DeclKind::EnumElement: {
    auto EED = cast<EnumElementDecl>(D);

    if (!EED->hasArgumentType())
      return;
    checkTypeAccessibility(TC, EED->getArgumentTypeLoc(), EED,
                           [&](AccessScope typeAccessScope,
                               const TypeRepr *complainRepr,
                               DowngradeToWarning downgradeToWarning) {
      auto typeAccess = typeAccessScope.accessibilityForDiagnostics();
      auto diagID = diag::enum_case_access;
      if (downgradeToWarning == DowngradeToWarning::Yes)
        diagID = diag::enum_case_access_warn;
      auto diag = TC.diagnose(EED, diagID,
                              EED->getFormalAccess(), typeAccess);
      highlightOffendingType(TC, diag, complainRepr);
    });

    return;
  }
  }
}

/// Figure out if a declaration should be exported to Objective-C.
static Optional<ObjCReason> shouldMarkAsObjC(TypeChecker &TC,
                                             const ValueDecl *VD,
                                             bool allowImplicit = false){
  assert(!isa<ClassDecl>(VD));

  ProtocolDecl *protocolContext =
      dyn_cast<ProtocolDecl>(VD->getDeclContext());
  bool isMemberOfObjCProtocol =
      protocolContext && protocolContext->isObjC();

  // explicitly declared @objc.
  if (VD->getAttrs().hasAttribute<ObjCAttr>())
    return ObjCReason::ExplicitlyObjC;
  // dynamic, @IBOutlet, @IBAction, and @NSManaged imply @objc.
  else if (VD->getAttrs().hasAttribute<DynamicAttr>())
    return ObjCReason::ExplicitlyDynamic;
  else if (VD->getAttrs().hasAttribute<IBOutletAttr>())
    return ObjCReason::ExplicitlyIBOutlet;
  else if (VD->getAttrs().hasAttribute<IBActionAttr>())
    return ObjCReason::ExplicitlyIBAction;
  else if (VD->getAttrs().hasAttribute<NSManagedAttr>())
    return ObjCReason::ExplicitlyNSManaged;
  // A member of an @objc protocol is implicitly @objc.
  else if (isMemberOfObjCProtocol)
    return ObjCReason::MemberOfObjCProtocol;
  // A @nonobjc is not @objc, even if it is an override of an @objc, so check
  // for @nonobjc first.
  else if (VD->getAttrs().hasAttribute<NonObjCAttr>())
    return None;
  // An override of an @objc declaration is implicitly @objc.
  else if (VD->getOverriddenDecl() && VD->getOverriddenDecl()->isObjC())
    return ObjCReason::OverridesObjC;
  // A witness to an @objc protocol requirement is implicitly @objc.
  else if (VD->getDeclContext()->getAsClassOrClassExtensionContext() &&
           !TC.findWitnessedObjCRequirements(
             VD,
             /*anySingleRequirement=*/true).empty())
    return ObjCReason::WitnessToObjC;
  else if (VD->isInvalid())
    return None;
  else if (VD->isOperator())
    return None;
  // Implicitly generated declarations are not @objc, except for constructors.
  else if (!allowImplicit && VD->isImplicit())
    return None;
  else if (VD->getFormalAccess() <= Accessibility::FilePrivate)
    return None;

  // If this declaration is part of a class with implicitly @objc members,
  // make it implicitly @objc. However, if the declaration cannot be represented
  // as @objc, don't diagnose.
  if (auto classDecl = VD->getDeclContext()
          ->getAsClassOrClassExtensionContext()) {
    // One cannot define @objc members of any foreign classes.
    if (classDecl->isForeign())
      return None;

    if (classDecl->checkObjCAncestry() != ObjCClassKind::NonObjC)
      return ObjCReason::DoNotDiagnose;
  }

  return None;
}

/// If we need to infer 'dynamic', do so now.
///
/// This occurs when
/// - it is implied by an attribute like @NSManaged
/// - we need to dynamically dispatch to a method in an extension.
///
/// FIXME: The latter reason is a hack. We should figure out how to safely
/// put extension methods into the class vtable.
static void inferDynamic(ASTContext &ctx, ValueDecl *D) {
  // If we can't infer dynamic here, don't.
  if (!DeclAttribute::canAttributeAppearOnDecl(DAK_Dynamic, D))
    return;

  // Only 'objc' declarations use 'dynamic'.
  if (!D->isObjC() || D->hasClangNode())
    return;

  // Only introduce 'dynamic' on declarations...
  bool isNSManaged = D->getAttrs().hasAttribute<NSManagedAttr>();
  if (!isa<ExtensionDecl>(D->getDeclContext())) {
    // ...and in classes on decls marked @NSManaged.
    if (!isNSManaged)
      return;
  }

  // The presence of 'dynamic' or 'final' blocks the inference of 'dynamic'.
  if (D->isDynamic() || D->isFinal())
    return;

  // Variables declared with 'let' cannot be 'dynamic'.
  if (auto VD = dyn_cast<VarDecl>(D)) {
    if (VD->isLet() && !isNSManaged) return;
  }

  // Accessors should not infer 'dynamic' on their own; they can get it from
  // their storage decls.
  if (auto FD = dyn_cast<FuncDecl>(D))
    if (FD->isAccessor())
      return;

  // The presence of 'final' on a class prevents 'dynamic'.
  auto classDecl = D->getDeclContext()->getAsClassOrClassExtensionContext();
  if (!classDecl) return;
  if (!isNSManaged && classDecl->isFinal() &&
      !classDecl->requiresStoredPropertyInits())
    return;

  // Add the 'dynamic' attribute.
  D->getAttrs().add(new (ctx) DynamicAttr(/*IsImplicit=*/true));
}

/// Check runtime functions responsible for implicit bridging of Objective-C
/// types.
static void checkObjCBridgingFunctions(TypeChecker &TC,
                                       ModuleDecl *mod,
                                       StringRef bridgedTypeName,
                                       StringRef forwardConversion,
                                       StringRef reverseConversion) {
  assert(mod);
  ModuleDecl::AccessPathTy unscopedAccess = {};
  SmallVector<ValueDecl *, 4> results;
  
  auto &Ctx = TC.Context;
  mod->lookupValue(unscopedAccess, Ctx.getIdentifier(bridgedTypeName),
                   NLKind::QualifiedLookup, results);
  mod->lookupValue(unscopedAccess, Ctx.getIdentifier(forwardConversion),
                   NLKind::QualifiedLookup, results);
  mod->lookupValue(unscopedAccess, Ctx.getIdentifier(reverseConversion),
                   NLKind::QualifiedLookup, results);
  
  for (auto D : results)
    TC.validateDecl(D);
}

static void checkBridgedFunctions(TypeChecker &TC) {
  if (TC.HasCheckedBridgeFunctions)
    return;
  
  TC.HasCheckedBridgeFunctions = true;
  
  #define BRIDGE_TYPE(BRIDGED_MOD, BRIDGED_TYPE, _, NATIVE_TYPE, OPT) \
  Identifier ID_##BRIDGED_MOD = TC.Context.getIdentifier(#BRIDGED_MOD);\
  if (ModuleDecl *module = TC.Context.getLoadedModule(ID_##BRIDGED_MOD)) {\
    checkObjCBridgingFunctions(TC, module, #BRIDGED_TYPE, \
    "_convert" #BRIDGED_TYPE "To" #NATIVE_TYPE, \
    "_convert" #NATIVE_TYPE "To" #BRIDGED_TYPE); \
  }
  #include "swift/SIL/BridgedTypes.def"
  
  if (ModuleDecl *module = TC.Context.getLoadedModule(TC.Context.Id_Foundation)) {
    checkObjCBridgingFunctions(TC, module,
                               TC.Context.getSwiftName(
                                 KnownFoundationEntity::NSError),
                               "_convertNSErrorToError",
                               "_convertErrorToNSError");
  }
}

/// Infer the Objective-C name for a given declaration.
static void inferObjCName(TypeChecker &tc, ValueDecl *decl) {
  // If this declaration overrides an @objc declaration, use its name.
  if (auto overridden = decl->getOverriddenDecl()) {
    if (overridden->isObjC()) {
      // Handle methods first.
      if (auto overriddenFunc = dyn_cast<AbstractFunctionDecl>(overridden)) {
        // Determine the selector of the overridden method.
        ObjCSelector overriddenSelector = overriddenFunc->getObjCSelector(&tc);

        // Dig out the @objc attribute on the method, if it exists.
        auto attr = decl->getAttrs().getAttribute<ObjCAttr>();
        if (!attr) {
          // There was no @objc attribute; add one with the
          // appropriate name.
          decl->getAttrs().add(ObjCAttr::create(tc.Context,
                                                overriddenSelector,
                                                true));
          return;
        }

        // Determine whether there is a name conflict.
        bool shouldFixName = !attr->hasName();
        if (attr->hasName() && *attr->getName() != overriddenSelector) {
          // If the user explicitly wrote the incorrect name, complain.
          if (!attr->isNameImplicit()) {
            {
              auto diag = tc.diagnose(
                            attr->AtLoc,
                            diag::objc_override_method_selector_mismatch,
                            *attr->getName(), overriddenSelector);
              fixDeclarationObjCName(diag, decl, overriddenSelector);
            }

            tc.diagnose(overriddenFunc, diag::overridden_here);
          }

          shouldFixName = true;
        }

        // If we have to set the name, do so.
        if (shouldFixName) {
          // Override the name on the attribute.
          const_cast<ObjCAttr *>(attr)->setName(overriddenSelector,
                                                /*implicit=*/true);
        }
        return;
      }

      // Handle properties.
      if (auto overriddenProp = dyn_cast<VarDecl>(overridden)) {
        Identifier overriddenName = overriddenProp->getObjCPropertyName();
        ObjCSelector overriddenNameAsSel(tc.Context, 0, overriddenName);

        // Dig out the @objc attribute, if specified.
        auto attr = decl->getAttrs().getAttribute<ObjCAttr>();
        if (!attr) {
          // There was no @objc attribute; add one with the
          // appropriate name.
          decl->getAttrs().add(
            ObjCAttr::createNullary(tc.Context,
                                    overriddenName,
                                    /*isNameImplicit=*/true));
          return;
        }

        // Determine whether there is a name conflict.
        bool shouldFixName = !attr->hasName();
        if (attr->hasName() && *attr->getName() != overriddenNameAsSel) {
          // If the user explicitly wrote the wrong name, complain.
          if (!attr->isNameImplicit()) {
            tc.diagnose(attr->AtLoc,
                        diag::objc_override_property_name_mismatch,
                        attr->getName()->getSelectorPieces()[0],
                        overriddenName)
              .fixItReplaceChars(attr->getNameLocs().front(),
                                 attr->getRParenLoc(),
                                 overriddenName.str());
            tc.diagnose(overridden, diag::overridden_here);
          }

          shouldFixName = true;
        }

        // Fix the name, if needed.
        if (shouldFixName) {
          const_cast<ObjCAttr *>(attr)->setName(overriddenNameAsSel,
                                                /*implicit=*/true);
        }
        return;
      }
    }
  }

  // Dig out the @objc attribute. If it already has a name, do
  // nothing; the protocol conformance checker will handle any
  // mismatches.
  auto attr = decl->getAttrs().getAttribute<ObjCAttr>();
  if (attr && attr->hasName()) return;

  // When no override determined the Objective-C name, look for
  // requirements for which this declaration is a witness.
  Optional<ObjCSelector> requirementObjCName;
  ValueDecl *firstReq = nullptr;
  for (auto req : tc.findWitnessedObjCRequirements(decl)) {
    // If this is the first requirement, take its name.
    if (!requirementObjCName) {
      requirementObjCName = req->getObjCRuntimeName();
      firstReq = req;
      continue;
    }

    // If this requirement has a different name from one we've seen,
    // note the ambiguity.
    if (*requirementObjCName != *req->getObjCRuntimeName()) {
      tc.diagnose(decl, diag::objc_ambiguous_inference,
                  decl->getDescriptiveKind(), decl->getFullName(),
                  *requirementObjCName, *req->getObjCRuntimeName());
      
      // Note the candidates and what Objective-C names they provide.
      auto diagnoseCandidate = [&](ValueDecl *req) {
        auto proto = cast<ProtocolDecl>(req->getDeclContext());
        auto diag = tc.diagnose(decl,
                                diag::objc_ambiguous_inference_candidate,
                                req->getFullName(),
                                proto->getFullName(),
                                *req->getObjCRuntimeName());
        fixDeclarationObjCName(diag, decl, req->getObjCRuntimeName());
      };
      diagnoseCandidate(firstReq);
      diagnoseCandidate(req);

      // Suggest '@nonobjc' to suppress this error, and not try to
      // infer @objc for anything.
      tc.diagnose(decl, diag::optional_req_near_match_nonobjc, true)
        .fixItInsert(decl->getAttributeInsertionLoc(false), "@nonobjc ");
      break;
    }
  }

  // If we have a name, install it via an @objc attribute.
  if (requirementObjCName) {
    if (attr)
      const_cast<ObjCAttr *>(attr)->setName(*requirementObjCName,
                                            /*implicit=*/true);
    else
      decl->getAttrs().add(
        ObjCAttr::create(tc.Context, *requirementObjCName,
                         /*implicitName=*/true));
  }
}

/// Mark the given declaration as being Objective-C compatible (or
/// not) as appropriate.
///
/// If the declaration has a @nonobjc attribute, diagnose an error
/// using the given Reason, if present.
void swift::markAsObjC(TypeChecker &TC, ValueDecl *D,
                       Optional<ObjCReason> isObjC,
                       Optional<ForeignErrorConvention> errorConvention) {
  D->setIsObjC(isObjC.hasValue());

  if (!isObjC) {
    // FIXME: For now, only @objc declarations can be dynamic.
    if (auto attr = D->getAttrs().getAttribute<DynamicAttr>(D))
      attr->setInvalid();
    return;
  }

  // By now, the caller will have handled the case where an implicit @objc
  // could be overridden by @nonobjc. If we see a @nonobjc and we are trying
  // to add an @objc for whatever reason, diagnose an error.
  if (auto *attr = D->getAttrs().getAttribute<NonObjCAttr>()) {
    if (*isObjC == ObjCReason::DoNotDiagnose)
      isObjC = ObjCReason::ImplicitlyObjC;

    TC.diagnose(D->getStartLoc(), diag::nonobjc_not_allowed,
                getObjCDiagnosticAttrKind(*isObjC));

    attr->setInvalid();
  }

  // Make sure we have the appropriate bridging operations.
  if (!isa<DestructorDecl>(D))
    checkBridgedFunctions(TC);
  TC.useObjectiveCBridgeableConformances(D->getInnermostDeclContext(),
                                         D->getInterfaceType());

  // Record the name of this Objective-C method in its class.
  if (auto classDecl
        = D->getDeclContext()->getAsClassOrClassExtensionContext()) {
    if (auto method = dyn_cast<AbstractFunctionDecl>(D)) {
      // Determine the foreign error convention.
      if (auto baseMethod = method->getOverriddenDecl()) {
        // If the overridden method has a foreign error convention,
        // adopt it.  Set the foreign error convention for a throwing
        // method.  Note that the foreign error convention affects the
        // selector, so we perform this before inferring a selector.
        if (method->hasThrows()) {
          if (auto baseErrorConvention
                = baseMethod->getForeignErrorConvention()) {
            errorConvention = baseErrorConvention;
          }

          assert(errorConvention && "Missing error convention");
          method->setForeignErrorConvention(*errorConvention);
        }
      } else if (method->hasThrows()) {
        // Attach the foreign error convention.
        assert(errorConvention && "Missing error convention");
        method->setForeignErrorConvention(*errorConvention);
      }

      // Infer the Objective-C name for this method.
      inferObjCName(TC, method);

      // ... then record it.
      classDecl->recordObjCMethod(method);

      // Swift does not permit class methods with Objective-C selectors 'load',
      // 'alloc', or 'allocWithZone:'.
      if (!method->isInstanceMember()) {
        auto isForbiddenSelector = [&TC](ObjCSelector sel)
        -> Optional<Diag<unsigned, DeclName, ObjCSelector>> {
          switch (sel.getNumArgs()) {
          case 0:
            if (sel.getSelectorPieces().front() == TC.Context.Id_load ||
                sel.getSelectorPieces().front() == TC.Context.Id_alloc)
              return diag::objc_class_method_not_permitted;
            // Swift 3 and earlier allowed you to override `initialize`, but
            // Swift's semantics do not guarantee that it will be called at
            // the point you expect. It is disallowed in Swift 4 and later.
            if (sel.getSelectorPieces().front() == TC.Context.Id_initialize) {
              if (TC.getLangOpts().isSwiftVersion3())
                return
                  diag::objc_class_method_not_permitted_swift3_compat_warning;
              else
                return diag::objc_class_method_not_permitted;
            }
            return None;
          case 1:
            if (sel.getSelectorPieces().front() == TC.Context.Id_allocWithZone)
              return diag::objc_class_method_not_permitted;
            return None;
          default:
            return None;
          }
        };
        auto sel = method->getObjCSelector(&TC);
        if (auto diagID = isForbiddenSelector(sel)) {
          auto diagInfo = getObjCMethodDiagInfo(method);
          TC.diagnose(method, *diagID,
                      diagInfo.first, diagInfo.second, sel);
        }
      }
    } else if (isa<VarDecl>(D)) {
      // Infer the Objective-C name for this property.
      inferObjCName(TC, D);
    }
  } else if (auto method = dyn_cast<AbstractFunctionDecl>(D)) {
    if (method->hasThrows()) {
      // Attach the foreign error convention.
      assert(errorConvention && "Missing error convention");
      method->setForeignErrorConvention(*errorConvention);
    }
  }

  // Record this method in the source-file-specific Objective-C method
  // table.
  if (auto method = dyn_cast<AbstractFunctionDecl>(D)) {
    if (auto sourceFile = method->getParentSourceFile()) {
      sourceFile->ObjCMethods[method->getObjCSelector()].push_back(method);
    }
  }
}

namespace {
  /// How to generate the raw value for each element of an enum that doesn't
  /// have one explicitly specified.
  enum class AutomaticEnumValueKind {
    /// Raw values cannot be automatically generated.
    None,
    /// The raw value is the enum element's name.
    String,
    /// The raw value is the previous element's raw value, incremented.
    ///
    /// For the first element in the enum, the raw value is 0.
    Integer,
  };
} // end anonymous namespace

/// Given the raw value literal expression for an enum case, produces the
/// auto-incremented raw value for the subsequent case, or returns null if
/// the value is not auto-incrementable.
static LiteralExpr *getAutomaticRawValueExpr(TypeChecker &TC,
                                             AutomaticEnumValueKind valueKind,
                                             EnumElementDecl *forElt,
                                             LiteralExpr *prevValue) {
  switch (valueKind) {
  case AutomaticEnumValueKind::None:
    TC.diagnose(forElt->getLoc(),
                diag::enum_non_integer_convertible_raw_type_no_value);
    return nullptr;

  case AutomaticEnumValueKind::String:
    return new (TC.Context) StringLiteralExpr(forElt->getNameStr(), SourceLoc(),
                                              /*Implicit=*/true);

  case AutomaticEnumValueKind::Integer:
    // If there was no previous value, start from zero.
    if (!prevValue) {
      return new (TC.Context) IntegerLiteralExpr("0", SourceLoc(),
                                                 /*Implicit=*/true);
    }
    
    if (auto intLit = dyn_cast<IntegerLiteralExpr>(prevValue)) {
      APInt nextVal = intLit->getValue() + 1;
      bool negative = nextVal.slt(0);
      if (negative)
        nextVal = -nextVal;

      llvm::SmallString<10> nextValStr;
      nextVal.toStringSigned(nextValStr);
      auto expr = new (TC.Context)
        IntegerLiteralExpr(TC.Context.AllocateCopy(StringRef(nextValStr)),
                           forElt->getLoc(), /*Implicit=*/true);
      if (negative)
        expr->setNegative(forElt->getLoc());

      return expr;
    }

    TC.diagnose(forElt->getLoc(),
                diag::enum_non_integer_raw_value_auto_increment);
    return nullptr;
  }

  llvm_unreachable("Unhandled AutomaticEnumValueKind in switch.");
}

static void checkEnumRawValues(TypeChecker &TC, EnumDecl *ED) {
  Type rawTy = ED->getRawType();

  if (!rawTy) {
    // @objc enums must have a raw type.
    if (ED->isObjC())
      TC.diagnose(ED->getNameLoc(), diag::objc_enum_no_raw_type);
    return;
  }

  if (ED->getGenericEnvironmentOfContext() != nullptr)
    rawTy = ED->mapTypeIntoContext(rawTy);
  if (rawTy->hasError())
    return;

  AutomaticEnumValueKind valueKind;

  if (ED->isObjC()) {
    // @objc enums must have a raw type that's an ObjC-representable
    // integer type.
    if (!TC.isCIntegerType(ED, rawTy)) {
      TC.diagnose(ED->getInherited().front().getSourceRange().Start,
                  diag::objc_enum_raw_type_not_integer,
                  rawTy);
      ED->getInherited().front().setInvalidType(TC.Context);
      return;
    }
    valueKind = AutomaticEnumValueKind::Integer;
  } else {
    // Swift enums require that the raw type is convertible from one of the
    // primitive literal protocols.
    auto conformsToProtocol = [&](KnownProtocolKind protoKind) {
        ProtocolDecl *proto = TC.getProtocol(ED->getLoc(), protoKind);
        return TC.conformsToProtocol(rawTy, proto, ED->getDeclContext(), None);
    };

    static auto otherLiteralProtocolKinds = {
      KnownProtocolKind::ExpressibleByFloatLiteral,
      KnownProtocolKind::ExpressibleByUnicodeScalarLiteral,
      KnownProtocolKind::ExpressibleByExtendedGraphemeClusterLiteral,
    };

    if (conformsToProtocol(KnownProtocolKind::ExpressibleByIntegerLiteral)) {
      valueKind = AutomaticEnumValueKind::Integer;
    } else if (conformsToProtocol(KnownProtocolKind::ExpressibleByStringLiteral)){
      valueKind = AutomaticEnumValueKind::String;
    } else if (std::any_of(otherLiteralProtocolKinds.begin(),
                           otherLiteralProtocolKinds.end(),
                           conformsToProtocol)) {
      valueKind = AutomaticEnumValueKind::None;
    } else {
      TC.diagnose(ED->getInherited().front().getSourceRange().Start,
                  diag::raw_type_not_literal_convertible,
                  rawTy);
      ED->getInherited().front().setInvalidType(TC.Context);
      return;
    }
  }

  // We need at least one case to have a raw value.
  if (ED->getAllElements().empty()) {
    TC.diagnose(ED->getInherited().front().getSourceRange().Start,
                diag::empty_enum_raw_type);
    return;
  }

  // Check the raw values of the cases.
  LiteralExpr *prevValue = nullptr;
  EnumElementDecl *lastExplicitValueElt = nullptr;

  // Keep a map we can use to check for duplicate case values.
  llvm::SmallDenseMap<RawValueKey, RawValueSource, 8> uniqueRawValues;

  for (auto elt : ED->getAllElements()) {
    // Skip if the raw value expr has already been checked.
    if (elt->getTypeCheckedRawValueExpr())
      continue;

    // Make sure the element is checked out before we poke at it.
    TC.validateDecl(elt);
    
    if (elt->isInvalid())
      continue;

    // We don't yet support raw values on payload cases.
    if (elt->hasArgumentType()) {
      TC.diagnose(elt->getLoc(),
                  diag::enum_with_raw_type_case_with_argument);
      TC.diagnose(ED->getInherited().front().getSourceRange().Start,
                  diag::enum_raw_type_here, rawTy);
      elt->setInvalid();
      continue;
    }
    
    // Check the raw value expr, if we have one.
    if (auto *rawValue = elt->getRawValueExpr()) {
      Expr *typeCheckedExpr = rawValue;
      if (!TC.typeCheckExpression(typeCheckedExpr, ED, 
                                  TypeLoc::withoutLoc(rawTy),
                                  CTP_EnumCaseRawValue)) {
        elt->setTypeCheckedRawValueExpr(typeCheckedExpr);
      }
      lastExplicitValueElt = elt;
    } else {
      // If the enum element has no explicit raw value, try to
      // autoincrement from the previous value, or start from zero if this
      // is the first element.
      auto nextValue = getAutomaticRawValueExpr(TC, valueKind, elt, prevValue);
      if (!nextValue) {
        elt->setInvalid();
        break;
      }
      elt->setRawValueExpr(nextValue);
      Expr *typeChecked = nextValue;
      if (!TC.typeCheckExpression(typeChecked, ED, 
                                  TypeLoc::withoutLoc(rawTy),
                                  CTP_EnumCaseRawValue))
        elt->setTypeCheckedRawValueExpr(typeChecked);
    }
    prevValue = elt->getRawValueExpr();
    assert(prevValue && "continued without setting raw value of enum case");

    // If we didn't find a valid initializer (maybe the initial value was
    // incompatible with the raw value type) mark the entry as being erroneous.
    if (!elt->getTypeCheckedRawValueExpr()) {
      elt->setInvalid();
      continue;
    }

    TC.checkEnumElementErrorHandling(elt);

    // Find the type checked version of the LiteralExpr used for the raw value.
    // this is unfortunate, but is needed because we're digging into the
    // literals to get information about them, instead of accepting general
    // expressions.
    LiteralExpr *rawValue = elt->getRawValueExpr();
    if (!rawValue->getType()) {
      elt->getTypeCheckedRawValueExpr()->forEachChildExpr([&](Expr *E)->Expr* {
        if (E->getKind() == rawValue->getKind())
          rawValue = cast<LiteralExpr>(E);
        return E;
      });
      elt->setRawValueExpr(rawValue);
    }

    prevValue = rawValue;
    assert(prevValue && "continued without setting raw value of enum case");

    // Check that the raw value is unique.
    RawValueKey key(rawValue);
    RawValueSource source{elt, lastExplicitValueElt};

    auto insertIterPair = uniqueRawValues.insert({key, source});
    if (insertIterPair.second)
      continue;

    // Diagnose the duplicate value.
    SourceLoc diagLoc = elt->getRawValueExpr()->isImplicit()
        ? elt->getLoc() : elt->getRawValueExpr()->getLoc();
    TC.diagnose(diagLoc, diag::enum_raw_value_not_unique);
    assert(lastExplicitValueElt &&
           "should not be able to have non-unique raw values when "
           "relying on autoincrement");
    if (lastExplicitValueElt != elt &&
        valueKind == AutomaticEnumValueKind::Integer) {
      TC.diagnose(lastExplicitValueElt->getRawValueExpr()->getLoc(),
                  diag::enum_raw_value_incrementing_from_here);
    }

    RawValueSource prevSource = insertIterPair.first->second;
    auto foundElt = prevSource.sourceElt;
    diagLoc = foundElt->getRawValueExpr()->isImplicit()
        ? foundElt->getLoc() : foundElt->getRawValueExpr()->getLoc();
    TC.diagnose(diagLoc, diag::enum_raw_value_used_here);
    if (foundElt != prevSource.lastExplicitValueElt &&
        valueKind == AutomaticEnumValueKind::Integer) {
      if (prevSource.lastExplicitValueElt)
        TC.diagnose(prevSource.lastExplicitValueElt
                      ->getRawValueExpr()->getLoc(),
                    diag::enum_raw_value_incrementing_from_here);
      else
        TC.diagnose(ED->getAllElements().front()->getLoc(),
                    diag::enum_raw_value_incrementing_from_zero);
    }
  }
}

/// Walks up the override chain for \p CD until it finds an initializer that is
/// required and non-implicit. If no such initializer exists, returns the
/// declaration where \c required was introduced (i.e. closest to the root
/// class).
static const ConstructorDecl *
findNonImplicitRequiredInit(const ConstructorDecl *CD) {
  while (CD->isImplicit()) {
    auto *overridden = CD->getOverriddenDecl();
    if (!overridden || !overridden->isRequired())
      break;
    CD = overridden;
  }
  return CD;
}

static void checkVarBehavior(VarDecl *decl, TypeChecker &TC) {
  // No behavior, no problems.
  if (!decl->hasBehavior())
    return;
  
  // Don't try to check the behavior if we already encountered an error.
  if (decl->getType()->hasError())
    return;
  
  auto behavior = decl->getMutableBehavior() ;
  // We should have set up the conformance during validation.
  assert(behavior->Conformance.hasValue());
  // If the behavior couldn't be resolved during validation, we can't really
  // do much more.
  auto *conformance = behavior->Conformance.getValue();
  if (!conformance)
    return;
  
  assert(behavior->ValueDecl);
  
  auto dc = decl->getDeclContext();
  auto behaviorSelf = conformance->getType();
  auto behaviorInterfaceSelf = conformance->getInterfaceType();
  auto behaviorProto = conformance->getProtocol();
  auto behaviorProtoTy = behaviorProto->getDeclaredType();
  
  // Treat any inherited protocols as constraints on `Self`, and gather
  // conformances from the containing type.
  //
  // It'd probably be cleaner to model as `where Self: ...` constraints when
  // we have those.
  //
  // TODO: Handle non-protocol requirements ('class', base class, etc.)
  for (auto refinedProto : behaviorProto->getInheritedProtocols(&TC)) {
    // A behavior in non-type or static context is never going to be able to
    // satisfy Self constraints (until we give structural types that ability).
    // Give a tailored error message for this case.
    if (!dc->isTypeContext() || decl->isStatic()) {
      TC.diagnose(behavior->getLoc(),
                  diag::property_behavior_with_self_requirement_not_in_type,
                  behaviorProto->getName());
      break;
    }
    
    // Blame conformance failures on the containing type.
    SourceLoc blameLoc;
    if (auto nomTy = dyn_cast<NominalTypeDecl>(dc)) {
      blameLoc = nomTy->getLoc();
    } else if (auto ext = dyn_cast<ExtensionDecl>(dc)) {
      blameLoc = ext->getLoc();
    } else {
      llvm_unreachable("unknown type context type?!");
    }

    auto inherited = TC.conformsToProtocol(behaviorSelf, refinedProto, dc,
                                           ConformanceCheckFlags::Used,
                                           blameLoc);
    if (inherited && inherited->isConcrete()) {
      conformance->setInheritedConformance(refinedProto,
                                           inherited->getConcrete());
    } else {
      // Add some notes that the conformance is behavior-driven.
      TC.diagnose(behavior->getLoc(),
                  diag::self_conformance_required_by_property_behavior,
                  refinedProto->getName(),
                  behaviorProto->getName());
      conformance->setInvalid();
    }
  }
  
  // Try to satisfy the protocol requirements from the property's traits.
  
  auto unknownRequirement = [&](ValueDecl *requirement) {
    // Diagnose requirements that can't be satisfied from the behavior decl.
    TC.diagnose(behavior->getLoc(),
                diag::property_behavior_unknown_requirement,
                behaviorProto->getName(),
                requirement->getName());
    TC.diagnose(requirement->getLoc(),
                diag::property_behavior_unknown_requirement_here);
    conformance->setInvalid();
  };
  
  conformance->setState(ProtocolConformanceState::CheckingTypeWitnesses);
  
  // First, satisfy any associated type requirements.
  Substitution valueSub;
  AssociatedTypeDecl *valueReqt = nullptr;
  for (auto requirementDecl : behaviorProto->getMembers()) {
    auto assocTy = dyn_cast<AssociatedTypeDecl>(requirementDecl);
    if (!assocTy)
      continue;
  
    // Match a Value associated type requirement to the property type.
    if (assocTy->getName() != TC.Context.Id_Value) {
      unknownRequirement(assocTy);
      continue;
    }
    
    valueReqt = assocTy;
    
    // Check for required protocol conformances.
    // TODO: Handle secondary 'where' constraints on the associated types.
    // TODO: Handle non-protocol constraints ('class', base class)
    auto propTy = decl->getType();
    SmallVector<ProtocolConformanceRef, 4> valueConformances;
    for (auto proto : assocTy->getConformingProtocols()) {
      auto valueConformance = TC.conformsToProtocol(propTy, proto, dc,
                                                    ConformanceCheckFlags::Used,
                                                    decl->getLoc());
      if (!valueConformance) {
        conformance->setInvalid();
        TC.diagnose(behavior->getLoc(),
                    diag::value_conformance_required_by_property_behavior,
                    proto->getName(),
                    behaviorProto->getName());
        goto next_requirement;
      }
      valueConformances.push_back(*valueConformance);
    }
    
    {
      auto conformancesCopy = TC.Context.AllocateCopy(valueConformances);
      valueSub = Substitution(propTy, conformancesCopy);
      // FIXME: Maybe we should synthesize an implicit TypeAliasDecl? We
      // really don't want the behavior conformances to show up in the
      // enclosing namespace though.
      conformance->setTypeWitness(assocTy, valueSub, /*typeDecl*/ nullptr);
    }
  next_requirement:;
  }
  
  // Bail out if we didn't resolve type witnesses.
  if (conformance->isInvalid()) {
    decl->markInvalid();
    return;
  }

  // Build a Substitution vector from the conformance.
  auto conformanceMem =
    TC.Context.AllocateUninitialized<ProtocolConformanceRef>(1);
  auto selfConformance = new ((void*)conformanceMem.data())
    ProtocolConformanceRef(conformance);
  Substitution allInterfaceSubs[] = {
    Substitution(behaviorInterfaceSelf, *selfConformance),
    Substitution(decl->getInterfaceType(), valueSub.getConformances()),
  };
  Substitution allContextSubs[] = {
    Substitution(behaviorSelf, *selfConformance),
    Substitution(decl->getType(), valueSub.getConformances()),
  };

  ArrayRef<Substitution> interfaceSubs = allInterfaceSubs;
  if (interfaceSubs.back().getConformances().empty())
    interfaceSubs = interfaceSubs.drop_back();

  ArrayRef<Substitution> contextSubs = allContextSubs;
  if (contextSubs.back().getConformances().empty())
    contextSubs = contextSubs.drop_back();

  // Now that type witnesses are done, satisfy property and method requirements.
  conformance->setState(ProtocolConformanceState::Checking);

  bool requiresParameter = false;
  for (auto requirementDecl : behaviorProto->getMembers()) {
    auto requirement = dyn_cast<ValueDecl>(requirementDecl);
    if (!requirement)
      continue;
    if (isa<AssociatedTypeDecl>(requirement))
      continue;
    
    if (auto varReqt = dyn_cast<VarDecl>(requirement)) {
      // Match a storage requirement.
      if (varReqt->getName() == TC.Context.Id_storage) {
        TC.validateDecl(varReqt);

        auto storageTy = varReqt->getInterfaceType();
        // We need an initStorage extension method to initialize this storage.
        // Should have the signature:
        //   static func initStorage() -> Storage
        // for default initialization, or:
        //   static func initStorage(_: Value) -> Storage
        // for parameterized initialization.
        auto expectedDefaultInitStorageTy =
          FunctionType::get(TC.Context.TheEmptyTupleType, storageTy);
        Type valueTy = DependentMemberType::get(
                                          behaviorProto->getSelfInterfaceType(),
                                          valueReqt);
        
        auto expectedParameterizedInitStorageTy =
          FunctionType::get(valueTy, storageTy);

        auto lookup = TC.lookupMember(dc, behaviorProtoTy,
                                      TC.Context.Id_initStorage);
        FuncDecl *defaultInitStorageDecl = nullptr;
        FuncDecl *parameterizedInitStorageDecl = nullptr;
        for (auto found : lookup) {
          if (auto foundFunc = dyn_cast<FuncDecl>(found.Decl)) {
            if (!foundFunc->isStatic())
              continue;
            auto methodTy = foundFunc->getInterfaceType()
              ->castTo<AnyFunctionType>()
              ->getResult();
            if (methodTy->isEqual(expectedDefaultInitStorageTy))
              defaultInitStorageDecl = foundFunc;
            else if (methodTy->isEqual(expectedParameterizedInitStorageTy))
              parameterizedInitStorageDecl = foundFunc;
          }
        }
        
        if (defaultInitStorageDecl && parameterizedInitStorageDecl) {
          TC.diagnose(behavior->getLoc(),
                      diag::property_behavior_protocol_reqt_ambiguous,
                      TC.Context.Id_initStorage);
          TC.diagnose(defaultInitStorageDecl->getLoc(),
                      diag::property_behavior_protocol_reqt_here,
                      TC.Context.Id_initStorage);
          TC.diagnose(parameterizedInitStorageDecl->getLoc(),
                      diag::property_behavior_protocol_reqt_here,
                      TC.Context.Id_initStorage);
          conformance->setInvalid();
          continue;
        }
        
        if (!defaultInitStorageDecl && !parameterizedInitStorageDecl) {
          TC.diagnose(behavior->getLoc(),
                      diag::property_behavior_protocol_no_initStorage,
                      expectedDefaultInitStorageTy,
                      expectedParameterizedInitStorageTy);
          for (auto found : lookup)
            TC.diagnose(found.Decl->getLoc(),
                        diag::found_candidate);
          conformance->setInvalid();
          continue;
        }
        
        // TODO: Support storage-backed behaviors in non-type contexts.
        if (!dc->isTypeContext()) {
          TC.diagnose(behavior->getLoc(),
                      diag::property_behavior_with_feature_not_supported,
                      behaviorProto->getName(), "storage");
          conformance->setInvalid();
          continue;
        }
        
        // TODO: Support destructured initializers such as
        // `var (a, b) = tuple __behavior blah`. This ought to be supportable
        // if the behavior allows for DI-like initialization.
        if (parameterizedInitStorageDecl
            && !isa<NamedPattern>(decl->getParentPattern()
                                      ->getSemanticsProvidingPattern())) {
          TC.diagnose(decl->getLoc(),
                      diag::property_behavior_unsupported_initializer);
          auto PBD = decl->getParentPatternBinding();
          unsigned entryIndex = PBD->getPatternEntryIndexForVarDecl(decl);
          PBD->setInit(entryIndex, nullptr);
          PBD->setInitializerChecked(entryIndex);
          continue;
        }
        
        // Instantiate the storage next to us in the enclosing scope.
        TC.completePropertyBehaviorStorage(decl, varReqt,
                                           defaultInitStorageDecl,
                                           parameterizedInitStorageDecl,
                                           behaviorSelf,
                                           storageTy,
                                           conformance,
                                           interfaceSubs,
                                           contextSubs);
        continue;
      }
    } else if (auto func = dyn_cast<FuncDecl>(requirement)) {
      // Handle accessors as part of their property.
      if (func->isAccessor())
        continue;
      
      // Handle a parameter block requirement.
      if (func->getName() == TC.Context.Id_parameter) {
        requiresParameter = true;
        
        TC.validateDecl(func);
        
        // The requirement should be for a nongeneric, nonmutating instance
        // method.
        if (func->isStatic() || func->isGeneric() || func->isMutating()) {
          TC.diagnose(behavior->getLoc(),
                      diag::property_behavior_invalid_parameter_reqt,
                      behaviorProto->getName());
          TC.diagnose(varReqt->getLoc(),
                      diag::property_behavior_protocol_reqt_here,
                      TC.Context.Id_parameter);
          conformance->setInvalid();
          continue;
        }
        
        // The declaration must have a parameter.
        if (!decl->getBehavior()->Param) {
          TC.diagnose(behavior->getLoc(),
                      diag::property_behavior_requires_parameter,
                      behaviorProto->getName(),
                      decl->getName());
          conformance->setInvalid();
          continue;
        }
        
        // TODO: Support parameter requirements in non-type contexts.
        if (!dc->isTypeContext()) {
          TC.diagnose(behavior->getLoc(),
                      diag::property_behavior_with_feature_not_supported,
                      behaviorProto->getName(), "parameter requirement");
          conformance->setInvalid();
          continue;
        }
        
        // Build the parameter witness method.
        TC.completePropertyBehaviorParameter(decl, func,
                                             conformance,
                                             interfaceSubs,
                                             contextSubs);
        continue;
      }
    }

    unknownRequirement(requirement);
  }
  
  // If the property was declared with an initializer, but the behavior
  // didn't use it, complain.
  if (decl->getParentInitializer()) {
    TC.diagnose(decl->getParentInitializer()->getLoc(),
                diag::property_behavior_invalid_initializer,
                behaviorProto->getName());
  }
  
  // If the property was declared with a parameter, but the behavior didn't
  // use it, complain.
  // TODO: The initializer could eventually be consumed by DI-style
  // initialization.
  if (!requiresParameter && decl->getBehavior()->Param) {
    TC.diagnose(decl->getBehavior()->Param->getLoc(),
                diag::property_behavior_invalid_parameter,
                behaviorProto->getName());
  }
  
  // Bail out if we didn't resolve method witnesses.
  if (conformance->isInvalid()) {
    decl->markInvalid();
    return;
  }

  conformance->setState(ProtocolConformanceState::Complete);

  // Check that the 'value' property from the protocol matches the
  // declared property type in context.
  auto sig = behaviorProto->getGenericSignatureOfContext();
  auto map = sig->getSubstitutionMap(interfaceSubs);
  auto substValueTy = behavior->ValueDecl->getInterfaceType()
    .subst(map, SubstOptions());
  
  if (!substValueTy->isEqual(decl->getInterfaceType())) {
    TC.diagnose(behavior->getLoc(),
                diag::property_behavior_value_type_doesnt_match,
                behaviorProto->getName(),
                substValueTy,
                decl->getName(),
                decl->getInterfaceType());
    TC.diagnose(behavior->ValueDecl->getLoc(),
                diag::property_behavior_value_decl_here);
    decl->markInvalid();
    return;
  }

  // Synthesize the bodies of the property's accessors now, forwarding to the
  // 'value' implementation.
  TC.completePropertyBehaviorAccessors(decl, behavior->ValueDecl,
                                       decl->getType(),
                                       interfaceSubs, contextSubs);
  
  return;
}

/// For building the higher-than component of the diagnostic path,
/// we use the visited set, which we've embellished with information
/// about how we reached a particular node.  This is reasonable because
/// we need to maintain the set anyway.
static void buildHigherThanPath(PrecedenceGroupDecl *last,
                          const llvm::DenseMap<PrecedenceGroupDecl *,
                                        PrecedenceGroupDecl *> &visitedFrom,
                          raw_ostream &out) {
  auto it = visitedFrom.find(last);
  assert(it != visitedFrom.end());
  auto from = it->second;
  if (from) {
    buildHigherThanPath(from, visitedFrom, out);
  }
  out << last->getName() << " -> ";
}

/// For building the lower-than component of the diagnostic path,
/// we just do a depth-first search to find a path.
static bool buildLowerThanPath(PrecedenceGroupDecl *start,
                               PrecedenceGroupDecl *target,
                               raw_ostream &out) {
  if (start == target) {
    out << start->getName();
    return true;
  }

  if (start->isInvalid())
    return false;

  for (auto &rel : start->getLowerThan()) {
    if (rel.Group && buildLowerThanPath(rel.Group, target, out)) {
      out << " -> " << start->getName();
      return true;
    }
  }

  return false;
}

static void checkPrecedenceCircularity(TypeChecker &TC,
                                       PrecedenceGroupDecl *PGD) {
  // Don't diagnose if this group is already marked invalid.
  if (PGD->isInvalid()) return;

  // The cycle doesn't necessarily go through this specific group,
  // so we need a proper visited set to avoid infinite loops.  We
  // also record a back-reference so that we can easily reconstruct
  // the cycle.
  llvm::DenseMap<PrecedenceGroupDecl*, PrecedenceGroupDecl*> visitedFrom;
  SmallVector<PrecedenceGroupDecl*, 4> stack;

  // Fill out the targets set.
  llvm::SmallPtrSet<PrecedenceGroupDecl*, 4> targets;
  stack.push_back(PGD);
  do {
    auto cur = stack.pop_back_val();

    // If we reach an invalid node, just bail out.
    if (cur->isInvalid()) {
      PGD->setInvalid();
      return;
    }

    targets.insert(cur);

    for (auto &rel : cur->getLowerThan()) {
      if (!rel.Group) continue;

      // We can't have cycles in the lower-than relationship
      // because it has to point outside of the module.

      stack.push_back(rel.Group);
    }
  } while (!stack.empty());

  // Make sure that the PGD is its own source.
  visitedFrom.insert({PGD, nullptr});

  stack.push_back(PGD);
  do {
    auto cur = stack.pop_back_val();

    // If we reach an invalid node, just bail out.
    if (cur->isInvalid()) {
      PGD->setInvalid();
      return;
    }

    for (auto &rel : cur->getHigherThan()) {
      if (!rel.Group) continue;

      // Check whether we've reached a target declaration.
      if (!targets.count(rel.Group)) {
        // If not, check whether we've visited this group before.
        if (visitedFrom.insert({rel.Group, cur}).second) {
          // If not, add it to the queue.
          stack.push_back(rel.Group);
        }

        // Note that we'll silently ignore cycles that don't go through PGD.
        // We should eventually process the groups that are involved.
        continue;
      }

      // Otherwise, we have something to report.
      SmallString<128> path;
      {
        llvm::raw_svector_ostream str(path);

        // Build the higherThan portion of the path (PGD -> cur).
        buildHigherThanPath(cur, visitedFrom, str);

        // Build the lowerThan portion of the path (rel.Group -> PGD).
        buildLowerThanPath(PGD, rel.Group, str);
      }
      
      TC.diagnose(PGD->getHigherThanLoc(), diag::precedence_group_cycle, path);
      PGD->setInvalid();
      return;
    }
  } while (!stack.empty());
}

/// Do a primitive lookup for the given precedence group.  This does
/// not validate the precedence group or diagnose if the lookup fails
/// (other than via ambiguity); for that, use
/// TypeChecker::lookupPrecedenceGroup.
///
/// Pass an invalid source location to suppress diagnostics.
static PrecedenceGroupDecl *
lookupPrecedenceGroupPrimitive(DeclContext *dc, Identifier name,
                               SourceLoc nameLoc) {
  if (auto sf = dc->getParentSourceFile()) {
    bool cascading = dc->isCascadingContextForLookup(false);
    return sf->lookupPrecedenceGroup(name, cascading, nameLoc);
  } else {
    return dc->getParentModule()->lookupPrecedenceGroup(name, nameLoc);
  }
}

void TypeChecker::validateDecl(PrecedenceGroupDecl *PGD) {
  checkDeclAttributesEarly(PGD);
  checkDeclAttributes(PGD);

  if (PGD->isInvalid() || PGD->isBeingValidated())
    return;
  PGD->setIsBeingValidated();
  SWIFT_DEFER { PGD->setIsBeingValidated(false); };

  bool isInvalid = false;

  // Validate the higherThan relationships.
  bool addedHigherThan = false;
  for (auto &rel : PGD->getMutableHigherThan()) {
    if (rel.Group) continue;

    auto group = lookupPrecedenceGroupPrimitive(PGD->getDeclContext(),
                                                rel.Name, rel.NameLoc);
    if (group) {
      rel.Group = group;
      validateDecl(group);
      addedHigherThan = true;
    } else if (!PGD->isInvalid()) {
      diagnose(rel.NameLoc, diag::unknown_precedence_group, rel.Name);
      isInvalid = true;
    }
  }

  // Validate the lowerThan relationships.
  for (auto &rel : PGD->getMutableLowerThan()) {
    if (rel.Group) continue;

    auto dc = PGD->getDeclContext();
    auto group = lookupPrecedenceGroupPrimitive(dc, rel.Name, rel.NameLoc);
    if (group) {
      if (group->getDeclContext()->getParentModule()
            == dc->getParentModule()) {
        if (!PGD->isInvalid()) {
          diagnose(rel.NameLoc, diag::precedence_group_lower_within_module);
          diagnose(group->getNameLoc(), diag::precedence_group_declared_here);
          isInvalid = true;
        }
      } else {
        rel.Group = group;
        validateDecl(group);
      }
    } else if (!PGD->isInvalid()) {
      diagnose(rel.NameLoc, diag::unknown_precedence_group, rel.Name);
      isInvalid = true;
    }
  }

  // Check for circularity.
  if (addedHigherThan) {
    checkPrecedenceCircularity(*this, PGD);
  }

  if (isInvalid) PGD->setInvalid();
}

PrecedenceGroupDecl *TypeChecker::lookupPrecedenceGroup(DeclContext *dc,
                                                        Identifier name,
                                                        SourceLoc nameLoc) {
  auto group = lookupPrecedenceGroupPrimitive(dc, name, nameLoc);
  if (group) {
    validateDecl(group);
  } else if (nameLoc.isValid()) {
    // FIXME: avoid diagnosing this multiple times per source file.
    diagnose(nameLoc, diag::unknown_precedence_group, name);
  }
  return group;
}

/// Validate the given operator declaration.
///
/// This establishes key invariants, such as an InfixOperatorDecl's
/// reference to its precedence group and the transitive validity of that
/// group.
void TypeChecker::validateDecl(OperatorDecl *OD) {
  checkDeclAttributesEarly(OD);
  checkDeclAttributes(OD);

  if (auto IOD = dyn_cast<InfixOperatorDecl>(OD)) {
    if (!IOD->getPrecedenceGroup()) {
      PrecedenceGroupDecl *group = nullptr;

      // If a name was given, try to look it up.
      Identifier name = IOD->getPrecedenceGroupName();
      if (!name.empty()) {
        auto loc = IOD->getPrecedenceGroupNameLoc();
        group = lookupPrecedenceGroupPrimitive(OD->getDeclContext(), name, loc);
        if (!group && !IOD->isInvalid()) {
          diagnose(loc, diag::unknown_precedence_group, name);
          IOD->setInvalid();
        }
      }

      // If that fails, or if a name was not given, use the default
      // precedence group.
      if (!group) {
        group = lookupPrecedenceGroupPrimitive(OD->getDeclContext(),
                                               Context.Id_DefaultPrecedence,
                                               SourceLoc());
        if (!group && name.empty() && !IOD->isInvalid()) {
          diagnose(OD->getLoc(), diag::missing_builtin_precedence_group,
                   Context.Id_DefaultPrecedence);
        }
      }

      // Validate the precedence group.
      if (group) {
        validateDecl(group);
        IOD->setPrecedenceGroup(group);
      }
    }
  }
}

static void validateAbstractStorageDecl(AbstractStorageDecl *ASD,
                                        TypeChecker &TC) {
  if (ASD->hasAccessorFunctions())
    maybeAddMaterializeForSet(ASD, TC);

  if (ASD->isFinal()) {
    makeFinal(TC.Context, ASD->getGetter());
    makeFinal(TC.Context, ASD->getSetter());
    makeFinal(TC.Context, ASD->getMaterializeForSetFunc());
  }

  if (auto getter = ASD->getGetter())
    TC.validateDecl(getter);
  if (auto setter = ASD->getSetter())
    TC.validateDecl(setter);
  if (auto materializeForSet = ASD->getMaterializeForSetFunc())
    TC.validateDecl(materializeForSet);
  if (ASD->hasAddressors()) {
    if (auto addressor = ASD->getAddressor())
      TC.validateDecl(addressor);
    if (auto addressor = ASD->getMutableAddressor())
      TC.validateDecl(addressor);
  }

}

namespace {
class DeclChecker : public DeclVisitor<DeclChecker> {
public:
  TypeChecker &TC;

  // For library-style parsing, we need to make two passes over the global
  // scope.  These booleans indicate whether this is currently the first or
  // second pass over the global scope (or neither, if we're in a context where
  // we only visit each decl once).
  unsigned IsFirstPass : 1;
  unsigned IsSecondPass : 1;

  DeclChecker(TypeChecker &TC, bool IsFirstPass, bool IsSecondPass)
      : TC(TC), IsFirstPass(IsFirstPass), IsSecondPass(IsSecondPass) {}

  void visit(Decl *decl) {
    PrettyStackTraceDecl StackTrace("type-checking", decl);
    
    DeclVisitor<DeclChecker>::visit(decl);

    if (auto VD = dyn_cast<ValueDecl>(decl)) {
      checkRedeclaration(TC, VD);
      
      // If this is a member of a nominal type, don't allow it to have a name of
      // "Type" or "Protocol" since we reserve the X.Type and X.Protocol
      // expressions to mean something builtin to the language.  We *do* allow
      // these if they are escaped with backticks though.
      auto &Context = TC.Context;
      if (VD->getDeclContext()->isTypeContext() &&
          (VD->getFullName().isSimpleName(Context.Id_Type) ||
           VD->getFullName().isSimpleName(Context.Id_Protocol)) &&
          VD->getNameLoc().isValid() &&
          Context.SourceMgr.extractText({VD->getNameLoc(), 1}) != "`") {
        TC.diagnose(VD->getNameLoc(), diag::reserved_member_name,
                    VD->getFullName(), VD->getNameStr());
        TC.diagnose(VD->getNameLoc(), diag::backticks_to_escape)
          .fixItReplace(VD->getNameLoc(), "`"+VD->getNameStr().str()+"`");
      }
    }

    if (!IsFirstPass) {
      TC.checkUnsupportedProtocolType(decl);
      if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
        TC.checkDeclCircularity(nominal);
      }
    }
  }

  //===--------------------------------------------------------------------===//
  // Visit Methods.
  //===--------------------------------------------------------------------===//

  void visitGenericTypeParamDecl(GenericTypeParamDecl *D) {
    llvm_unreachable("cannot reach here");
  }
  
  void visitImportDecl(ImportDecl *ID) {
    TC.checkDeclAttributesEarly(ID);
    TC.checkDeclAttributes(ID);
  }

  void visitOperatorDecl(OperatorDecl *OD) {
    TC.validateDecl(OD);
  }

  void visitPrecedenceGroupDecl(PrecedenceGroupDecl *PGD) {
    TC.validateDecl(PGD);
  }

  void visitBoundVariable(VarDecl *VD) {
    TC.validateDecl(VD);
    
    if (!VD->getType()->isMaterializable()) {
      TC.diagnose(VD->getStartLoc(), diag::var_type_not_materializable,
                  VD->getType());
      VD->markInvalid();
    }

    // Check the behavior.
    checkVarBehavior(VD, TC);

    // WARNING: Anything you put in this function will only be run when the
    // VarDecl is fully type-checked within its own file. It will NOT be run
    // when the VarDecl is merely used from another file.

    // Reject cases where this is a variable that has storage but it isn't
    // allowed.
    if (VD->hasStorage()) {
      // Stored properties in protocols are diagnosed in
      // maybeAddAccessorsToVariable(), to ensure they run when a
      // protocol requirement is validated but not type checked.

      // Enums and extensions cannot have stored instance properties.
      // Static stored properties are allowed, with restrictions
      // enforced below.
      if (isa<EnumDecl>(VD->getDeclContext()) &&
          !VD->isStatic()) {
        // Enums can only have computed properties.
        TC.diagnose(VD->getLoc(), diag::enum_stored_property);
        VD->markInvalid();
      } else if (isa<ExtensionDecl>(VD->getDeclContext()) &&
                 !VD->isStatic()) {
        TC.diagnose(VD->getLoc(), diag::extension_stored_property);
        VD->markInvalid();
      }
      
      // We haven't implemented type-level storage in some contexts.
      if (VD->isStatic()) {
        auto PBD = VD->getParentPatternBinding();
        // Selector for unimplemented_static_var message.
        enum : unsigned {
          Misc,
          GenericTypes,
          Classes,
        };
        auto unimplementedStatic = [&](unsigned diagSel) {
          auto staticLoc = PBD->getStaticLoc();
          TC.diagnose(VD->getLoc(), diag::unimplemented_static_var,
                      diagSel, PBD->getStaticSpelling(),
                      diagSel == Classes)
            .highlight(staticLoc);
        };

        auto DC = VD->getDeclContext();

        // Non-stored properties are fine.
        if (!PBD->hasStorage()) {
          // do nothing

        // Stored type variables in a generic context need to logically
        // occur once per instantiation, which we don't yet handle.
        } else if (DC->isGenericContext()) {
          unimplementedStatic(GenericTypes);
        } else if (DC->getAsClassOrClassExtensionContext()) {
          auto StaticSpelling = PBD->getStaticSpelling();
          if (StaticSpelling != StaticSpellingKind::KeywordStatic)
            unimplementedStatic(Classes);
        }
      }
    }

    // Synthesize accessors for lazy, all checking already been performed.
    if (VD->getAttrs().hasAttribute<LazyAttr>() && !VD->isStatic() &&
        !VD->getGetter()->hasBody())
      TC.completeLazyVarImplementation(VD);

    // If this is a willSet/didSet property, synthesize the getter and setter
    // decl.
    if (VD->hasObservers() && !VD->getGetter()->getBody())
      synthesizeObservingAccessors(VD, TC);

    // If this is a get+mutableAddress property, synthesize the setter body.
    if (VD->getStorageKind() == VarDecl::ComputedWithMutableAddress &&
        !VD->getSetter()->getBody()) {
      synthesizeSetterForMutableAddressedStorage(VD, TC);
    }

    // Typecheck any accessors that were previously synthesized
    // (that were previously only validated at point of synthesis)
    if (auto getter = VD->getGetter()) {
      if (getter->hasBody()) {
        TC.typeCheckDecl(getter, true);
        TC.typeCheckDecl(getter, false);
      }
    }
    if (auto setter = VD->getSetter()) {
      if (setter->hasBody()) {
        TC.typeCheckDecl(setter, true);
        TC.typeCheckDecl(setter, false);
      }
    }

    TC.checkDeclAttributes(VD);
  }


  void visitBoundVars(Pattern *P) {
    P->forEachVariable([&] (VarDecl *VD) { this->visitBoundVariable(VD); });
  }

  void visitPatternBindingDecl(PatternBindingDecl *PBD) {
    // Check all the pattern/init pairs in the PBD.
    for (unsigned i = 0, e = PBD->getNumPatternEntries(); i != e; ++i)
      validatePatternBindingDecl(TC, PBD, i);

    if (PBD->isBeingValidated())
      return;

    // If the initializers in the PBD aren't checked yet, do so now.
    if (!IsFirstPass) {
      for (unsigned i = 0, e = PBD->getNumPatternEntries(); i != e; ++i) {
        if (!PBD->isInitializerChecked(i) && PBD->getInit(i))
          TC.typeCheckPatternBinding(PBD, i, /*skipClosures*/false);
      }
    }

    TC.checkDeclAttributesEarly(PBD);

    if (!IsSecondPass) {
      for (unsigned i = 0, e = PBD->getNumPatternEntries(); i != e; ++i) {
        // Type check each VarDecl that this PatternBinding handles.
        visitBoundVars(PBD->getPattern(i));

        // If we have a type but no initializer, check whether the type is
        // default-initializable. If so, do it.
        if (PBD->getPattern(i)->hasType() &&
            !PBD->getInit(i) &&
            PBD->getPattern(i)->hasStorage() &&
            !PBD->getPattern(i)->getType()->hasError()) {

          // If we have a type-adjusting attribute (like ownership), apply it now.
          if (auto var = PBD->getSingleVar())
            TC.checkTypeModifyingDeclAttributes(var);

          // Decide whether we should suppress default initialization.
          if (isNeverDefaultInitializable(PBD->getPattern(i)))
            continue;

          auto type = PBD->getPattern(i)->getType();
          if (auto defaultInit = buildDefaultInitializer(TC, type)) {
            // If we got a default initializer, install it and re-type-check it
            // to make sure it is properly coerced to the pattern type.
            PBD->setInit(i, defaultInit);
            TC.typeCheckPatternBinding(PBD, i, /*skipClosures*/false);
          }
        }
      }
    }

    bool isInSILMode = false;
    if (auto sourceFile = PBD->getDeclContext()->getParentSourceFile())
      isInSILMode = sourceFile->Kind == SourceFileKind::SIL;
    bool isTypeContext = PBD->getDeclContext()->isTypeContext();

    // If this is a declaration without an initializer, reject code if
    // uninitialized vars are not allowed.
    for (unsigned i = 0, e = PBD->getNumPatternEntries(); i != e; ++i) {
      auto entry = PBD->getPatternList()[i];
    
      if (entry.getInit() || isInSILMode) continue;
      
      entry.getPattern()->forEachVariable([&](VarDecl *var) {
        // If the variable has no storage, it never needs an initializer.
        if (!var->hasStorage())
          return;

        auto *varDC = var->getDeclContext();

        // Non-member observing properties need an initializer.
        if (var->getStorageKind() == VarDecl::StoredWithObservers &&
            !isTypeContext) {
          TC.diagnose(var->getLoc(), diag::observingprop_requires_initializer);
          PBD->setInvalid();
          var->setInvalid();
          if (!var->hasType()) {
            var->markInvalid();
          }
          return;
        }

        // Static/class declarations require an initializer unless in a
        // protocol.
        if (var->isStatic() && !isa<ProtocolDecl>(varDC) &&
            !var->isInvalid() && !PBD->isInvalid()) {
          TC.diagnose(var->getLoc(), diag::static_requires_initializer,
                      var->getCorrectStaticSpelling());
          PBD->setInvalid();
          var->setInvalid();
          if (!var->hasType()) {
            var->markInvalid();
          }
          return;
        }

        // Global variables require an initializer (except in top level code).
        if (varDC->isModuleScopeContext() &&
            !varDC->getParentSourceFile()->isScriptMode() &&
            !var->isInvalid() && !PBD->isInvalid()) {
          TC.diagnose(var->getLoc(),
                      diag::global_requires_initializer, var->isLet());
          PBD->setInvalid();
          var->setInvalid();
          if (!var->hasType()) {
            var->markInvalid();
          }
          return;
        }
      });
    }

    if (!IsFirstPass)
      checkAccessibility(TC, PBD);

    TC.checkDeclAttributes(PBD);
  }

  void visitSubscriptDecl(SubscriptDecl *SD) {
    if (IsSecondPass) {
      checkAccessibility(TC, SD);
      return;
    }

    if (SD->hasInterfaceType())
      return;

    auto dc = SD->getDeclContext();
    assert(dc->isTypeContext() &&
           "Decl parsing must prevent subscripts outside of types!");

    TC.checkDeclAttributesEarly(SD);
    TC.computeAccessibility(SD);

    GenericTypeToArchetypeResolver resolver(dc);

    bool isInvalid = TC.validateType(SD->getElementTypeLoc(), dc,
                                     TypeResolutionOptions(),
                                     &resolver);
    isInvalid |= TC.typeCheckParameterList(SD->getIndices(), dc,
                                           TypeResolutionOptions(),
                                           resolver);

    if (isInvalid) {
      SD->setInterfaceType(ErrorType::get(TC.Context));
      SD->setInvalid();
    } else {
      auto indicesIfaceTy = SD->getIndices()->getInterfaceType(TC.Context);

      auto elementTy = SD->getElementTypeLoc().getType();
      auto elementIfaceTy = dc->mapTypeOutOfContext(elementTy);

      SD->setInterfaceType(FunctionType::get(indicesIfaceTy, elementIfaceTy));
    }

    validateAttributes(TC, SD);

    if (!checkOverrides(TC, SD)) {
      // If a subscript has an override attribute but does not override
      // anything, complain.
      if (auto *OA = SD->getAttrs().getAttribute<OverrideAttr>()) {
        if (!SD->getOverriddenDecl()) {
          TC.diagnose(SD, diag::subscript_does_not_override)
              .highlight(OA->getLocation());
          OA->setInvalid();
        }
      }
    }

    // Member subscripts need some special validation logic.
    if (auto nominalDecl = dc->getAsNominalTypeOrNominalTypeExtensionContext()) {
      // If this is a class member, mark it final if the class is final.
      if (auto cls = dyn_cast<ClassDecl>(nominalDecl)) {
        if (cls->isFinal() && !SD->isFinal()) {
          makeFinal(TC.Context, SD);
        }
      }

      // A subscript is ObjC-compatible if it's explicitly @objc, or a
      // member of an ObjC-compatible class or protocol.
      Optional<ObjCReason> isObjC = shouldMarkAsObjC(TC, SD);

      if (isObjC && !TC.isRepresentableInObjC(SD, *isObjC))
        isObjC = None;
      markAsObjC(TC, SD, isObjC);

      // Infer 'dynamic' before touching accessors.
      inferDynamic(TC.Context, SD);
    }

    validateAbstractStorageDecl(SD, TC);

    // If this is a get+mutableAddress property, synthesize the setter body.
    if (SD->getStorageKind() == SubscriptDecl::ComputedWithMutableAddress &&
        !SD->getSetter()->getBody()) {
      synthesizeSetterForMutableAddressedStorage(SD, TC);
    }

    TC.checkDeclAttributes(SD);
  }

  void visitTypeAliasDecl(TypeAliasDecl *TAD) {
    TC.checkDeclAttributesEarly(TAD);
    TC.computeAccessibility(TAD);

    if (!IsSecondPass)
      TC.validateDecl(TAD);

    if (IsSecondPass)
      checkAccessibility(TC, TAD);

    TC.checkDeclAttributes(TAD);
  }
  
  void visitAssociatedTypeDecl(AssociatedTypeDecl *assocType) {
    if (!assocType->hasInterfaceType())
      TC.validateDecl(assocType);
  }

  void checkUnsupportedNestedType(NominalTypeDecl *NTD) {
    TC.diagnoseInlineableLocalType(NTD);

    // We don't support protocols outside the top level of a file.
    if (isa<ProtocolDecl>(NTD) &&
        !NTD->getParent()->isModuleScopeContext()) {
      TC.diagnose(NTD->getLoc(),
                  diag::unsupported_nested_protocol,
                  NTD->getName());
      return;
    }

    // We don't support nested types in generics yet.
    if (NTD->isGenericContext()) {
      auto DC = NTD->getDeclContext();
      if (auto proto = DC->getAsProtocolOrProtocolExtensionContext()) {
        if (DC->getAsProtocolExtensionContext()) {
          TC.diagnose(NTD->getLoc(),
                      diag::unsupported_type_nested_in_protocol_extension,
                      NTD->getName(),
                      proto->getName());
        } else {
          TC.diagnose(NTD->getLoc(),
                      diag::unsupported_type_nested_in_protocol,
                      NTD->getName(),
                      proto->getName());
        }
      }

      if (DC->isLocalContext() && DC->isGenericContext()) {
        // A local generic context is a generic function.
        if (auto AFD = dyn_cast<AbstractFunctionDecl>(DC)) {
          TC.diagnose(NTD->getLoc(),
                      diag::unsupported_type_nested_in_generic_function,
                      NTD->getName(),
                      AFD->getName());
        } else {
          TC.diagnose(NTD->getLoc(),
                      diag::unsupported_type_nested_in_generic_closure,
                      NTD->getName());
        }
      }
    }
  }

  void visitEnumDecl(EnumDecl *ED) {
    TC.checkDeclAttributesEarly(ED);
    TC.computeAccessibility(ED);

    if (!IsSecondPass) {
      checkUnsupportedNestedType(ED);

      TC.validateDecl(ED);

      TC.ValidatedTypes.remove(ED);

      {
        // Check for circular inheritance of the raw type.
        SmallVector<EnumDecl *, 8> path;
        path.push_back(ED);
        checkCircularity(TC, ED, diag::circular_enum_inheritance,
                         diag::enum_here, path);
      }
      {
        // Check for duplicate enum members.
        llvm::DenseMap<Identifier, EnumElementDecl *> Elements;
        for (auto *EED : ED->getAllElements()) {
          auto Res = Elements.insert({ EED->getName(), EED });
          if (!Res.second) {
            EED->setInterfaceType(ErrorType::get(TC.Context));
            EED->setInvalid();
            if (auto *RawValueExpr = EED->getRawValueExpr())
              RawValueExpr->setType(ErrorType::get(TC.Context));

            auto PreviousEED = Res.first->second;
            TC.diagnose(EED->getLoc(), diag::duplicate_enum_element);
            TC.diagnose(PreviousEED->getLoc(),
                        diag::previous_decldef, true, EED->getName());
          }
        }
      }
    }

    if (!IsFirstPass) {
      checkAccessibility(TC, ED);

      if (ED->hasRawType() && !ED->isObjC()) {
        // ObjC enums have already had their raw values checked, but pure Swift
        // enums haven't.
        checkEnumRawValues(TC, ED);
      }

      TC.checkConformancesInContext(ED, ED);
    }
    
    for (Decl *member : ED->getMembers())
      visit(member);
    

    TC.checkDeclAttributes(ED);
  }

  void visitStructDecl(StructDecl *SD) {
    TC.checkDeclAttributesEarly(SD);
    TC.computeAccessibility(SD);

    if (!IsSecondPass) {
      checkUnsupportedNestedType(SD);

      TC.validateDecl(SD);
      TC.ValidatedTypes.remove(SD);
      TC.addImplicitConstructors(SD);
    }

    if (!IsFirstPass) {
      checkAccessibility(TC, SD);

      if (!SD->isInvalid())
        TC.checkConformancesInContext(SD, SD);
    }

    // Visit each of the members.
    for (Decl *Member : SD->getMembers())
      visit(Member);

    TC.checkDeclAttributes(SD);
  }

  /// Check whether the given properties can be @NSManaged in this class.
  static bool propertiesCanBeNSManaged(ClassDecl *classDecl,
                                       ArrayRef<VarDecl *> vars) {
    // Check whether we have an Objective-C-defined class in our
    // inheritance chain.
    while (classDecl) {
      // If we found an Objective-C-defined class, continue checking.
      if (classDecl->hasClangNode())
        break;

      // If we ran out of superclasses, we're done.
      if (!classDecl->hasSuperclass())
        return false;

      classDecl = classDecl->getSuperclass()->getClassOrBoundGenericClass();
    }

    // If all of the variables are @objc, we can use @NSManaged.
    for (auto var : vars) {
      if (!var->isObjC())
        return false;
    }

    // Okay, we can use @NSManaged.
    return true;
  }

  /// Check that all stored properties have in-class initializers.
  void checkRequiredInClassInits(ClassDecl *cd) {
    ClassDecl *source = nullptr;
    for (auto member : cd->getMembers()) {
      auto pbd = dyn_cast<PatternBindingDecl>(member);
      if (!pbd)
        continue;

      if (pbd->isStatic() || !pbd->hasStorage() || 
          isDefaultInitializable(pbd) || pbd->isInvalid())
        continue;

      // The variables in this pattern have not been
      // initialized. Diagnose the lack of initial value.
      pbd->setInvalid();
      SmallVector<VarDecl *, 4> vars;
      for (auto entry : pbd->getPatternList())
        entry.getPattern()->collectVariables(vars);
      bool suggestNSManaged = propertiesCanBeNSManaged(cd, vars);
      switch (vars.size()) {
      case 0:
        llvm_unreachable("should have been marked invalid");

      case 1:
        TC.diagnose(pbd->getLoc(), diag::missing_in_class_init_1,
                    vars[0]->getName(), suggestNSManaged);
        break;

      case 2:
        TC.diagnose(pbd->getLoc(), diag::missing_in_class_init_2,
                    vars[0]->getName(), vars[1]->getName(), suggestNSManaged);
        break;

      case 3:
        TC.diagnose(pbd->getLoc(), diag::missing_in_class_init_3plus,
                    vars[0]->getName(), vars[1]->getName(), vars[2]->getName(),
                    false, suggestNSManaged);
        break;

      default:
        TC.diagnose(pbd->getLoc(), diag::missing_in_class_init_3plus,
                    vars[0]->getName(), vars[1]->getName(), vars[2]->getName(),
                    true, suggestNSManaged);
        break;
      }

      // Figure out where this requirement came from.
      if (!source) {
        source = cd;
        while (true) {
          // If this class had the 'requires_stored_property_inits'
          // attribute, diagnose here.
          if (source->getAttrs().
                hasAttribute<RequiresStoredPropertyInitsAttr>())
            break;

          // If the superclass doesn't require in-class initial
          // values, the requirement was introduced at this point, so
          // stop here.
          auto superclass = cast<ClassDecl>(
                              source->getSuperclass()->getAnyNominal());
          if (!superclass->requiresStoredPropertyInits())
            break;

          // Keep looking.
          source = superclass;
        }
      }

      // Add a note describing why we need an initializer.
      TC.diagnose(source, diag::requires_stored_property_inits_here,
                  source->getDeclaredType(), cd == source, suggestNSManaged);
    }
  }


  void visitClassDecl(ClassDecl *CD) {
    TC.checkDeclAttributesEarly(CD);
    TC.computeAccessibility(CD);

    if (!IsSecondPass) {
      checkUnsupportedNestedType(CD);

      TC.validateDecl(CD);
      if (!CD->hasValidSignature())
        return;

      TC.ValidatedTypes.remove(CD);

      {
        // Check for circular inheritance.
        SmallVector<ClassDecl *, 8> path;
        path.push_back(CD);
        checkCircularity(TC, CD, diag::circular_class_inheritance,
                         diag::class_here, path);
      }
    }

    // If this class needs an implicit constructor, add it.
    if (!IsFirstPass)
      TC.addImplicitConstructors(CD);

    TC.addImplicitDestructor(CD);

    if (!IsFirstPass && !CD->isInvalid())
      TC.checkConformancesInContext(CD, CD);

    for (Decl *Member : CD->getMembers())
      visit(Member);

    // If this class requires all of its stored properties to have
    // in-class initializers, diagnose this now.
    if (CD->requiresStoredPropertyInits())
      checkRequiredInClassInits(CD);

    if (!IsFirstPass) {
      if (auto superclassTy = CD->getSuperclass()) {
        ClassDecl *Super = superclassTy->getClassOrBoundGenericClass();

        if (auto *SF = CD->getParentSourceFile()) {
          if (auto *tracker = SF->getReferencedNameTracker()) {
            bool isPrivate =
                CD->getFormalAccess() <= Accessibility::FilePrivate;
            tracker->addUsedMember({Super, Identifier()}, !isPrivate);
          }
        }

        bool isInvalidSuperclass = false;

        if (Super->isFinal()) {
          TC.diagnose(CD, diag::inheritance_from_final_class,
                      Super->getName());
          // FIXME: should this really be skipping the rest of decl-checking?
          return;
        }

        if (Super->hasClangNode() && Super->getGenericParams()
            && superclassTy->hasTypeParameter()) {
          TC.diagnose(CD,
                      diag::inheritance_from_unspecialized_objc_generic_class,
                      Super->getName());
        }

        switch (Super->getForeignClassKind()) {
        case ClassDecl::ForeignKind::Normal:
          break;
        case ClassDecl::ForeignKind::CFType:
          TC.diagnose(CD, diag::inheritance_from_cf_class,
                      Super->getName());
          isInvalidSuperclass = true;
          break;
        case ClassDecl::ForeignKind::RuntimeOnly:
          TC.diagnose(CD, diag::inheritance_from_objc_runtime_visible_class,
                      Super->getName());
          isInvalidSuperclass = true;
          break;
        }

        // Require the superclass to be open if this is outside its
        // defining module.  But don't emit another diagnostic if we
        // already complained about the class being inherently
        // un-subclassable.
        if (!isInvalidSuperclass &&
            Super->getFormalAccess(CD->getDeclContext())
              < Accessibility::Open &&
            Super->getModuleContext() != CD->getModuleContext()) {
          TC.diagnose(CD, diag::superclass_not_open, superclassTy);
          isInvalidSuperclass = true;
        }

        // Require superclasses to be open if the subclass is open.
        // This is a restriction we can consider lifting in the future,
        // e.g. to enable a "sealed" superclass whose subclasses are all
        // of one of several alternatives.
        if (!isInvalidSuperclass &&
            CD->getFormalAccess() == Accessibility::Open &&
            Super->getFormalAccess() != Accessibility::Open) {
          TC.diagnose(CD, diag::superclass_of_open_not_open, superclassTy);
          TC.diagnose(Super, diag::superclass_here);
        }

      }

      checkAccessibility(TC, CD);
    }

    TC.checkDeclAttributes(CD);
  }

  void visitProtocolDecl(ProtocolDecl *PD) {
    TC.checkDeclAttributesEarly(PD);
    TC.computeAccessibility(PD);

    if (!IsSecondPass)
      checkUnsupportedNestedType(PD);

    if (IsSecondPass) {
      checkAccessibility(TC, PD);
      for (auto member : PD->getMembers()) {
        TC.checkUnsupportedProtocolType(member);
        checkAccessibility(TC, member);
      }
      TC.checkInheritanceClause(PD);
      return;
    }

    TC.validateDecl(PD);
    if (!PD->hasValidSignature())
      return;

    {
      // Check for circular inheritance within the protocol.
      SmallVector<ProtocolDecl *, 8> path;
      path.push_back(PD);
      checkCircularity(TC, PD, diag::circular_protocol_def,
                       diag::protocol_here, path);

      // Make sure the parent protocols have been fully validated.
      for (auto inherited : PD->getLocalProtocols()) {
        TC.validateDecl(inherited);
        for (auto *member : inherited->getMembers())
          if (auto *requirement = dyn_cast<ValueDecl>(member))
            TC.validateDecl(requirement);
      }

      if (auto *SF = PD->getParentSourceFile()) {
        if (auto *tracker = SF->getReferencedNameTracker()) {
          bool isNonPrivate =
              (PD->getFormalAccess() > Accessibility::FilePrivate);
          for (auto *parentProto : PD->getInheritedProtocols(nullptr))
            tracker->addUsedMember({parentProto, Identifier()}, isNonPrivate);
        }
      }
    }

    // Check the members.
    for (auto Member : PD->getMembers())
      visit(Member);

    TC.checkDeclAttributes(PD);
  }

  void visitVarDecl(VarDecl *VD) {
    // Delay type-checking on VarDecls until we see the corresponding
    // PatternBindingDecl.
  }

  bool semaFuncParamPatterns(AbstractFunctionDecl *fd,
                             GenericTypeResolver &resolver) {
    bool hadError = false;
    for (auto paramList : fd->getParameterLists()) {
      hadError |= TC.typeCheckParameterList(paramList, fd,
                                            TypeResolutionOptions(),
                                            resolver);
    }

    return hadError;
  }

  bool semaFuncDecl(FuncDecl *FD, GenericTypeResolver &resolver) {
    TC.checkForForbiddenPrefix(FD);

    bool badType = false;
    if (!FD->getBodyResultTypeLoc().isNull()) {
      TypeResolutionOptions options;
      if (FD->hasDynamicSelf())
        options |= TR_DynamicSelfResult;
      if (TC.validateType(FD->getBodyResultTypeLoc(), FD, options,
                          &resolver)) {
        badType = true;
      }
    }

    badType |= semaFuncParamPatterns(FD, resolver);

    if (badType) {
      FD->setInterfaceType(ErrorType::get(TC.Context));
      FD->setInvalid();
      return true;
    }

    return false;
  }

  /// Bind the given function declaration, which declares an operator, to
  /// the corresponding operator declaration.
  void bindFuncDeclToOperator(FuncDecl *FD) {
    OperatorDecl *op = nullptr;
    auto operatorName = FD->getFullName().getBaseName();

    // Check for static/final/class when we're in a type.
    auto dc = FD->getDeclContext();
    if (dc->isTypeContext()) {
      // Within a class, operator functions must be 'static' or 'final'.
      if (auto classDecl = dc->getAsClassOrClassExtensionContext()) {
        // For a class, we also need the function or class to be 'final'.
        if (!classDecl->isFinal() && !FD->isFinal() &&
            FD->getStaticSpelling() != StaticSpellingKind::KeywordStatic) {
          if (!FD->isStatic()) {
            TC.diagnose(FD->getLoc(), diag::nonstatic_operator_in_type,
                        operatorName,
                        dc->getDeclaredInterfaceType())
              .fixItInsert(FD->getAttributeInsertionLoc(/*forModifier=*/true),
                           "static ");

            FD->setStatic();
          } else {
            TC.diagnose(FD->getLoc(), diag::nonfinal_operator_in_class,
                        operatorName, dc->getDeclaredInterfaceType())
              .fixItInsert(FD->getAttributeInsertionLoc(/*forModifier=*/true),
                           "final ");
            FD->getAttrs().add(new (TC.Context) FinalAttr(/*IsImplicit=*/true));
          }
        }
      } else if (!FD->isStatic()) {
        // Operator functions must be static.
        TC.diagnose(FD, diag::nonstatic_operator_in_type,
                    operatorName,
                    dc->getDeclaredInterfaceType())
          .fixItInsert(FD->getAttributeInsertionLoc(/*forModifier=*/true),
                       "static ");
        FD->setStatic();
      }
    } else if (!dc->isModuleScopeContext()) {
      TC.diagnose(FD, diag::operator_in_local_scope);
    }

    SourceFile &SF = *FD->getDeclContext()->getParentSourceFile();
    if (FD->isUnaryOperator()) {
      if (FD->getAttrs().hasAttribute<PrefixAttr>()) {
        op = SF.lookupPrefixOperator(operatorName,
                                     FD->isCascadingContextForLookup(false),
                                     FD->getLoc());
      } else if (FD->getAttrs().hasAttribute<PostfixAttr>()) {
        op = SF.lookupPostfixOperator(operatorName,
                                      FD->isCascadingContextForLookup(false),
                                      FD->getLoc());
      } else {
        auto prefixOp =
            SF.lookupPrefixOperator(operatorName,
                                    FD->isCascadingContextForLookup(false),
                                    FD->getLoc());
        auto postfixOp =
            SF.lookupPostfixOperator(operatorName,
                                     FD->isCascadingContextForLookup(false),
                                     FD->getLoc());

        // If we found both prefix and postfix, or neither prefix nor postfix,
        // complain. We can't fix this situation.
        if (static_cast<bool>(prefixOp) == static_cast<bool>(postfixOp)) {
          TC.diagnose(FD, diag::declared_unary_op_without_attribute);

          // If we found both, point at them.
          if (prefixOp) {
            TC.diagnose(prefixOp, diag::unary_operator_declaration_here, false)
              .fixItInsert(FD->getLoc(), "prefix ");
            TC.diagnose(postfixOp, diag::unary_operator_declaration_here, true)
              .fixItInsert(FD->getLoc(), "postfix ");
          } else {
            // FIXME: Introduce a Fix-It that adds the operator declaration?
          }

          // FIXME: Errors could cascade here, because name lookup for this
          // operator won't find this declaration.
          return;
        }

        // We found only one operator declaration, so we know whether this
        // should be a prefix or a postfix operator.

        // Fix the AST and determine the insertion text.
        const char *insertionText;
        auto &C = FD->getASTContext();
        if (postfixOp) {
          insertionText = "postfix ";
          op = postfixOp;
          FD->getAttrs().add(new (C) PostfixAttr(/*implicit*/false));
        } else {
          insertionText = "prefix ";
          op = prefixOp;
          FD->getAttrs().add(new (C) PrefixAttr(/*implicit*/false));
        }

        // Emit diagnostic with the Fix-It.
        TC.diagnose(FD->getFuncLoc(), diag::unary_op_missing_prepos_attribute,
                    static_cast<bool>(postfixOp))
          .fixItInsert(FD->getFuncLoc(), insertionText);
        TC.diagnose(op, diag::unary_operator_declaration_here,
                    static_cast<bool>(postfixOp));
      }
    } else if (FD->isBinaryOperator()) {
      op = SF.lookupInfixOperator(operatorName,
                                  FD->isCascadingContextForLookup(false),
                                  FD->getLoc());
    } else {
      TC.diagnose(FD, diag::invalid_arg_count_for_operator);
      return;
    }

    if (!op) {
      // FIXME: Add Fix-It introducing an operator declaration?
      TC.diagnose(FD, diag::declared_operator_without_operator_decl);
      return;
    }

    FD->setOperatorDecl(op);
  }

  /// Determine whether the given declaration requires a definition.
  ///
  /// Only valid for declarations that can have definitions, i.e.,
  /// functions, initializers, etc.
  static bool requiresDefinition(Decl *decl) {
    // Invalid, implicit, and Clang-imported declarations never
    // require a definition.
    if (decl->isInvalid() || decl->isImplicit() || decl->hasClangNode())
      return false;

    // Functions can have _silgen_name, semantics, and NSManaged attributes.
    if (auto func = dyn_cast<AbstractFunctionDecl>(decl)) {
      if (func->getAttrs().hasAttribute<SILGenNameAttr>() ||
          func->getAttrs().hasAttribute<SemanticsAttr>() ||
          func->getAttrs().hasAttribute<NSManagedAttr>())
        return false;
    }

    // Declarations in SIL don't require definitions.
    if (auto sourceFile = decl->getDeclContext()->getParentSourceFile()) {
      if (sourceFile->Kind == SourceFileKind::SIL)
        return false;
    }

    // Everything else requires a definition.
    return true;
  }

  /// Check for methods that return 'DynamicResult'.
  bool checkDynamicSelfReturn(FuncDecl *func) {
    // Check whether we have a specified result type.
    auto typeRepr = func->getBodyResultTypeLoc().getTypeRepr();
    if (!typeRepr)
      return false;

    // 'Self' on a property accessor is not dynamic 'Self'...even on a read-only
    // property. We could implement it as such in the future.
    if (func->isAccessor())
      return false;

    return checkDynamicSelfReturn(func, typeRepr, 0);
  }

  bool checkDynamicSelfReturn(FuncDecl *func, TypeRepr *typeRepr,
                              unsigned optionalDepth) {
    // Look through parentheses.
    if (auto parenRepr = dyn_cast<TupleTypeRepr>(typeRepr)) {
      if (!parenRepr->isParenType()) return false;
      return checkDynamicSelfReturn(func, parenRepr->getElement(0),
                                    optionalDepth);
    }

    // Look through attributes.
    if (auto attrRepr = dyn_cast<AttributedTypeRepr>(typeRepr)) {
      TypeAttributes attrs = attrRepr->getAttrs();
      if (!attrs.empty())
        return false;
      return checkDynamicSelfReturn(func, attrRepr->getTypeRepr(),
                                    optionalDepth);
    }

    // Look through optional types.
    if (auto attrRepr = dyn_cast<OptionalTypeRepr>(typeRepr)) {
      // But only one level.
      if (optionalDepth != 0) return false;
      return checkDynamicSelfReturn(func, attrRepr->getBase(),
                                    optionalDepth + 1);
    }

    // Check whether we have a simple identifier type.
    auto simpleRepr = dyn_cast<SimpleIdentTypeRepr>(typeRepr);
    if (!simpleRepr)
      return false;

    // Check whether it is 'Self'.
    if (simpleRepr->getIdentifier() != TC.Context.Id_Self)
      return false;

    // 'Self' in protocol extensions is not dynamic 'Self'.
    DeclContext *dc = func->getDeclContext();
    for (auto parentDC = dc; !parentDC->isModuleScopeContext();
         parentDC = parentDC->getParent()) {
      if (parentDC->getAsProtocolExtensionContext()) {
        return false;
      }
    }

    // Dynamic 'Self' is only permitted on methods.
    if (!dc->isTypeContext()) {
      TC.diagnose(simpleRepr->getIdLoc(), diag::dynamic_self_non_method,
                  dc->isLocalContext());
      simpleRepr->setInvalid();
      return true;
    }

    // 'Self' is only a dynamic self on class methods and
    // protocol requirements.
    auto declaredType = dc->getDeclaredTypeOfContext();
    if (declaredType->hasError())
      return false;

    auto nominal = declaredType->getAnyNominal();
    if (!isa<ClassDecl>(nominal) && !isa<ProtocolDecl>(nominal)) {
      int which;
      if (isa<StructDecl>(nominal))
        which = 0;
      else if (isa<EnumDecl>(nominal))
        which = 1;
      else
        llvm_unreachable("Unknown nominal type");
      TC.diagnose(simpleRepr->getIdLoc(), diag::dynamic_self_struct_enum,
                  which, nominal->getName())
        .fixItReplace(simpleRepr->getIdLoc(), nominal->getName().str());
      simpleRepr->setInvalid();
      return true;
    }

    // Note that the function has a dynamic Self return type and set
    // the return type component to the dynamic self type.
    func->setDynamicSelf(true);
    return false;
  }

  /// Determine whether this is an unparenthesized closure type.
  static AnyFunctionType *isUnparenthesizedTrailingClosure(Type type) {
    if (isa<ParenType>(type.getPointer()))
      return nullptr;

    // Only consider the rvalue type.
    type = type->getRValueType();

    // Look through one level of optionality.
    if (auto objectType = type->getAnyOptionalObjectType())
      type = objectType;

    // Is it a function type?
    return type->getAs<AnyFunctionType>();
  }

  void checkMemberOperator(FuncDecl *FD) {
    // Check that member operators reference the type of 'Self'.
    if (FD->getNumParameterLists() != 2 || FD->isInvalid()) return;

    auto *DC = FD->getDeclContext();
    auto selfNominal = DC->getAsNominalTypeOrNominalTypeExtensionContext();
    if (!selfNominal) return;

    // Check the parameters for a reference to 'Self'.
    bool isProtocol = isa<ProtocolDecl>(selfNominal);
    for (auto param : *FD->getParameterList(1)) {
      auto paramType = param->getInterfaceType();
      if (!paramType) break;

      // Look through 'inout'.
      paramType = paramType->getInOutObjectType();
      // Look through a metatype reference, if there is one.
      if (auto metatypeType = paramType->getAs<AnyMetatypeType>())
        paramType = metatypeType->getInstanceType();

      // Is it the same nominal type?
      if (paramType->getAnyNominal() == selfNominal) return;

      if (isProtocol) {
        // For a protocol, is it the 'Self' type parameter?
        if (auto genericParam = paramType->getAs<GenericTypeParamType>())
          if (genericParam->isEqual(DC->getSelfInterfaceType()))
            return;
      }
    }

    // We did not find 'Self'. Complain.
    TC.diagnose(FD, diag::operator_in_unrelated_type,
                FD->getDeclContext()->getDeclaredInterfaceType(),
                isProtocol, FD->getFullName());
  }

  void visitFuncDecl(FuncDecl *FD) {
    if (!IsFirstPass) {
      if (FD->hasBody()) {
        // Record the body.
        TC.definedFunctions.push_back(FD);
      } else if (requiresDefinition(FD)) {
        // Complain if we should have a body.
        TC.diagnose(FD->getLoc(), diag::func_decl_without_brace);
      }
    }

    if (IsSecondPass) {
      checkAccessibility(TC, FD);
      return;
    }

    TC.checkDeclAttributesEarly(FD);
    TC.computeAccessibility(FD);

    if (FD->hasInterfaceType() || FD->isBeingValidated())
      return;

    FD->setIsBeingValidated();

    // Bind operator functions to the corresponding operator declaration.
    if (FD->isOperator())
      bindFuncDeclToOperator(FD);

    // Validate 'static'/'class' on functions in extensions.
    auto StaticSpelling = FD->getStaticSpelling();
    if (StaticSpelling != StaticSpellingKind::None &&
        FD->getDeclContext()->isExtensionContext()) {
      if (auto *NTD = FD->getDeclContext()
              ->getAsNominalTypeOrNominalTypeExtensionContext()) {
        if (!isa<ClassDecl>(NTD)) {
          if (StaticSpelling == StaticSpellingKind::KeywordClass) {
            TC.diagnose(FD, diag::class_func_not_in_class)
                .fixItReplace(FD->getStaticLoc(), "static");
            TC.diagnose(NTD, diag::extended_type_declared_here);
          }
        }
      }
    }

    // Validate the mutating attribute if present, and install it into the bit
    // on funcdecl (instead of just being a DeclAttribute).
    if (FD->getAttrs().hasAttribute<MutatingAttr>())
      FD->setMutating(true);
    else if (FD->getAttrs().hasAttribute<NonMutatingAttr>())
      FD->setMutating(false);

    // Check whether the return type is dynamic 'Self'.
    if (checkDynamicSelfReturn(FD))
      FD->setInvalid();

    // Observing accessors (and their generated regular accessors) may have
    // the type of the var inferred.
    if (auto *VD = dyn_cast_or_null<VarDecl>(FD->getAccessorStorageDecl())) {
      if (VD->hasObservers()) {
        TC.validateDecl(VD);
        Type valueTy = VD->getType()->getReferenceStorageReferent();
        Type valueIfaceTy = VD->getInterfaceType()->getReferenceStorageReferent();
        if (FD->isObservingAccessor() || (FD->isSetter() && FD->isImplicit())) {
          unsigned firstParamIdx = FD->getParent()->isTypeContext();
          auto *firstParamPattern = FD->getParameterList(firstParamIdx);
          auto *newValueParam = firstParamPattern->get(0);
          newValueParam->setType(valueTy);
          newValueParam->setInterfaceType(valueIfaceTy);
        } else if (FD->isGetter() && FD->isImplicit()) {
          FD->getBodyResultTypeLoc().setType(valueIfaceTy, true);
        }
      }
    }

    // Before anything else, set up the 'self' argument correctly if present.
    if (FD->getDeclContext()->isTypeContext())
      configureImplicitSelf(TC, FD);

    // If we have generic parameters, check the generic signature now.
    if (auto gp = FD->getGenericParams()) {
      gp->setOuterParameters(FD->getDeclContext()->getGenericParamsOfContext());

      auto *sig = TC.validateGenericFuncSignature(FD);

      // Create a fresh archetype builder.
      ArchetypeBuilder builder =
        TC.createArchetypeBuilder(FD->getModuleContext());
      auto *parentSig = FD->getDeclContext()->getGenericSignatureOfContext();
      TC.checkGenericParamList(&builder, gp, parentSig, nullptr);

      // Infer requirements from parameter patterns.
      for (auto pattern : FD->getParameterLists()) {
        builder.inferRequirements(pattern, gp);
      }

      // Infer requirements from the result type.
      if (!FD->getBodyResultTypeLoc().isNull()) {
        unsigned depth = gp->getDepth();
        builder.inferRequirements(FD->getBodyResultTypeLoc(),
                                  /*minDepth=*/depth,
                                  /*maxDepth=*/depth);
      }

      // Revert the types within the signature so it can be type-checked with
      // archetypes below.
      TC.revertGenericFuncSignature(FD);

      // Assign archetypes.
      auto *env = builder.getGenericEnvironment(sig);
      FD->setGenericEnvironment(env);
    } else if (FD->getDeclContext()->getGenericSignatureOfContext()) {
      (void)TC.validateGenericFuncSignature(FD);
      // Revert all of the types within the signature of the function.
      TC.revertGenericFuncSignature(FD);
      FD->setGenericEnvironment(
          FD->getDeclContext()->getGenericEnvironmentOfContext());
    }

    // Set the context type of 'self'.
    if (FD->getDeclContext()->isTypeContext())
      recordSelfContextType(FD);

    // Type check the parameters and return type again, now with archetypes.
    GenericTypeToArchetypeResolver resolver(FD);
    if (semaFuncDecl(FD, resolver)) {
      FD->setIsBeingValidated(false);
      return;
    }

    if (!FD->getGenericSignatureOfContext())
      TC.configureInterfaceType(FD, FD->getGenericSignature());

    // We want the function to be available for name lookup as soon
    // as it has a valid interface type.
    FD->setIsBeingValidated(false);

    if (FD->isInvalid())
      return;

    validateAttributes(TC, FD);

    // Member functions need some special validation logic.
    if (FD->getDeclContext()->isTypeContext()) {
      if (!checkOverrides(TC, FD)) {
        // If a method has an 'override' keyword but does not
        // override anything, complain.
        if (auto *OA = FD->getAttrs().getAttribute<OverrideAttr>()) {
          if (!FD->getOverriddenDecl()) {
            TC.diagnose(FD, diag::method_does_not_override)
              .highlight(OA->getLocation());
            OA->setInvalid();
          }
        }
      }

      if (FD->isOperator())
        checkMemberOperator(FD);

      Optional<ObjCReason> isObjC = shouldMarkAsObjC(TC, FD);

      ProtocolDecl *protocolContext = dyn_cast<ProtocolDecl>(
          FD->getDeclContext());
      if (protocolContext && FD->isAccessor()) {
        // Don't complain about accessors in protocols.  We will emit a
        // diagnostic about the property itself.
        if (isObjC)
          isObjC = ObjCReason::DoNotDiagnose;
      }

      if (FD->isGetterOrSetter()) {
        // If the property decl is an instance property, its accessors will
        // be instance methods and the above condition will mark them ObjC.
        // The only additional condition we need to check is if the var decl
        // had an @objc or @iboutlet property.

        AbstractStorageDecl *storage = FD->getAccessorStorageDecl();
        // Validate the subscript or property because it might not be type
        // checked yet.
        TC.validateDecl(storage);

        if (storage->getAttrs().hasAttribute<NonObjCAttr>())
          isObjC = None;
        else if (!isObjC && storage->isObjC())
          isObjC = ObjCReason::DoNotDiagnose;

        // If the storage is dynamic, propagate to this accessor.
        if (isObjC && storage->isDynamic() && !FD->isDynamic())
          FD->getAttrs().add(new (TC.Context) DynamicAttr(/*implicit*/ true));
      }

      Optional<ForeignErrorConvention> errorConvention;
      if (isObjC &&
          (FD->isInvalid() || !TC.isRepresentableInObjC(FD, *isObjC,
                                                        errorConvention)))
        isObjC = None;
      markAsObjC(TC, FD, isObjC, errorConvention);
    }
    
    // If the function is exported to C, it must be representable in (Obj-)C.
    if (auto CDeclAttr = FD->getAttrs().getAttribute<swift::CDeclAttr>()) {
      Optional<ForeignErrorConvention> errorConvention;
      if (TC.isRepresentableInObjC(FD, ObjCReason::ExplicitlyCDecl,
                                   errorConvention)) {
        if (FD->hasThrows()) {
          FD->setForeignErrorConvention(*errorConvention);
          TC.diagnose(CDeclAttr->getLocation(), diag::cdecl_throws);
        }
      }
    }
    
    inferDynamic(TC.Context, FD);

    TC.checkDeclAttributes(FD);

    // If this is a class member, mark it final if the class is final.
    if (auto cls = FD->getDeclContext()->getAsClassOrClassExtensionContext()) {
      if (cls->isFinal() && !FD->isAccessor() &&
          !FD->isFinal() && !FD->isDynamic()) {
        makeFinal(TC.Context, FD);
      }
      // static func declarations in classes are synonyms
      // for `class final func` declarations.
      if (FD->getStaticSpelling() == StaticSpellingKind::KeywordStatic) {
        auto finalAttr = FD->getAttrs().getAttribute<FinalAttr>();
        if (finalAttr) {
          auto finalRange = finalAttr->getRange();
          if (finalRange.isValid())
            TC.diagnose(finalRange.Start, diag::decl_already_final)
            .highlight(finalRange)
            .fixItRemove(finalRange);
        }
        makeFinal(TC.Context, FD);
      }
    }

    // Check whether we have parameters with default arguments that follow a
    // closure parameter; warn about such things, because the closure will not
    // be treated as a trailing closure.
    if (!FD->isImplicit()) {
      auto paramList = FD->getParameterList(FD->getImplicitSelfDecl() ? 1 : 0);
      bool anyDefaultArguments = false;
      for (unsigned i = paramList->size(); i != 0; --i) {
        // Determine whether the parameter is of (possibly lvalue, possibly
        // optional), non-autoclosure function type, which could receive a
        // closure. We look at the type sugar directly, so that one can
        // suppress this warning by adding parentheses.
        auto &param = paramList->get(i-1);
        auto paramType = param->getType();

        if (auto *funcTy = isUnparenthesizedTrailingClosure(paramType)) {
          // If we saw any default arguments before this, complain.
          // This doesn't apply to autoclosures.
          if (anyDefaultArguments && !funcTy->getExtInfo().isAutoClosure()) {
            TC.diagnose(param->getStartLoc(),
                        diag::non_trailing_closure_before_default_args)
              .highlight(param->getSourceRange());
          }

          break;
        }

        // If we have a default argument, keep going.
        if (param->isDefaultArgument()) {
          anyDefaultArguments = true;
          continue;
        }

        // We're done.
        break;
      }
    }
  }

  void visitModuleDecl(ModuleDecl *) { }

  /// Perform basic checking to determine whether a declaration can override a
  /// declaration in a superclass.
  static bool areOverrideCompatibleSimple(ValueDecl *decl,
                                          ValueDecl *parentDecl) {
    // If the number of argument labels does not match, these overrides cannot
    // be compatible.
    if (decl->getFullName().getArgumentNames().size() !=
          parentDecl->getFullName().getArgumentNames().size())
      return false;

    if (auto func = dyn_cast<FuncDecl>(decl)) {
      // Specific checking for methods.
      auto parentFunc = cast<FuncDecl>(parentDecl);
      if (func->isStatic() != parentFunc->isStatic())
        return false;
      if (!!func->getGenericParams() != !!parentFunc->getGenericParams())
        return false;
    } else if (auto var = dyn_cast<VarDecl>(decl)) {
      auto parentVar = cast<VarDecl>(parentDecl);
      if (var->isStatic() != parentVar->isStatic())
        return false;
    }

    return true;
  }

  /// Drop the optionality of the result type of the given function type.
  static Type dropResultOptionality(Type type, unsigned uncurryLevel) {
    // We've hit the result type.
    if (uncurryLevel == 0) {
      if (auto objectTy = type->getAnyOptionalObjectType())
        return objectTy;

      return type;
    }

    // Determine the input and result types of this function.
    auto fnType = type->castTo<AnyFunctionType>();
    Type inputType = fnType->getInput();
    Type resultType = dropResultOptionality(fnType->getResult(), 
                                            uncurryLevel - 1);
    
    // Produce the resulting function type.
    if (auto genericFn = dyn_cast<GenericFunctionType>(fnType)) {
      return GenericFunctionType::get(genericFn->getGenericSignature(),
                                      inputType, resultType,
                                      fnType->getExtInfo());
    }
    
    return FunctionType::get(inputType, resultType, fnType->getExtInfo());    
  }

  static bool
  diagnoseMismatchedOptionals(TypeChecker &TC,
                              const ValueDecl *member,
                              const ParameterList *params,
                              TypeLoc resultTL,
                              const ValueDecl *parentMember,
                              Type owningTy,
                              bool treatIUOResultAsError) {
    bool emittedError = false;
    Type plainParentTy = owningTy->adjustSuperclassMemberDeclType(
        parentMember, member, parentMember->getInterfaceType(), &TC);
    const auto *parentTy = plainParentTy->castTo<AnyFunctionType>();
    if (isa<AbstractFunctionDecl>(parentMember))
      parentTy = parentTy->getResult()->castTo<AnyFunctionType>();

    // Check the parameter types.
    auto checkParam = [&](const ParamDecl *decl, Type parentParamTy) {
      Type paramTy = decl->getType();
      if (!paramTy || !parentParamTy)
        return;

      OptionalTypeKind paramOTK;
      (void)paramTy->getAnyOptionalObjectType(paramOTK);
      if (paramOTK == OTK_Optional)
        return;

      OptionalTypeKind parentOTK;
      (void)parentParamTy->getAnyOptionalObjectType(parentOTK);

      TypeLoc TL = decl->getTypeLoc();
      if (!TL.getTypeRepr())
        return;

      if (paramOTK == OTK_None) {
        switch (parentOTK) {
        case OTK_None:
          return;
        case OTK_ImplicitlyUnwrappedOptional:
          if (!treatIUOResultAsError)
            return;
          break;
        case OTK_Optional:
          break;
        }

        emittedError = true;
        auto diag = TC.diagnose(decl->getStartLoc(),
                                diag::override_optional_mismatch,
                                member->getDescriptiveKind(),
                                isa<SubscriptDecl>(member),
                                parentParamTy, paramTy);
        if (TL.getTypeRepr()->isSimple()) {
          diag.fixItInsertAfter(TL.getSourceRange().End, "?");
        } else {
          diag.fixItInsert(TL.getSourceRange().Start, "(");
          diag.fixItInsertAfter(TL.getSourceRange().End, ")?");
        }
        return;
      }

      if (parentOTK != OTK_None)
        return;

      // Allow silencing this warning using parens.
      if (isa<ParenType>(TL.getType().getPointer()))
        return;

      TC.diagnose(decl->getStartLoc(), diag::override_unnecessary_IUO,
                  member->getDescriptiveKind(), parentParamTy, paramTy)
        .highlight(TL.getSourceRange());

      auto sugaredForm =
        dyn_cast<ImplicitlyUnwrappedOptionalTypeRepr>(TL.getTypeRepr());
      if (sugaredForm) {
        TC.diagnose(sugaredForm->getExclamationLoc(),
                    diag::override_unnecessary_IUO_remove)
          .fixItRemove(sugaredForm->getExclamationLoc());
      }

      TC.diagnose(TL.getSourceRange().Start,
                  diag::override_unnecessary_IUO_silence)
        .fixItInsert(TL.getSourceRange().Start, "(")
        .fixItInsertAfter(TL.getSourceRange().End, ")");
    };

    auto parentInput = parentTy->getInput();
    
    if (auto parentTupleInput = parentInput->getAs<TupleType>()) {
      // FIXME: If we ever allow argument reordering, this is incorrect.
      ArrayRef<ParamDecl*> sharedParams = params->getArray();
      sharedParams = sharedParams.slice(0, parentTupleInput->getNumElements());
      for_each(sharedParams, parentTupleInput->getElementTypes(), checkParam);
    } else {
      // Otherwise, the parent has a single parameter with no label.
      checkParam(params->get(0), parentInput);
    }

    if (!resultTL.getTypeRepr())
      return emittedError;

    auto checkResult = [&](TypeLoc resultTL, Type parentResultTy) {
      Type resultTy = resultTL.getType();
      if (!resultTy || !parentResultTy)
        return;

      OptionalTypeKind resultOTK;
      if (!resultTy->getAnyOptionalObjectType(resultOTK))
        return;

      TypeRepr *TR = resultTL.getTypeRepr();

      if (resultOTK == OTK_Optional || treatIUOResultAsError) {
        if (parentResultTy->getAnyOptionalObjectType())
          return;
        emittedError = true;
        auto diag = TC.diagnose(resultTL.getSourceRange().Start,
                                diag::override_optional_result_mismatch,
                                member->getDescriptiveKind(),
                                isa<SubscriptDecl>(member),
                                parentResultTy, resultTy);
        if (auto optForm = dyn_cast<OptionalTypeRepr>(TR)) {
          diag.fixItRemove(optForm->getQuestionLoc());
        } else if (auto iuoForm =
            dyn_cast<ImplicitlyUnwrappedOptionalTypeRepr>(TR)) {
          diag.fixItRemove(iuoForm->getExclamationLoc());
        }
        return;
      }

      if (!parentResultTy->getAnyOptionalObjectType())
        return;

      // Allow silencing this warning using parens.
      if (isa<ParenType>(resultTy.getPointer()))
        return;

      TC.diagnose(resultTL.getSourceRange().Start,
                  diag::override_unnecessary_result_IUO,
                  member->getDescriptiveKind(), parentResultTy, resultTy)
        .highlight(resultTL.getSourceRange());

      auto sugaredForm = dyn_cast<ImplicitlyUnwrappedOptionalTypeRepr>(TR);
      if (sugaredForm) {
        TC.diagnose(sugaredForm->getExclamationLoc(),
                    diag::override_unnecessary_IUO_use_strict)
          .fixItReplace(sugaredForm->getExclamationLoc(), "?");
      }

      TC.diagnose(resultTL.getSourceRange().Start,
                  diag::override_unnecessary_IUO_silence)
        .fixItInsert(resultTL.getSourceRange().Start, "(")
        .fixItInsertAfter(resultTL.getSourceRange().End, ")");
    };

    checkResult(resultTL, parentTy->getResult());
    return emittedError;
  }

  /// Make sure that there is an invalid 'override' attribute on the
  /// given declaration.
  static void makeInvalidOverrideAttr(TypeChecker &TC, ValueDecl *decl) {
    if (auto overrideAttr = decl->getAttrs().getAttribute<OverrideAttr>()) {
      overrideAttr->setInvalid();
    } else {
      auto attr = new (TC.Context) OverrideAttr(true);
      decl->getAttrs().add(attr);
      attr->setInvalid();
    }

    if (auto storage = dyn_cast<AbstractStorageDecl>(decl)) {
      if (auto getter = storage->getGetter())
        makeInvalidOverrideAttr(TC, getter);
      if (auto setter = storage->getSetter())
        makeInvalidOverrideAttr(TC, setter);
    }
  }

  static void adjustFunctionTypeForOverride(Type &type) {
    // Drop 'throws'.
    // FIXME: Do we want to allow overriding a function returning a value
    // with one returning Never?
    auto fnType = type->castTo<AnyFunctionType>();
    auto extInfo = fnType->getExtInfo();
    extInfo = extInfo.withThrows(false);
    if (fnType->getExtInfo() != extInfo)
      type = fnType->withExtInfo(extInfo);
  }

  /// If the difference between the types of \p decl and \p base is something
  /// we feel confident about fixing (even partially), emit a note with fix-its
  /// attached. Otherwise, no note will be emitted.
  ///
  /// \returns true iff a diagnostic was emitted.
  static bool noteFixableMismatchedTypes(TypeChecker &TC, ValueDecl *decl,
                                         const ValueDecl *base) {
    DiagnosticTransaction tentativeDiags(TC.Diags);

    {
      Type baseTy = base->getInterfaceType();
      if (baseTy->hasError())
        return false;

      Optional<InFlightDiagnostic> activeDiag;
      if (auto *baseInit = dyn_cast<ConstructorDecl>(base)) {
        // Special-case initializers, whose "type" isn't useful besides the
        // input arguments.
        baseTy = baseTy->getAs<AnyFunctionType>()->getResult();
        Type argTy = baseTy->getAs<AnyFunctionType>()->getInput();
        auto diagKind = diag::override_type_mismatch_with_fixits_init;
        unsigned numArgs = baseInit->getParameters()->size();
        activeDiag.emplace(TC.diagnose(decl, diagKind,
                                       /*plural*/std::min(numArgs, 2U),
                                       argTy));
      } else {
        if (isa<AbstractFunctionDecl>(base))
          baseTy = baseTy->getAs<AnyFunctionType>()->getResult();

        activeDiag.emplace(TC.diagnose(decl,
                                       diag::override_type_mismatch_with_fixits,
                                       base->getDescriptiveKind(), baseTy));
      }

      if (fixItOverrideDeclarationTypes(*activeDiag, decl, base))
        return true;
    }

    // There weren't any fixes we knew how to make. Drop this diagnostic.
    tentativeDiags.abort();
    return false;
  }

  enum class OverrideCheckingAttempt {
    PerfectMatch,
    MismatchedOptional,
    MismatchedTypes,
    BaseName,
    BaseNameWithMismatchedOptional,
    Final
  };

  friend OverrideCheckingAttempt &operator++(OverrideCheckingAttempt &attempt) {
    assert(attempt != OverrideCheckingAttempt::Final);
    attempt = static_cast<OverrideCheckingAttempt>(1+static_cast<int>(attempt));
    return attempt;
  }

  struct OverrideMatch {
    ValueDecl *Decl;
    bool IsExact;
    Type SubstType;
  };

  static void diagnoseGeneralOverrideFailure(TypeChecker &TC,
                                             ValueDecl *decl,
                                             ArrayRef<OverrideMatch> matches,
                                             OverrideCheckingAttempt attempt) {
    switch (attempt) {
    case OverrideCheckingAttempt::PerfectMatch:
      TC.diagnose(decl, diag::override_multiple_decls_base,
                  decl->getFullName());
      break;
    case OverrideCheckingAttempt::BaseName:
      TC.diagnose(decl, diag::override_multiple_decls_arg_mismatch,
                  decl->getFullName());
      break;
    case OverrideCheckingAttempt::MismatchedOptional:
    case OverrideCheckingAttempt::MismatchedTypes:
    case OverrideCheckingAttempt::BaseNameWithMismatchedOptional:
      if (isa<ConstructorDecl>(decl))
        TC.diagnose(decl, diag::initializer_does_not_override);
      else if (isa<SubscriptDecl>(decl))
        TC.diagnose(decl, diag::subscript_does_not_override);
      else if (isa<VarDecl>(decl))
        TC.diagnose(decl, diag::property_does_not_override);
      else
        TC.diagnose(decl, diag::method_does_not_override);
      break;
    case OverrideCheckingAttempt::Final:
      llvm_unreachable("should have exited already");
    }

    for (auto match : matches) {
      auto matchDecl = match.Decl;
      if (attempt == OverrideCheckingAttempt::PerfectMatch) {
        TC.diagnose(matchDecl, diag::overridden_here);
        continue;
      }

      auto diag = TC.diagnose(matchDecl, diag::overridden_near_match_here,
                              matchDecl->getDescriptiveKind(),
                              matchDecl->getFullName());
      if (attempt == OverrideCheckingAttempt::BaseName) {
        fixDeclarationName(diag, cast<AbstractFunctionDecl>(decl),
                           matchDecl->getFullName());
      }
    }
  }

  /// Determine which method or subscript this method or subscript overrides
  /// (if any).
  ///
  /// \returns true if an error occurred.
  static bool checkOverrides(TypeChecker &TC, ValueDecl *decl) {
    if (decl->isInvalid() || decl->getOverriddenDecl())
      return false;

    auto owningTy = decl->getDeclContext()->getDeclaredInterfaceType();
    if (!owningTy)
      return false;

    auto classDecl = owningTy->getClassOrBoundGenericClass();
    if (!classDecl)
      return false;

    Type superclass = classDecl->getSuperclass();
    if (!superclass)
      return false;

    // Ignore accessor methods (e.g. getters and setters), they will be handled
    // when their storage decl is processed.
    if (auto *fd = dyn_cast<FuncDecl>(decl))
      if (fd->isAccessor())
        return false;
    
    auto method = dyn_cast<AbstractFunctionDecl>(decl);
    ConstructorDecl *ctor = nullptr;
    if (method)
      ctor = dyn_cast<ConstructorDecl>(method);

    auto abstractStorage = dyn_cast<AbstractStorageDecl>(decl);
    assert((method || abstractStorage) && "Not a method or abstractStorage?");
    SubscriptDecl *subscript = nullptr;
    if (abstractStorage)
      subscript = dyn_cast<SubscriptDecl>(abstractStorage);

    // Figure out the type of the declaration that we're using for comparisons.
    auto declTy = decl->getInterfaceType()->getUnlabeledType(TC.Context);
    if (method) {
      declTy = declTy->castTo<AnyFunctionType>()->getResult();
      adjustFunctionTypeForOverride(declTy);
    } else {
      declTy = declTy->getReferenceStorageReferent();
    }

    // Ignore the optionality of initializers when comparing types;
    // we'll enforce this separately
    if (ctor) {
      declTy = dropResultOptionality(declTy, 1);
    }

    // Look for members with the same name and matching types as this
    // one.
    auto attempt = OverrideCheckingAttempt::PerfectMatch;
    SmallVector<OverrideMatch, 2> matches;
    DeclName name = decl->getFullName();
    bool hadExactMatch = false;
    LookupResult members;

    do {
      switch (attempt) {
      case OverrideCheckingAttempt::PerfectMatch:
        break;
      case OverrideCheckingAttempt::MismatchedOptional:
        // Don't keep looking if the user didn't indicate it's an override.
        if (!decl->getAttrs().hasAttribute<OverrideAttr>())
          return false;
        break;
      case OverrideCheckingAttempt::MismatchedTypes:
        break;
      case OverrideCheckingAttempt::BaseName:
        // Don't keep looking if this is already a simple name, or if there
        // are no arguments.
        if (name.isSimpleName() || name.getArgumentNames().empty())
          return false;
        name = name.getBaseName();
        members.clear();
        break;
      case OverrideCheckingAttempt::BaseNameWithMismatchedOptional:
        break;
      case OverrideCheckingAttempt::Final:
        // Give up.
        return false;
      }

      if (members.empty()) {
        auto lookupOptions = defaultMemberLookupOptions;

        // Class methods cannot override declarations only
        // visible via dynamic dispatch.
        lookupOptions -= NameLookupFlags::DynamicLookup;

        // Class methods cannot override declarations only
        // visible as protocol requirements or protocol
        // extension members.
        lookupOptions -= NameLookupFlags::ProtocolMembers;
        lookupOptions -= NameLookupFlags::PerformConformanceCheck;

        members = TC.lookupMember(decl->getDeclContext(), superclass,
                                  name, lookupOptions);
      }

      for (auto memberResult : members) {
        auto member = memberResult.Decl;

        if (member->isInvalid())
          continue;

        if (member->getKind() != decl->getKind())
          continue;

        if (!member->getDeclContext()->getAsClassOrClassExtensionContext())
          continue;

        auto parentDecl = cast<ValueDecl>(member);

        // Check whether there are any obvious reasons why the two given
        // declarations do not have an overriding relationship.
        if (!areOverrideCompatibleSimple(decl, parentDecl))
          continue;

        auto parentMethod = dyn_cast<AbstractFunctionDecl>(parentDecl);
        auto parentStorage = dyn_cast<AbstractStorageDecl>(parentDecl);
        assert(parentMethod || parentStorage);

        // If both are Objective-C, then match based on selectors or
        // subscript kind and check the types separately.
        bool objCMatch = false;
        if (parentDecl->isObjC() && decl->isObjC()) {
          if (method) {
            if (method->getObjCSelector(&TC)
                  == parentMethod->getObjCSelector(&TC))
              objCMatch = true;
          } else if (auto *parentSubscript =
                       dyn_cast<SubscriptDecl>(parentStorage)) {
            // If the subscript kinds don't match, it's not an override.
            if (subscript->getObjCSubscriptKind(&TC)
                  == parentSubscript->getObjCSubscriptKind(&TC))
              objCMatch = true;
          }

          // Properties don't need anything here since they are always
          // checked by name.
        }

        // Check whether the types are identical.
        // FIXME: It's wrong to use the uncurried types here for methods.
        auto parentDeclTy = owningTy->adjustSuperclassMemberDeclType(
            parentDecl, decl, parentDecl->getInterfaceType(), &TC);
        parentDeclTy = parentDeclTy->getUnlabeledType(TC.Context);
        if (method) {
          parentDeclTy = parentDeclTy->castTo<AnyFunctionType>()->getResult();
          adjustFunctionTypeForOverride(parentDeclTy);
        } else {
          parentDeclTy = parentDeclTy->getReferenceStorageReferent();
        }

        // Ignore the optionality of initializers when comparing types;
        // we'll enforce this separately
        if (ctor) {
          parentDeclTy = dropResultOptionality(parentDeclTy, 1);

          // Factory methods cannot be overridden.
          auto parentCtor = cast<ConstructorDecl>(parentDecl);
          if (parentCtor->isFactoryInit())
            continue;
        }

        if (declTy->isEqual(parentDeclTy)) {
          matches.push_back({parentDecl, true, parentDeclTy});
          hadExactMatch = true;
          continue;
        }
        
        // If this is a property, we accept the match and then reject it below
        // if the types don't line up, since you can't overload properties based
        // on types.
        if (isa<VarDecl>(parentDecl) ||
            attempt == OverrideCheckingAttempt::MismatchedTypes) {
          matches.push_back({parentDecl, false, parentDeclTy});
          continue;
        }

        // Failing that, check for subtyping.
        auto matchMode = OverrideMatchMode::Strict;
        if (attempt == OverrideCheckingAttempt::MismatchedOptional ||
            attempt == OverrideCheckingAttempt::BaseNameWithMismatchedOptional){
          matchMode = OverrideMatchMode::AllowTopLevelOptionalMismatch;
        } else if (parentDecl->isObjC()) {
          matchMode = OverrideMatchMode::AllowNonOptionalForIUOParam;
        }

        if (declTy->canOverride(parentDeclTy, matchMode, &TC)) {
          // If the Objective-C selectors match, always call it exact.
          matches.push_back({parentDecl, objCMatch, parentDeclTy});
          hadExactMatch |= objCMatch;
          continue;
        }

        // Not a match. If we had an Objective-C match, this is a serious
        // problem.
        if (objCMatch) {
          if (method) {
            TC.diagnose(decl, diag::override_objc_type_mismatch_method,
                        method->getObjCSelector(&TC), declTy);
          } else {
            TC.diagnose(decl, diag::override_objc_type_mismatch_subscript,
                        static_cast<unsigned>(
                          subscript->getObjCSubscriptKind(&TC)),
                        declTy);
          }
          TC.diagnose(parentDecl, diag::overridden_here_with_type,
                      parentDeclTy);
          
          // Put an invalid 'override' attribute here.
          makeInvalidOverrideAttr(TC, decl);

          return true;
        }
      }
      if (!matches.empty())
        break;

      ++attempt;
    } while (true);

    assert(!matches.empty());

    // If we had an exact match, throw away any non-exact matches.
    if (hadExactMatch)
      matches.erase(std::remove_if(matches.begin(), matches.end(),
                                   [&](OverrideMatch &match) {
                                     return !match.IsExact;
                                   }), matches.end());

    // If we have a single match (exact or not), take it.
    if (matches.size() == 1) {
      auto matchDecl = matches.front().Decl;
      auto matchType = matches.front().SubstType;
      bool emittedMatchError = false;

      // If the name of our match differs from the name we were looking for,
      // complain.
      if (decl->getFullName() != matchDecl->getFullName()) {
        auto diag = TC.diagnose(decl, diag::override_argument_name_mismatch,
                                isa<ConstructorDecl>(decl),
                                decl->getFullName(),
                                matchDecl->getFullName());
        fixDeclarationName(diag, cast<AbstractFunctionDecl>(decl),
                           matchDecl->getFullName());
        emittedMatchError = true;
      }

      // If we have an explicit ownership modifier and our parent doesn't,
      // complain.
      auto parentAttr = matchDecl->getAttrs().getAttribute<OwnershipAttr>();
      if (auto ownershipAttr = decl->getAttrs().getAttribute<OwnershipAttr>()) {
        Ownership parentOwnership;
        if (parentAttr)
          parentOwnership = parentAttr->get();
        else
          parentOwnership = Ownership::Strong;
        if (parentOwnership != ownershipAttr->get()) {
          TC.diagnose(decl, diag::override_ownership_mismatch,
                      (unsigned)parentOwnership,
                      (unsigned)ownershipAttr->get());
          TC.diagnose(matchDecl, diag::overridden_here);
        }
      }

      // Check that the override has the required accessibility.
      // Overrides have to be at least as accessible as what they
      // override, except:
      //   - they don't have to be more accessible than their class and
      //   - a final method may be public instead of open.
      // Also diagnose attempts to override a non-open method from outside its
      // defining module.  This is not required for constructors, which are
      // never really "overridden" in the intended sense here, because of
      // course derived classes will change how the class is initialized.
      Accessibility matchAccess =
          matchDecl->getFormalAccess(decl->getDeclContext());
      if (matchAccess < Accessibility::Open &&
          matchDecl->getModuleContext() != decl->getModuleContext() &&
          !isa<ConstructorDecl>(decl)) {
        TC.diagnose(decl, diag::override_of_non_open,
                    decl->getDescriptiveKind());

      } else if (matchAccess == Accessibility::Open &&
                 classDecl->getFormalAccess(decl->getDeclContext()) ==
                   Accessibility::Open &&
                 decl->getFormalAccess() != Accessibility::Open &&
                 !decl->isFinal()) {
        {
          auto diag = TC.diagnose(decl, diag::override_not_accessible,
                                  /*setter*/false,
                                  decl->getDescriptiveKind(),
                                  /*fromOverridden*/true);
          fixItAccessibility(diag, decl, Accessibility::Open);
        }
        TC.diagnose(matchDecl, diag::overridden_here);

      } else {
        auto matchAccessScope =
          matchDecl->getFormalAccessScope(decl->getDeclContext());
        auto classAccessScope =
          classDecl->getFormalAccessScope(decl->getDeclContext());
        auto requiredAccessScope =
          matchAccessScope.intersectWith(classAccessScope);


        bool shouldDiagnose = false;
        bool shouldDiagnoseSetter = false;
        if (!isa<ConstructorDecl>(decl)) {
          auto DC = requiredAccessScope->getDeclContext();
          shouldDiagnose = !decl->isAccessibleFrom(DC);

          if (!shouldDiagnose && matchDecl->isSettable(decl->getDeclContext())){
            auto matchASD = cast<AbstractStorageDecl>(matchDecl);
            if (matchASD->isSetterAccessibleFrom(decl->getDeclContext())) {
              const auto *ASD = cast<AbstractStorageDecl>(decl);
              shouldDiagnoseSetter =
                  ASD->isSettable(DC) && !ASD->isSetterAccessibleFrom(DC);
            }
          }
        }
        if (shouldDiagnose || shouldDiagnoseSetter) {
          bool overriddenForcesAccess =
            (requiredAccessScope->hasEqualDeclContextWith(matchAccessScope) &&
             matchAccess != Accessibility::Open);
          Accessibility requiredAccess =
            requiredAccessScope->requiredAccessibilityForDiagnostics();
          {
            auto diag = TC.diagnose(decl, diag::override_not_accessible,
                                    shouldDiagnoseSetter,
                                    decl->getDescriptiveKind(),
                                    overriddenForcesAccess);
            fixItAccessibility(diag, decl, requiredAccess,
                               shouldDiagnoseSetter);
          }
          TC.diagnose(matchDecl, diag::overridden_here);
        }
      }

      bool mayHaveMismatchedOptionals =
          (attempt == OverrideCheckingAttempt::MismatchedOptional ||
           attempt == OverrideCheckingAttempt::BaseNameWithMismatchedOptional);

      // If this is an exact type match, we're successful!
      if (declTy->isEqual(matchType)) {
        // Nothing to do.
        
      } else if (method) {
        if (attempt == OverrideCheckingAttempt::MismatchedTypes) {
          auto diagKind = diag::method_does_not_override;
          if (ctor)
            diagKind = diag::initializer_does_not_override;
          TC.diagnose(decl, diagKind);
          noteFixableMismatchedTypes(TC, decl, matchDecl);
          TC.diagnose(matchDecl, diag::overridden_near_match_here,
                      matchDecl->getDescriptiveKind(),
                      matchDecl->getFullName());
          emittedMatchError = true;

        } else if ((!isa<FuncDecl>(method) ||
                    !cast<FuncDecl>(method)->isAccessor()) &&
                   (matchDecl->isObjC() || mayHaveMismatchedOptionals)) {
          // Private migration help for overrides of Objective-C methods.
          TypeLoc resultTL;
          if (auto *methodAsFunc = dyn_cast<FuncDecl>(method))
            resultTL = methodAsFunc->getBodyResultTypeLoc();
          emittedMatchError |=
              diagnoseMismatchedOptionals(TC, method,
                                          method->getParameterList(1), resultTL,
                                          matchDecl, owningTy,
                                          mayHaveMismatchedOptionals);
        }
      } else if (auto subscript =
                   dyn_cast_or_null<SubscriptDecl>(abstractStorage)) {
        // Otherwise, if this is a subscript, validate that covariance is ok.
        // If the parent is non-mutable, it's okay to be covariant.
        auto parentSubscript = cast<SubscriptDecl>(matchDecl);
        if (parentSubscript->getSetter()) {
          TC.diagnose(subscript, diag::override_mutable_covariant_subscript,
                      declTy, matchType);
          TC.diagnose(matchDecl, diag::subscript_override_here);
          return true;
        }

        if (attempt == OverrideCheckingAttempt::MismatchedTypes) {
          TC.diagnose(decl, diag::subscript_does_not_override);
          noteFixableMismatchedTypes(TC, decl, matchDecl);
          TC.diagnose(matchDecl, diag::overridden_near_match_here,
                      matchDecl->getDescriptiveKind(),
                      matchDecl->getFullName());
          emittedMatchError = true;

        } else if (mayHaveMismatchedOptionals) {
          emittedMatchError |=
              diagnoseMismatchedOptionals(TC, subscript,
                                          subscript->getIndices(),
                                          subscript->getElementTypeLoc(),
                                          matchDecl, owningTy,
                                          mayHaveMismatchedOptionals);
        }
      } else if (auto property = dyn_cast_or_null<VarDecl>(abstractStorage)) {
        auto propertyTy = property->getInterfaceType();
        auto parentPropertyTy = superclass->adjustSuperclassMemberDeclType(
            matchDecl, decl, matchDecl->getInterfaceType(), &TC);
        
        if (!propertyTy->canOverride(parentPropertyTy,
                                     OverrideMatchMode::Strict,
                                     &TC)) {
          TC.diagnose(property, diag::override_property_type_mismatch,
                      property->getName(), propertyTy, parentPropertyTy);
          noteFixableMismatchedTypes(TC, decl, matchDecl);
          TC.diagnose(matchDecl, diag::property_override_here);
          return true;
        }
        
        // Differing only in Optional vs. ImplicitlyUnwrappedOptional is fine.
        bool IsSilentDifference = false;
        if (auto propertyTyNoOptional = propertyTy->getAnyOptionalObjectType())
          if (auto parentPropertyTyNoOptional =
              parentPropertyTy->getAnyOptionalObjectType())
            if (propertyTyNoOptional->isEqual(parentPropertyTyNoOptional))
              IsSilentDifference = true;
        
        // The overridden property must not be mutable.
        if (cast<AbstractStorageDecl>(matchDecl)->getSetter() &&
            !IsSilentDifference) {
          TC.diagnose(property, diag::override_mutable_covariant_property,
                      property->getName(), parentPropertyTy, propertyTy);
          TC.diagnose(matchDecl, diag::property_override_here);
          return true;
        }
      }

      // Catch-all to make sure we don't silently accept something we shouldn't.
      if (attempt != OverrideCheckingAttempt::PerfectMatch &&
          !emittedMatchError) {
        diagnoseGeneralOverrideFailure(TC, decl, matches, attempt);
      }

      return recordOverride(TC, decl, matchDecl);
    }

    // We override more than one declaration. Complain.
    diagnoseGeneralOverrideFailure(TC, decl, matches, attempt);
    return true;
  }

  /// Attribute visitor that checks how the given attribute should be
  /// considered when overriding a declaration.
  ///
  /// Note that the attributes visited are those of the base
  /// declaration, so if you need to check that the overriding
  /// declaration doesn't have an attribute if the base doesn't have
  /// it, this isn't sufficient.
  class AttributeOverrideChecker
          : public AttributeVisitor<AttributeOverrideChecker> {
    TypeChecker &TC;
    ValueDecl *Base;
    ValueDecl *Override;

  public:
    AttributeOverrideChecker(TypeChecker &tc, ValueDecl *base,
                             ValueDecl *override)
      : TC(tc), Base(base), Override(override) { }

    /// Deleting this ensures that all attributes are covered by the visitor
    /// below.
    void visitDeclAttribute(DeclAttribute *A) = delete;

#define UNINTERESTING_ATTR(CLASS)                                              \
    void visit##CLASS##Attr(CLASS##Attr *) {}

    UNINTERESTING_ATTR(Accessibility)
    UNINTERESTING_ATTR(Alignment)
    UNINTERESTING_ATTR(CDecl)
    UNINTERESTING_ATTR(SILGenName)
    UNINTERESTING_ATTR(Exported)
    UNINTERESTING_ATTR(GKInspectable)
    UNINTERESTING_ATTR(IBAction)
    UNINTERESTING_ATTR(IBDesignable)
    UNINTERESTING_ATTR(IBInspectable)
    UNINTERESTING_ATTR(IBOutlet)
    UNINTERESTING_ATTR(Indirect)
    UNINTERESTING_ATTR(Inline)
    UNINTERESTING_ATTR(Inlineable)
    UNINTERESTING_ATTR(Effects)
    UNINTERESTING_ATTR(FixedLayout)
    UNINTERESTING_ATTR(Lazy)
    UNINTERESTING_ATTR(LLDBDebuggerFunction)
    UNINTERESTING_ATTR(Mutating)
    UNINTERESTING_ATTR(NonMutating)
    UNINTERESTING_ATTR(NonObjC)
    UNINTERESTING_ATTR(NoReturn)
    UNINTERESTING_ATTR(NSApplicationMain)
    UNINTERESTING_ATTR(NSCopying)
    UNINTERESTING_ATTR(NSManaged)
    UNINTERESTING_ATTR(ObjC)
    UNINTERESTING_ATTR(ObjCBridged)
    UNINTERESTING_ATTR(Optional)
    UNINTERESTING_ATTR(Override)
    UNINTERESTING_ATTR(RawDocComment)
    UNINTERESTING_ATTR(Required)
    UNINTERESTING_ATTR(Convenience)
    UNINTERESTING_ATTR(Semantics)
    UNINTERESTING_ATTR(SetterAccessibility)
    UNINTERESTING_ATTR(UIApplicationMain)
    UNINTERESTING_ATTR(Versioned)
    UNINTERESTING_ATTR(ObjCNonLazyRealization)
    UNINTERESTING_ATTR(UnsafeNoObjCTaggedPointer)
    UNINTERESTING_ATTR(SwiftNativeObjCRuntimeBase)
    UNINTERESTING_ATTR(ShowInInterface)
    UNINTERESTING_ATTR(Specialize)

    // These can't appear on overridable declarations.
    UNINTERESTING_ATTR(AutoClosure)
    UNINTERESTING_ATTR(NoEscape)

    UNINTERESTING_ATTR(Prefix)
    UNINTERESTING_ATTR(Postfix)
    UNINTERESTING_ATTR(Infix)
    UNINTERESTING_ATTR(Ownership)

    UNINTERESTING_ATTR(SynthesizedProtocol)
    UNINTERESTING_ATTR(RequiresStoredPropertyInits)
    UNINTERESTING_ATTR(Transparent)
    UNINTERESTING_ATTR(SILStored)
    UNINTERESTING_ATTR(Testable)

    UNINTERESTING_ATTR(WarnUnqualifiedAccess)
    UNINTERESTING_ATTR(DiscardableResult)

#undef UNINTERESTING_ATTR

    void visitAvailableAttr(AvailableAttr *attr) {
      // FIXME: Check that this declaration is at least as available as the
      // one it overrides.
    }

    void visitRethrowsAttr(RethrowsAttr *attr) {
      // 'rethrows' functions are a subtype of ordinary 'throws' functions.
      // Require 'rethrows' on the override if it was there on the base,
      // unless the override is completely non-throwing.
      if (!Override->getAttrs().hasAttribute<RethrowsAttr>() &&
          cast<AbstractFunctionDecl>(Override)->hasThrows()) {
        TC.diagnose(Override, diag::override_rethrows_with_non_rethrows,
                    isa<ConstructorDecl>(Override));
        TC.diagnose(Base, diag::overridden_here);
      }
    }

    void visitFinalAttr(FinalAttr *attr) {
      // If this is an accessor, don't complain if we would have
      // complained about the storage declaration.
      if (auto func = dyn_cast<FuncDecl>(Override)) {
        if (auto storageDecl = func->getAccessorStorageDecl()) {
          if (storageDecl->getOverriddenDecl() &&
              storageDecl->getOverriddenDecl()->isFinal())
            return;
        }
      }

      // FIXME: Customize message to the kind of thing.
      auto baseKind = Base->getDescriptiveKind();
      switch (baseKind) {
      case DescriptiveDeclKind::StaticLet:
      case DescriptiveDeclKind::StaticVar:
      case DescriptiveDeclKind::StaticMethod:
        TC.diagnose(Override, diag::override_static, baseKind);
        break;
      default:
        TC.diagnose(Override, diag::override_final,
                    Override->getDescriptiveKind(), baseKind);
        break;
      }

      TC.diagnose(Base, diag::overridden_here);
    }

    void visitDynamicAttr(DynamicAttr *attr) {
      if (!Override->getAttrs().hasAttribute<DynamicAttr>())
        // Dynamic is inherited.
        Override->getAttrs().add(
                                new (TC.Context) DynamicAttr(/*implicit*/true));
    }
  };

  /// Determine whether overriding the given declaration requires a keyword.
  static bool overrideRequiresKeyword(ValueDecl *overridden) {
    if (auto ctor = dyn_cast<ConstructorDecl>(overridden)) {
      return ctor->isDesignatedInit() && !ctor->isRequired();
    }

    return true;
  }

  /// Returns true if a diagnostic about an accessor being less available
  /// than the accessor it overrides would be redundant because we will
  /// already emit another diagnostic.
  static bool
  isRedundantAccessorOverrideAvailabilityDiagnostic(TypeChecker &TC,
                                                    ValueDecl *override,
                                                    ValueDecl *base) {

    auto *overrideFn = dyn_cast<FuncDecl>(override);
    auto *baseFn = dyn_cast<FuncDecl>(base);
    if (!overrideFn || !baseFn)
      return false;

    AbstractStorageDecl *overrideASD = overrideFn->getAccessorStorageDecl();
    AbstractStorageDecl *baseASD = baseFn->getAccessorStorageDecl();
    if (!overrideASD || !baseASD)
      return false;

    if (overrideASD->getOverriddenDecl() != baseASD)
      return false;

    // If we have already emitted a diagnostic about an unsafe override
    // for the property, don't complain about the accessor.
    if (!TC.isAvailabilitySafeForOverride(overrideASD, baseASD)) {
      return true;
    }

    // Returns true if we will already diagnose a bad override
    // on the property's accessor of the given kind.
    auto accessorOverrideAlreadyDiagnosed = [&](AccessorKind kind) {
      FuncDecl *overrideAccessor = overrideASD->getAccessorFunction(kind);
      FuncDecl *baseAccessor = baseASD->getAccessorFunction(kind);
      if (overrideAccessor && baseAccessor &&
          !TC.isAvailabilitySafeForOverride(overrideAccessor, baseAccessor)) {
        return true;
      }
      return false;
    };

    // If we have already emitted a diagnostic about an unsafe override
    // for a getter or a setter, no need to complain about materializeForSet,
    // which is synthesized to be as available as both the getter and
    // the setter.
    if (overrideFn->getAccessorKind() == AccessorKind::IsMaterializeForSet) {
      if (accessorOverrideAlreadyDiagnosed(AccessorKind::IsGetter) ||
          accessorOverrideAlreadyDiagnosed(AccessorKind::IsSetter)) {
        return true;
      }
    }

    return false;
  }

  /// Diagnose an override for potential availability. Returns true if
  /// a diagnostic was emitted and false otherwise.
  static bool diagnoseOverrideForAvailability(TypeChecker &TC,
                                              ValueDecl *override,
                                              ValueDecl *base) {
    if (TC.isAvailabilitySafeForOverride(override, base))
      return false;

    // Suppress diagnostics about availability overrides for accessors
    // if they would be redundant with other diagnostics.
    if (isRedundantAccessorOverrideAvailabilityDiagnostic(TC, override, base))
      return false;

    if (auto *FD = dyn_cast<FuncDecl>(override)) {
      if (FD->isAccessor()) {
        TC.diagnose(override, diag::override_accessor_less_available,
                    FD->getDescriptiveKind(),
                    FD->getAccessorStorageDecl()->getName());
        TC.diagnose(base, diag::overridden_here);
        return true;
      }
    }

    TC.diagnose(override, diag::override_less_available, override->getName());
    TC.diagnose(base, diag::overridden_here);

    return true;
  }

  /// Record that the \c overriding declarations overrides the
  /// \c overridden declaration.
  ///
  /// \returns true if an error occurred.
  static bool recordOverride(TypeChecker &TC, ValueDecl *override,
                             ValueDecl *base, bool isKnownObjC = false) {
    // Check property and subscript overriding.
    if (auto *baseASD = dyn_cast<AbstractStorageDecl>(base)) {
      auto *overrideASD = cast<AbstractStorageDecl>(override);
      
      // Make sure that the overriding property doesn't have storage.
      if (overrideASD->hasStorage() && !overrideASD->hasObservers()) {
        TC.diagnose(overrideASD, diag::override_with_stored_property,
                    overrideASD->getName());
        TC.diagnose(baseASD, diag::property_override_here);
        return true;
      }

      // Make sure that an observing property isn't observing something
      // read-only.  Observing properties look at change, read-only properties
      // have nothing to observe!
      bool baseIsSettable = baseASD->isSettable(baseASD->getDeclContext());
      if (baseIsSettable && TC.Context.LangOpts.EnableAccessControl) {
        baseIsSettable =
           baseASD->isSetterAccessibleFrom(overrideASD->getDeclContext());
      }
      if (overrideASD->hasObservers() && !baseIsSettable) {
        TC.diagnose(overrideASD, diag::observing_readonly_property,
                    overrideASD->getName());
        TC.diagnose(baseASD, diag::property_override_here);
        return true;
      }

      // Make sure we're not overriding a settable property with a non-settable
      // one.  The only reasonable semantics for this would be to inherit the
      // setter but override the getter, and that would be surprising at best.
      if (baseIsSettable && !override->isSettable(override->getDeclContext())) {
        TC.diagnose(overrideASD, diag::override_mutable_with_readonly_property,
                    overrideASD->getName());
        TC.diagnose(baseASD, diag::property_override_here);
        return true;
      }
      
      
      // Make sure a 'let' property is only overridden by 'let' properties.  A
      // let property provides more guarantees than the getter of a 'var'
      // property.
      if (isa<VarDecl>(baseASD) && cast<VarDecl>(baseASD)->isLet()) {
        TC.diagnose(overrideASD, diag::override_let_property,
                    overrideASD->getName());
        TC.diagnose(baseASD, diag::property_override_here);
        return true;
      }
    }
    
    // Non-Objective-C declarations in extensions cannot override or
    // be overridden.
    if ((base->getDeclContext()->isExtensionContext() ||
         override->getDeclContext()->isExtensionContext()) &&
        !base->isObjC() && !isKnownObjC) {
      TC.diagnose(override, diag::override_decl_extension,
                  !override->getDeclContext()->isExtensionContext());
      TC.diagnose(base, diag::overridden_here);
      return true;
    }
    
    // If the overriding declaration does not have the 'override' modifier on
    // it, complain.
    if (!override->getAttrs().hasAttribute<OverrideAttr>() &&
        overrideRequiresKeyword(base)) {
      // FIXME: rdar://16320042 - For properties, we don't have a useful
      // location for the 'var' token.  Instead of emitting a bogus fixit, only
      // emit the fixit for 'func's.
      if (!isa<VarDecl>(override))
        TC.diagnose(override, diag::missing_override)
            .fixItInsert(override->getStartLoc(), "override ");
      else
        TC.diagnose(override, diag::missing_override);
      TC.diagnose(base, diag::overridden_here);
      override->getAttrs().add(
          new (TC.Context) OverrideAttr(SourceLoc()));
    }

    // If the overridden method is declared in a Swift Class Declaration,
    // dispatch will use table dispatch. If the override is in an extension
    // warn, since it is not added to the class vtable.
    //
    // FIXME: Only warn if the extension is in another module, and if
    // it is in the same module, update the vtable.
    if (auto *baseDecl = dyn_cast<ClassDecl>(base->getDeclContext())) {
      if (baseDecl->hasKnownSwiftImplementation() && 
          !base->isDynamic() &&
          override->getDeclContext()->isExtensionContext()) {
        // For compatibility, only generate a warning in Swift 3
        TC.diagnose(override, (TC.Context.isSwiftVersion3()
          ? diag::override_class_declaration_in_extension_warning
          : diag::override_class_declaration_in_extension));
        TC.diagnose(base, diag::overridden_here);
      }
    }
    // If the overriding declaration is 'throws' but the base is not,
    // complain.
    if (auto overrideFn = dyn_cast<AbstractFunctionDecl>(override)) {
      if (overrideFn->hasThrows() &&
          !cast<AbstractFunctionDecl>(base)->hasThrows()) {
        TC.diagnose(override, diag::override_throws,
                    isa<ConstructorDecl>(override));
        TC.diagnose(base, diag::overridden_here);
      }

      if (!overrideFn->hasThrows() && base->isObjC() &&
          cast<AbstractFunctionDecl>(base)->hasThrows()) {
        TC.diagnose(override, diag::override_throws_objc,
                    isa<ConstructorDecl>(override));
        TC.diagnose(base, diag::overridden_here);
      }
    }

    // FIXME: Possibly should extend to more availability checking.
    if (auto *attr = base->getAttrs().getUnavailable(TC.Context)) {
      TC.diagnoseUnavailableOverride(override, base, attr);
    }
    
    if (!TC.getLangOpts().DisableAvailabilityChecking) {
      diagnoseOverrideForAvailability(TC, override, base);
    }

    /// Check attributes associated with the base; some may need to merged with
    /// or checked against attributes in the overriding declaration.
    AttributeOverrideChecker attrChecker(TC, base, override);
    for (auto attr : base->getAttrs()) {
      attrChecker.visit(attr);
    }

    if (auto overridingFunc = dyn_cast<FuncDecl>(override)) {
      overridingFunc->setOverriddenDecl(cast<FuncDecl>(base));
    } else if (auto overridingCtor = dyn_cast<ConstructorDecl>(override)) {
      overridingCtor->setOverriddenDecl(cast<ConstructorDecl>(base));
    } else if (auto overridingASD = dyn_cast<AbstractStorageDecl>(override)) {
      auto *baseASD = cast<AbstractStorageDecl>(base);
      overridingASD->setOverriddenDecl(baseASD);

      // Make sure we get consistent overrides for the accessors as well.
      assert(baseASD->hasAccessorFunctions());

      auto recordAccessorOverride = [&](AccessorKind kind) {
        // We need the same accessor on both.
        auto baseAccessor = baseASD->getAccessorFunction(kind);
        if (!baseAccessor) return;
        auto overridingAccessor = overridingASD->getAccessorFunction(kind);
        if (!overridingAccessor) return;

        // For setter accessors, we need the base's setter to be
        // accessible from the overriding context, or it's not an override.
        if ((kind == AccessorKind::IsSetter ||
             kind == AccessorKind::IsMaterializeForSet) &&
            !baseASD->isSetterAccessibleFrom(overridingASD->getDeclContext()))
          return;

        // FIXME: Egregious hack to set an 'override' attribute.
        if (!overridingAccessor->getAttrs().hasAttribute<OverrideAttr>()) {
          auto loc = overridingASD->getOverrideLoc();
          overridingAccessor->getAttrs().add(
              new (TC.Context) OverrideAttr(loc));
        }

        recordOverride(TC, overridingAccessor, baseAccessor,
                       baseASD->isObjC());
      };

      recordAccessorOverride(AccessorKind::IsGetter);
      recordAccessorOverride(AccessorKind::IsSetter);
      recordAccessorOverride(AccessorKind::IsMaterializeForSet);
    } else {
      llvm_unreachable("Unexpected decl");
    }
    
    return false;
  }

  void visitEnumCaseDecl(EnumCaseDecl *ECD) {
    // The type-checker doesn't care about how these are grouped.
  }

  void visitEnumElementDecl(EnumElementDecl *EED) {
    if (IsSecondPass) {
      checkAccessibility(TC, EED);
      return;
    }
    if (EED->hasInterfaceType() || EED->isBeingValidated())
      return;
    
    TC.checkDeclAttributesEarly(EED);
    TC.validateAccessibility(EED);

    // Only attempt to validate the argument type or raw value if the element
    // is not currently being validated.
    if (EED->getRecursiveness() == ElementRecursiveness::NotRecursive) {
      EED->setRecursiveness(ElementRecursiveness::PotentiallyRecursive);
      
      validateAttributes(TC, EED);
      
      if (!EED->getArgumentTypeLoc().isNull()) {
        if (TC.validateType(EED->getArgumentTypeLoc(), EED->getDeclContext(),
                            TR_EnumCase)) {
          EED->setInterfaceType(ErrorType::get(TC.Context));
          EED->setInvalid();
          return;
        }
      }

      // If we have a raw value, make sure there's a raw type as well.
      if (auto *rawValue = EED->getRawValueExpr()) {
        EnumDecl *ED = EED->getParentEnum();
        if (!ED->hasRawType()) {
          TC.diagnose(rawValue->getLoc(),diag::enum_raw_value_without_raw_type);
          // Recover by setting the raw type as this element's type.
          Expr *typeCheckedExpr = rawValue;
          if (!TC.typeCheckExpression(typeCheckedExpr, ED)) {
            EED->setTypeCheckedRawValueExpr(typeCheckedExpr);
            TC.checkEnumElementErrorHandling(EED);
          }
        } else {
          // Wait until the second pass, when all the raw value expressions
          // can be checked together.
        }
      }
    } else if (EED->getRecursiveness() ==
                ElementRecursiveness::PotentiallyRecursive) {
      EED->setRecursiveness(ElementRecursiveness::Recursive);
    }
    
    // If the element was not already marked as recursive by a re-entrant call,
    // we can be sure it's not recursive.
    if (EED->getRecursiveness() == ElementRecursiveness::PotentiallyRecursive) {
      EED->setRecursiveness(ElementRecursiveness::NotRecursive);
    }

    // Now that we have an argument type we can set the element's declared
    // type.
    if (!EED->hasInterfaceType() && !EED->computeType())
      return;

    // Require the carried type to be materializable.
    if (EED->getArgumentType() &&
        !EED->getArgumentType()->isMaterializable()) {
      TC.diagnose(EED->getLoc(), diag::enum_element_not_materializable);
      EED->setInterfaceType(ErrorType::get(TC.Context));
      EED->setInvalid();
    }
    TC.checkDeclAttributes(EED);
  }

  void visitExtensionDecl(ExtensionDecl *ED) {
    TC.validateExtension(ED);

    TC.checkDeclAttributesEarly(ED);

    if (!IsSecondPass) {
      if (auto extendedTy = ED->getExtendedType()) {
        if (!extendedTy->is<NominalType>() &&
            !extendedTy->is<BoundGenericType>() &&
            !extendedTy->hasError()) {
          // FIXME: Redundant diagnostic test here?
          TC.diagnose(ED->getStartLoc(), diag::non_nominal_extension,
                      extendedTy);
          // FIXME: It would be nice to point out where we found the named type
          // declaration, if any.
          ED->setInvalid();
        }
      }

      TC.checkInheritanceClause(ED);
      if (auto extendedTy = ED->getExtendedType()) {
        if (auto nominal = extendedTy->getAnyNominal()) {
          TC.validateDecl(nominal);
          // Check the raw values of an enum, since we might synthesize
          // RawRepresentable while checking conformances on this extension.
          if (auto enumDecl = dyn_cast<EnumDecl>(nominal)) {
            if (enumDecl->hasRawType())
              checkEnumRawValues(TC, enumDecl);
          }
        }
      }

      validateAttributes(TC, ED);
    }

    // Check conformances before visiting members, since we might
    // synthesize bodies for derived conformances
    if (!IsFirstPass) {
      TC.computeDefaultAccessibility(ED);
      if (auto *AA = ED->getAttrs().getAttribute<AccessibilityAttr>()) {
        const auto access = AA->getAccess();
        AccessScope desiredAccessScope = AccessScope::getPublic();
        switch (access) {
        case Accessibility::Private:
          assert((ED->isInvalid() ||
                  ED->getDeclContext()->isModuleScopeContext()) &&
                 "non-top-level extensions make 'private' != 'fileprivate'");
          SWIFT_FALLTHROUGH;
        case Accessibility::FilePrivate: {
          const DeclContext *DC = ED->getModuleScopeContext();
          bool isPrivate = access == Accessibility::Private;
          desiredAccessScope = AccessScope(DC, isPrivate);
          break;
        }
        case Accessibility::Internal:
          desiredAccessScope = AccessScope(ED->getModuleContext());
          break;
        case Accessibility::Public:
        case Accessibility::Open:
          break;
        }
        checkGenericParamAccessibility(TC, ED->getGenericParams(), ED,
                                       desiredAccessScope, access);
      }
      TC.checkConformancesInContext(ED, ED);
    }

    for (Decl *Member : ED->getMembers())
      visit(Member);

    if (!ED->isInvalid())
      TC.checkDeclAttributes(ED);
 }

  void visitTopLevelCodeDecl(TopLevelCodeDecl *TLCD) {
    // See swift::performTypeChecking for TopLevelCodeDecl handling.
    llvm_unreachable("TopLevelCodeDecls are handled elsewhere");
  }
  
  void visitIfConfigDecl(IfConfigDecl *ICD) {
    // The active members of the #if block will be type checked along with
    // their enclosing declaration.
    TC.checkDeclAttributesEarly(ICD);
    TC.checkDeclAttributes(ICD);
  }

  void visitConstructorDecl(ConstructorDecl *CD) {
    if (CD->isInvalid()) {
      CD->setInterfaceType(ErrorType::get(TC.Context));
      CD->setInvalid();
      return;
    }

    if (!IsFirstPass) {
      if (CD->getBody()) {
        TC.definedFunctions.push_back(CD);
      } else if (requiresDefinition(CD)) {
        // Complain if we should have a body.
        TC.diagnose(CD->getLoc(), diag::missing_initializer_def);
      }
    }

    if (IsSecondPass) {
      checkAccessibility(TC, CD);
      return;
    }
    if (CD->hasInterfaceType() || CD->isBeingValidated())
      return;

    CD->setIsBeingValidated();

    TC.checkDeclAttributesEarly(CD);
    TC.computeAccessibility(CD);

    assert(CD->getDeclContext()->isTypeContext()
           && "Decl parsing must prevent constructors outside of types!");

    // convenience initializers are only allowed on classes and in
    // extensions thereof.
    if (CD->isConvenienceInit()) {
      if (auto extType = CD->getDeclContext()->getDeclaredInterfaceType()) {
        auto extClass = extType->getClassOrBoundGenericClass();

        // Forbid convenience inits on Foreign CF types, as Swift does not yet
        // support user-defined factory inits.
        if (extClass &&
            extClass->getForeignClassKind() == ClassDecl::ForeignKind::CFType) {
          TC.diagnose(CD->getLoc(), diag::cfclass_convenience_init);
        }

        if (!extClass && !extType->hasError()) {
          auto ConvenienceLoc =
            CD->getAttrs().getAttribute<ConvenienceAttr>()->getLocation();

          // Produce a tailored diagnostic for structs and enums.
          bool isStruct = extType->getStructOrBoundGenericStruct() != nullptr;
          if (isStruct || extType->getEnumOrBoundGenericEnum()) {
            TC.diagnose(CD->getLoc(), diag::enumstruct_convenience_init,
                        isStruct ? "structs" : "enums")
              .fixItRemove(ConvenienceLoc);
          } else {
            TC.diagnose(CD->getLoc(), diag::nonclass_convenience_init, extType)
              .fixItRemove(ConvenienceLoc);
          }
          CD->setInitKind(CtorInitializerKind::Designated);
        }
      }
    } else if (auto extType = CD->getDeclContext()->getDeclaredInterfaceType()) {
      // A designated initializer for a class must be written within the class
      // itself.
      if (extType->getClassOrBoundGenericClass() &&
          isa<ExtensionDecl>(CD->getDeclContext())) {
        TC.diagnose(CD->getLoc(), diag::designated_init_in_extension, extType)
          .fixItInsert(CD->getLoc(), "convenience ");
        CD->setInitKind(CtorInitializerKind::Convenience);
      } else if (CD->getDeclContext()->getAsProtocolExtensionContext()) {
        CD->setInitKind(CtorInitializerKind::Convenience);
      }
    }

    configureImplicitSelf(TC, CD);

    if (auto gp = CD->getGenericParams()) {
      // Write up generic parameters and check the generic parameter list.
      gp->setOuterParameters(CD->getDeclContext()->getGenericParamsOfContext());

      auto *sig = TC.validateGenericFuncSignature(CD);

      auto builder = TC.createArchetypeBuilder(CD->getModuleContext());
      auto *parentSig = CD->getDeclContext()->getGenericSignatureOfContext();
      TC.checkGenericParamList(&builder, gp, parentSig, nullptr);

      // Infer requirements from the parameters of the constructor.
      builder.inferRequirements(CD->getParameterList(1), gp);

      // Revert the types within the signature so it can be type-checked with
      // archetypes below.
      TC.revertGenericFuncSignature(CD);

      // Assign archetypes.
      auto *env = builder.getGenericEnvironment(sig);
      CD->setGenericEnvironment(env);
    } else if (CD->getDeclContext()->getGenericSignatureOfContext()) {
      (void)TC.validateGenericFuncSignature(CD);

      // Revert all of the types within the signature of the constructor.
      TC.revertGenericFuncSignature(CD);

      CD->setGenericEnvironment(
          CD->getDeclContext()->getGenericEnvironmentOfContext());
    }

    // Set the context type of 'self'.
    recordSelfContextType(CD);

    // Type check the constructor parameters.
    GenericTypeToArchetypeResolver resolver(CD);
    if (CD->isInvalid() || semaFuncParamPatterns(CD, resolver)) {
      CD->setInterfaceType(ErrorType::get(TC.Context));
      CD->setInvalid();
    } else {
      if (!CD->getGenericSignatureOfContext())
        TC.configureInterfaceType(CD, CD->getGenericSignature());
    }

    // We want the constructor to be available for name lookup as soon
    // as it has a valid interface type.
    CD->setIsBeingValidated(false);

    validateAttributes(TC, CD);

    // Check whether this initializer overrides an initializer in its
    // superclass.
    if (!checkOverrides(TC, CD)) {
      // If an initializer has an override attribute but does not override
      // anything or overrides something that doesn't need an 'override'
      // keyword (e.g., a convenience initializer), complain.
      // anything, or overrides something that complain.
      if (auto *attr = CD->getAttrs().getAttribute<OverrideAttr>()) {
        if (!CD->getOverriddenDecl()) {
          TC.diagnose(CD, diag::initializer_does_not_override)
            .highlight(attr->getLocation());
          attr->setInvalid();
        } else if (!overrideRequiresKeyword(CD->getOverriddenDecl())) {
          // Special case: we are overriding a 'required' initializer, so we
          // need (only) the 'required' keyword.
          if (cast<ConstructorDecl>(CD->getOverriddenDecl())->isRequired()) {
            if (CD->getAttrs().hasAttribute<RequiredAttr>()) {
              TC.diagnose(CD, diag::required_initializer_override_keyword)
                .fixItRemove(attr->getLocation());
            } else {
              TC.diagnose(CD, diag::required_initializer_override_wrong_keyword)
                .fixItReplace(attr->getLocation(), "required");
              CD->getAttrs().add(
                new (TC.Context) RequiredAttr(/*IsImplicit=*/true));
            }

            TC.diagnose(findNonImplicitRequiredInit(CD->getOverriddenDecl()),
                        diag::overridden_required_initializer_here);
          } else {
            // We tried to override a convenience initializer.
            TC.diagnose(CD, diag::initializer_does_not_override)
              .highlight(attr->getLocation());
            TC.diagnose(CD->getOverriddenDecl(),
                        diag::convenience_init_override_here);
          }
        }
      }

      // A failable initializer cannot override a non-failable one.
      // This would normally be diagnosed by the covariance rules;
      // however, those are disabled so that we can provide a more
      // specific diagnostic here.
      if (CD->getFailability() != OTK_None &&
          CD->getOverriddenDecl() &&
          CD->getOverriddenDecl()->getFailability() == OTK_None) {
        TC.diagnose(CD, diag::failable_initializer_override,
                    CD->getFullName());
        TC.diagnose(CD->getOverriddenDecl(), 
                    diag::nonfailable_initializer_override_here,
                    CD->getOverriddenDecl()->getFullName());
      }
    }

    // An initializer is ObjC-compatible if it's explicitly @objc or a member
    // of an ObjC-compatible class.
    if (CD->getDeclContext()->isTypeContext()) {
      Optional<ObjCReason> isObjC = shouldMarkAsObjC(TC, CD,
          /*allowImplicit=*/true);

      Optional<ForeignErrorConvention> errorConvention;
      if (isObjC &&
          (CD->isInvalid() ||
           !TC.isRepresentableInObjC(CD, *isObjC, errorConvention)))
        isObjC = None;
      markAsObjC(TC, CD, isObjC, errorConvention);
    }

    // If this initializer overrides a 'required' initializer, it must itself
    // be marked 'required'.
    if (!CD->getAttrs().hasAttribute<RequiredAttr>()) {
      if (CD->getOverriddenDecl() && CD->getOverriddenDecl()->isRequired()) {
        TC.diagnose(CD, diag::required_initializer_missing_keyword)
          .fixItInsert(CD->getLoc(), "required ");

        TC.diagnose(findNonImplicitRequiredInit(CD->getOverriddenDecl()),
                    diag::overridden_required_initializer_here);

        CD->getAttrs().add(
            new (TC.Context) RequiredAttr(/*IsImplicit=*/true));
      }
    }

    if (CD->isRequired()) {
      if (auto nominal = CD->getDeclContext()
              ->getAsNominalTypeOrNominalTypeExtensionContext()) {
        auto requiredAccess = std::min(nominal->getFormalAccess(),
                                       Accessibility::Public);
        if (requiredAccess == Accessibility::Private)
          requiredAccess = Accessibility::FilePrivate;
        if (CD->getFormalAccess() < requiredAccess) {
          auto diag = TC.diagnose(CD,
                                  diag::required_initializer_not_accessible);
          fixItAccessibility(diag, CD, requiredAccess);
        }
      }
    }

    inferDynamic(TC.Context, CD);

    TC.checkDeclAttributes(CD);
  }

  void visitDestructorDecl(DestructorDecl *DD) {
    if (DD->isInvalid()) {
      DD->setInterfaceType(ErrorType::get(TC.Context));
      return;
    }

    if (!IsFirstPass) {
      if (DD->getBody())
        TC.definedFunctions.push_back(DD);
    }

    if (IsSecondPass ||
        DD->hasInterfaceType() ||
        DD->isBeingValidated()) {
      return;
    }

    DD->setIsBeingValidated();

    assert(DD->getDeclContext()->isTypeContext()
           && "Decl parsing must prevent destructors outside of types!");

    TC.checkDeclAttributesEarly(DD);
    if (!DD->hasAccessibility()) {
      auto enclosingClass = cast<ClassDecl>(DD->getParent());
      DD->setAccessibility(enclosingClass->getFormalAccess());
    }

    configureImplicitSelf(TC, DD);

    if (DD->getDeclContext()->getGenericSignatureOfContext()) {
      (void)TC.validateGenericFuncSignature(DD);
      DD->setGenericEnvironment(
          DD->getDeclContext()->getGenericEnvironmentOfContext());
    }

    // Set the context type of 'self'.
    recordSelfContextType(DD);

    GenericTypeToArchetypeResolver resolver(DD);
    if (semaFuncParamPatterns(DD, resolver)) {
      DD->setInterfaceType(ErrorType::get(TC.Context));
      DD->setInvalid();
    }

    if (!DD->getGenericSignatureOfContext())
      TC.configureInterfaceType(DD, DD->getGenericSignature());

    DD->setIsBeingValidated(false);

    // Do this before markAsObjC() to diagnose @nonobjc better
    validateAttributes(TC, DD);

    // Destructors are always @objc, because their Objective-C entry point is
    // -dealloc.
    markAsObjC(TC, DD, ObjCReason::ImplicitlyObjC);

    TC.checkDeclAttributes(DD);
  }
};
} // end anonymous namespace

bool swift::checkOverrides(TypeChecker &TC, ValueDecl *decl) {
  return DeclChecker::checkOverrides(TC, decl);
}

bool TypeChecker::isAvailabilitySafeForOverride(ValueDecl *override,
                                                ValueDecl *base) {
  // API availability ranges are contravariant: make sure the version range
  // of an overridden declaration is fully contained in the range of the
  // overriding declaration.
  AvailabilityContext overrideInfo =
      AvailabilityInference::availableRange(override, Context);
  AvailabilityContext baseInfo =
      AvailabilityInference::availableRange(base, Context);

  return baseInfo.isContainedIn(overrideInfo);
}

bool TypeChecker::isAvailabilitySafeForConformance(
    ProtocolDecl *proto, ValueDecl *requirement, ValueDecl *witness,
    DeclContext *dc, AvailabilityContext &requirementInfo) {

  // We assume conformances in
  // non-SourceFiles have already been checked for availability.
  if (!dc->getParentSourceFile())
    return true;

  NominalTypeDecl *conformingDecl = dc->getAsNominalTypeOrNominalTypeExtensionContext();
  assert(conformingDecl && "Must have conforming declaration");

  // Make sure that any access of the witness through the protocol
  // can only occur when the witness is available. That is, make sure that
  // on every version where the conforming declaration is available, if the
  // requirement is available then the witness is available as well.
  // We do this by checking that (an over-approximation of) the intersection of
  // the requirement's available range with both the conforming declaration's
  // available range and the protocol's available range is fully contained in
  // (an over-approximation of) the intersection of the witnesses's available
  // range with both the conforming type's available range and the protocol
  // declaration's available range.
  AvailabilityContext witnessInfo =
      AvailabilityInference::availableRange(witness, Context);
  requirementInfo = AvailabilityInference::availableRange(requirement, Context);

  AvailabilityContext infoForConformingDecl =
      overApproximateAvailabilityAtLocation(conformingDecl->getLoc(),
                                            conformingDecl);

  // Constrain over-approximates intersection of version ranges.
  witnessInfo.constrainWith(infoForConformingDecl);
  requirementInfo.constrainWith(infoForConformingDecl);

  AvailabilityContext infoForProtocolDecl =
      overApproximateAvailabilityAtLocation(proto->getLoc(), proto);

  witnessInfo.constrainWith(infoForProtocolDecl);
  requirementInfo.constrainWith(infoForProtocolDecl);

  return requirementInfo.isContainedIn(witnessInfo);
}

void TypeChecker::typeCheckDecl(Decl *D, bool isFirstPass) {
  checkForForbiddenPrefix(D);
  bool isSecondPass =
    !isFirstPass && D->getDeclContext()->isModuleScopeContext();
  DeclChecker(*this, isFirstPass, isSecondPass).visit(D);
}

// A class is @objc if it does not have generic ancestry, and it either has
// an explicit @objc attribute, or its superclass is @objc.
static Optional<ObjCReason> shouldMarkClassAsObjC(TypeChecker &TC,
                                                  ClassDecl *CD) {
  ObjCClassKind kind = CD->checkObjCAncestry();

  if (auto attr = CD->getAttrs().getAttribute<ObjCAttr>()) {
    if (kind == ObjCClassKind::ObjCMembers) {
      TC.diagnose(attr->getLocation(), diag::objc_for_generic_class)
        .fixItRemove(attr->getRangeWithAt());
    }

    // Only allow ObjC-rooted classes to be @objc.
    // (Leave a hole for test cases.)
    if (kind == ObjCClassKind::ObjCWithSwiftRoot &&
        TC.getLangOpts().EnableObjCAttrRequiresFoundation) {
      TC.diagnose(attr->getLocation(), diag::invalid_objc_swift_rooted_class)
        .fixItRemove(attr->getRangeWithAt());
    }

    return ObjCReason::ExplicitlyObjC;
  }

  if (kind == ObjCClassKind::ObjCWithSwiftRoot ||
      kind == ObjCClassKind::ObjC)
    return ObjCReason::ImplicitlyObjC;

  return None;
}

using NominalDeclSet = llvm::SmallPtrSetImpl<NominalTypeDecl *>;
static bool checkEnumDeclCircularity(EnumDecl *E, NominalDeclSet &known,
                                     Type baseType, bool isGenericArg = false);

static bool checkStructDeclCircularity(StructDecl *S, NominalDeclSet &known,
                                       Type baseType,
                                       bool isGenericArg = false);

// dispatch arbitrary declaration to relevant circularity checks.
static bool checkNominalDeclCircularity(Decl *decl, NominalDeclSet &known,
                                        Type baseType,
                                        bool isGenericArg = false) {
  if (auto s = dyn_cast<StructDecl>(decl)) {
    if (checkStructDeclCircularity(s, known, baseType, isGenericArg)) {
      return true;
    }
  } else if (auto e = dyn_cast<EnumDecl>(decl)) {
    if (checkEnumDeclCircularity(e, known, baseType, isGenericArg)) {
      return true;
    }
  }
  return false;
}


// break down type that "contains" other types and check them each.
static bool deconstructTypeForDeclCircularity(Type type,
                                              NominalDeclSet &known) {

  if (auto tuple = type->getAs<TupleType>()) {
    // check each element in tuple
    for (auto Elt: tuple->getElements()) {
      if (deconstructTypeForDeclCircularity(Elt.getType(), known)) {
        return true;
      }
    }
  }

  if (auto decl = type->getAnyNominal()) {
    // Found a circularity! Stop checking from here on out.
    if (known.count(decl)) {
      return true;
    }

    bool isGenericArg = type->getAs<BoundGenericType>() != nullptr;
    if (checkNominalDeclCircularity(decl, known, type, isGenericArg)) {
      return true;
    }
  }

  return false;
}


static bool checkStructDeclCircularity(StructDecl *S, NominalDeclSet &known,
                                       Type baseType, bool isGenericArg) {

  // if we are checking a generic argument, don't make it a starting point
  // of a circularity.
  if (!isGenericArg) {
    known.insert(S);
  }

  for (auto field: S->getStoredProperties()) {
    if (auto vd = dyn_cast<VarDecl>(field)) {
      // skip uninteresting fields.
      if (vd->isStatic() || !vd->hasStorage() || !vd->hasType()) {
        continue;
      }

      auto vdt = baseType->getTypeOfMember(S->getModuleContext(), vd, nullptr);

      if (deconstructTypeForDeclCircularity(vdt, known)) {
        return true;
      }
    }
  }

  // we didn't find any circularity, clean up.
  if (!isGenericArg) {
    known.erase(S);
  }
  return false;
}

static bool checkEnumDeclCircularity(EnumDecl *E, NominalDeclSet &known,
                                     Type baseType, bool isGenericArg) {
  // enums marked as 'indirect' are safe
  if (E->isIndirect()) { return false; }


  // if we are checking a generic argument, don't make it a starting point
  // of a circularity.
  if (!isGenericArg) {
    known.insert(E);
  }

  for (auto elt: E->getAllElements()) {
    // skip uninteresting fields.
    if (!elt->hasArgumentType() || elt->isIndirect()) {
      continue;
    }
    auto eltType = baseType->getTypeOfMember(E->getModuleContext(), elt,
      nullptr, elt->getArgumentInterfaceType());

    if (!eltType) { continue; }

    if (deconstructTypeForDeclCircularity(eltType, known)) {
      return true;
    }
  }

  // we didn't find any circularity, clean up.
  if (!isGenericArg) {
    known.erase(E);
  }

  return false;
}

void TypeChecker::checkDeclCircularity(NominalTypeDecl *decl) {
  llvm::SmallPtrSet<NominalTypeDecl *, 16> known;
  auto baseType = decl->getDeclaredInterfaceType();

  if (checkNominalDeclCircularity(decl, known, baseType)) {
    if (isa<StructDecl>(decl)) {
      diagnose(decl->getLoc(),
               diag::unsupported_recursive_type,
               decl->getDeclaredInterfaceType());
    } else if (isa<EnumDecl>(decl)) {
      diagnose(decl->getLoc(),
               diag::recursive_enum_not_indirect,
               decl->getDeclaredInterfaceType())
        .fixItInsert(decl->getStartLoc(), "indirect ");
    }
  }
}

void TypeChecker::validateDecl(ValueDecl *D) {
  // Generic parameters are validated as part of their context.
  if (isa<GenericTypeParamDecl>(D))
    return;

  // Handling validation failure due to re-entrancy is left
  // up to the caller, who must call hasValidSignature() to
  // check that validateDecl() returned a fully-formed decl.
  if (D->isBeingValidated())
    return;

  PrettyStackTraceDecl StackTrace("validating", D);

  if (hasEnabledForbiddenTypecheckPrefix())
    checkForForbiddenPrefix(D);

  validateAccessibility(D);

  // Validate the context.
  auto dc = D->getDeclContext();
  if (auto nominal = dyn_cast<NominalTypeDecl>(dc)) {
    validateDecl(nominal);
    if (!nominal->hasValidSignature())
      return;
  } else if (auto ext = dyn_cast<ExtensionDecl>(dc)) {
    validateExtension(ext);
    if (!ext->hasValidSignature())
      return;
  }

  SWIFT_FUNC_STAT;

  switch (D->getKind()) {
  case DeclKind::Import:
  case DeclKind::Extension:
  case DeclKind::PatternBinding:
  case DeclKind::EnumCase:
  case DeclKind::TopLevelCode:
  case DeclKind::InfixOperator:
  case DeclKind::PrefixOperator:
  case DeclKind::PostfixOperator:
  case DeclKind::PrecedenceGroup:
  case DeclKind::IfConfig:
    llvm_unreachable("not a value decl");

  case DeclKind::Module:
    return;
      
  case DeclKind::GenericTypeParam:
    llvm_unreachable("handled above");

  case DeclKind::AssociatedType: {
    if (D->hasInterfaceType())
      return;

    auto assocType = cast<AssociatedTypeDecl>(D);

    assocType->setIsBeingValidated();
    SWIFT_DEFER { assocType->setIsBeingValidated(false); };

    checkDeclAttributesEarly(assocType);
    checkInheritanceClause(assocType);

    // Check the default definition, if there is one.
    TypeLoc &defaultDefinition = assocType->getDefaultDefinitionLoc();
    if (!defaultDefinition.isNull() &&
        validateType(defaultDefinition, assocType->getDeclContext())) {
      defaultDefinition.setInvalidType(Context);
    }

    // Finally, set the interface type.
    assocType->computeType();

    checkDeclAttributes(assocType);
    break;
  }

  case DeclKind::TypeAlias: {
    // Type aliases may not have an underlying type yet.
    auto typeAlias = cast<TypeAliasDecl>(D);

    if (typeAlias->hasCompletedValidation())
      return;

    typeAlias->setHasCompletedValidation();

    // Check generic parameters, if needed.
    typeAlias->setIsBeingValidated();
    SWIFT_DEFER { typeAlias->setIsBeingValidated(false); };

    validateGenericTypeSignature(typeAlias);

    TypeResolutionOptions options;
    if (typeAlias->getFormalAccess() <= Accessibility::FilePrivate)
      options |= TR_KnownNonCascadingDependency;

    if (typeAlias->getDeclContext()->isModuleScopeContext() &&
        typeAlias->getGenericParams() == nullptr) {
      IterativeTypeChecker ITC(*this);
      ITC.satisfy(requestResolveTypeDecl(typeAlias));
    } else {
      if (validateType(typeAlias->getUnderlyingTypeLoc(),
                       typeAlias, options)) {
        typeAlias->setInvalid();
        typeAlias->getUnderlyingTypeLoc().setInvalidType(Context);
      }

      typeAlias->setUnderlyingType(typeAlias->getUnderlyingTypeLoc().getType());
    }

    break;
  }

  case DeclKind::Enum:
  case DeclKind::Struct:
  case DeclKind::Class: {
    auto nominal = cast<NominalTypeDecl>(D);
    if (nominal->hasInterfaceType())
      return;
    nominal->computeType();

    // Check generic parameters, if needed.
    nominal->setIsBeingValidated();
    validateGenericTypeSignature(nominal);
    nominal->setIsBeingValidated(false);

    checkInheritanceClause(D);

    validateAttributes(*this, D);

    // Mark a class as @objc. This must happen before checking its members.
    if (auto CD = dyn_cast<ClassDecl>(nominal)) {
      Optional<ObjCReason> isObjC = shouldMarkClassAsObjC(*this, CD);
      markAsObjC(*this, CD, isObjC);

      // Determine whether we require in-class initializers.
      if (CD->getAttrs().hasAttribute<RequiresStoredPropertyInitsAttr>() ||
          (CD->hasSuperclass() &&
           CD->getSuperclass()->getClassOrBoundGenericClass()
             ->requiresStoredPropertyInits()))
        CD->setRequiresStoredPropertyInits(true);
    }

    if (auto *ED = dyn_cast<EnumDecl>(nominal)) {
      // @objc enums use their raw values as the value representation, so we
      // need to force the values to be checked.
      if (ED->isObjC())
        checkEnumRawValues(*this, ED);
    }

    ValidatedTypes.insert(nominal);
    break;
  }

  case DeclKind::Protocol: {
    auto proto = cast<ProtocolDecl>(D);
    if (proto->hasInterfaceType())
      return;
    proto->computeType();

    // Validate the generic type signature, which is just <Self : P>.
    proto->setIsBeingValidated();
    validateGenericTypeSignature(proto);
    proto->setIsBeingValidated(false);

    // Record inherited protocols.
    resolveInheritedProtocols(proto);

    validateAttributes(*this, D);

    // If the protocol is @objc, it may only refine other @objc protocols.
    // FIXME: Revisit this restriction.
    if (proto->getAttrs().hasAttribute<ObjCAttr>()) {
      Optional<ObjCReason> isObjC = ObjCReason::ImplicitlyObjC;

      for (auto inherited : proto->getInheritedProtocols(nullptr)) {
        if (!inherited->isObjC()) {
          diagnose(proto->getLoc(),
                   diag::objc_protocol_inherits_non_objc_protocol,
                   proto->getDeclaredType(), inherited->getDeclaredType());
          diagnose(inherited->getLoc(), diag::protocol_here,
                   inherited->getName());
          isObjC = None;
        }
      }

      markAsObjC(*this, proto, isObjC);
    }

    ValidatedTypes.insert(proto);
    break;
  }

  case DeclKind::Var:
  case DeclKind::Param: {
    auto VD = cast<VarDecl>(D);
    if (!VD->hasInterfaceType()) {
      if (VD->isSelfParameter()) {
        if (!VD->hasInterfaceType()) {
          VD->setInterfaceType(ErrorType::get(Context));
          VD->setInvalid();
        }
        recordSelfContextType(cast<AbstractFunctionDecl>(VD->getDeclContext()));
      } else if (PatternBindingDecl *PBD = VD->getParentPatternBinding()) {
        if (PBD->isBeingValidated()) {
          diagnose(VD, diag::pattern_used_in_type, VD->getName());

        } else {
          for (unsigned i = 0, e = PBD->getNumPatternEntries(); i != e; ++i)
            validatePatternBindingDecl(*this, PBD, i);
        }

        auto parentPattern = VD->getParentPattern();
        if (PBD->isInvalid() || !parentPattern->hasType()) {
          parentPattern->setType(ErrorType::get(Context));
          setBoundVarsTypeError(parentPattern, Context);
          
          // If no type has been set for the initializer, we need to diagnose
          // the failure.
          if (VD->getParentInitializer() &&
              !VD->getParentInitializer()->getType()) {
            diagnose(parentPattern->getLoc(), diag::identifier_init_failure,
                     parentPattern->getBoundName());
          }
          
          return;
        }
      } else {
        // FIXME: This case is hit when code completion occurs in a function
        // parameter list. Previous parameters are definitely in scope, but
        // we don't really know how to type-check them.
        // We can also hit this when code-completing in a closure body.
        assert(isa<AbstractFunctionDecl>(D->getDeclContext()) ||
               isa<AbstractClosureExpr>(D->getDeclContext()) ||
               isa<TopLevelCodeDecl>(D->getDeclContext()));
        VD->markInvalid();
      }
    }

    if (!VD->didEarlyAttrValidation()) {
      checkDeclAttributesEarly(VD);
      validateAttributes(*this, VD);

      // FIXME: Guarding the rest of these things together with early attribute
      // validation is a hack. It's necessary because properties can get types
      // before validateDecl is called.

      if (!DeclChecker::checkOverrides(*this, VD)) {
        // If a property has an override attribute but does not override
        // anything, complain.
        auto overridden = VD->getOverriddenDecl();
        if (auto *OA = VD->getAttrs().getAttribute<OverrideAttr>()) {
          if (!overridden) {
            diagnose(VD, diag::property_does_not_override)
              .highlight(OA->getLocation());
            OA->setInvalid();
          }
        }
      }

      // Properties need some special validation logic.
      if (auto *nominalDecl = VD->getDeclContext()
              ->getAsNominalTypeOrNominalTypeExtensionContext()) {
        // If this is a property, check if it needs to be exposed to
        // Objective-C.
        Optional<ObjCReason> isObjC = shouldMarkAsObjC(*this, VD);

        if (isObjC && !isRepresentableInObjC(VD, *isObjC))
          isObjC = None;

        markAsObjC(*this, VD, isObjC);

        // Infer 'dynamic' before touching accessors.
        inferDynamic(Context, VD);

        // If this variable is a class member, mark it final if the
        // class is final, or if it was declared with 'let'.
        if (auto cls = dyn_cast<ClassDecl>(nominalDecl)) {
          if (cls->isFinal() || VD->isLet()) {
            if (!VD->isFinal() && !VD->isDynamic()) {
              makeFinal(Context, VD);
            }
          }
          if (VD->isStatic()) {
            auto staticSpelling =
              VD->getParentPatternBinding()->getStaticSpelling();
            if (staticSpelling == StaticSpellingKind::KeywordStatic) {
              auto finalAttr = VD->getAttrs().getAttribute<FinalAttr>();
              if (finalAttr) {
                auto finalRange = finalAttr->getRange();
                if (finalRange.isValid())
                  diagnose(finalRange.Start, diag::decl_already_final)
                  .highlight(finalRange)
                  .fixItRemove(finalRange);
              }
              makeFinal(Context, VD);
            }
          }
        }
      }

      // Synthesize accessors as necessary.
      maybeAddAccessorsToVariable(VD, *this);

      // Make sure the getter and setter have valid types, since they will be
      // used by SILGen for any accesses to this variable.
      validateAbstractStorageDecl(VD, *this);

      if (VD->isDynamic()) {
        makeDynamic(Context, VD->getGetter());
        makeDynamic(Context, VD->getSetter());
        // Skip materializeForSet -- it won't be used with a dynamic property.
      }
    }

    break;
  }
      
  case DeclKind::Func: {
    if (D->hasInterfaceType())
      return;
    typeCheckDecl(D, true);
    break;
  }

  case DeclKind::Subscript:
  case DeclKind::Constructor:
    if (D->hasInterfaceType())
      return;
    typeCheckDecl(D, true);
    break;

  case DeclKind::Destructor:
  case DeclKind::EnumElement: {
    if (D->hasInterfaceType())
      return;
    if (auto container = dyn_cast<NominalTypeDecl>(D->getDeclContext())) {
      validateDecl(container);
      typeCheckDecl(D, true);
    } else {
      D->setInterfaceType(ErrorType::get(Context));
    }
    break;
  }
  }

  assert(D->hasValidSignature());
}


void TypeChecker::validateAccessibility(ValueDecl *D) {
  if (D->hasAccessibility())
    return;

  // FIXME: Encapsulate the following in computeAccessibility() ?

  switch (D->getKind()) {
  case DeclKind::Import:
  case DeclKind::Extension:
  case DeclKind::PatternBinding:
  case DeclKind::EnumCase:
  case DeclKind::TopLevelCode:
  case DeclKind::InfixOperator:
  case DeclKind::PrefixOperator:
  case DeclKind::PostfixOperator:
  case DeclKind::PrecedenceGroup:
  case DeclKind::IfConfig:
    llvm_unreachable("not a value decl");

  case DeclKind::Module:
    break;

  case DeclKind::TypeAlias:
    computeAccessibility(D);
    break;

  case DeclKind::GenericTypeParam:
    // Ultimately handled in generic signature validation.
    return;

  case DeclKind::AssociatedType: {
      auto assocType = cast<AssociatedTypeDecl>(D);
      auto prot = assocType->getProtocol();
      validateAccessibility(prot);
      assocType->setAccessibility(std::max(prot->getFormalAccess(),
                                           Accessibility::Internal));
      break;
    }

  case DeclKind::Enum:
  case DeclKind::Struct:
  case DeclKind::Class:
  case DeclKind::Protocol:
  case DeclKind::Var:
  case DeclKind::Param:
  case DeclKind::Func:
  case DeclKind::Subscript:
  case DeclKind::Constructor:
    computeAccessibility(D);
    break;

  case DeclKind::Destructor:
  case DeclKind::EnumElement: {
    if (D->isInvalid()) {
      D->setAccessibility(Accessibility::Private);
    } else {
      auto container = cast<NominalTypeDecl>(D->getDeclContext());
      validateAccessibility(container);
      D->setAccessibility(std::max(container->getFormalAccess(),
                                   Accessibility::Internal));
    }
    break;
  }
  }

  assert(D->hasAccessibility());
}

/// Form the interface type of an extension from the raw type and the
/// extension's list of generic parameters.
static Type formExtensionInterfaceType(Type type,
                                       GenericParamList *genericParams) {
  // Find the nominal type declaration and its parent type.
  Type parentType;
  NominalTypeDecl *nominal;
  if (auto unbound = type->getAs<UnboundGenericType>()) {
    parentType = unbound->getParent();
    nominal = cast<NominalTypeDecl>(unbound->getDecl());
  } else if (auto bound = type->getAs<BoundGenericType>()) {
    parentType = bound->getParent();
    nominal = bound->getDecl();
  } else {
    auto nominalType = type->castTo<NominalType>();
    parentType = nominalType->getParent();
    nominal = nominalType->getDecl();
  }

  // Reconstruct the parent, if there is one.
  if (parentType) {
    // Build the nested extension type.
    auto parentGenericParams = nominal->getGenericParams()
                                 ? genericParams->getOuterParameters()
                                 : genericParams;
    parentType = formExtensionInterfaceType(parentType, parentGenericParams);
  }

  // If we don't have generic parameters at this level, just build the result.
  if (!nominal->getGenericParams() || isa<ProtocolDecl>(nominal)) {
    Type resultType = NominalType::get(nominal, parentType,
                                       nominal->getASTContext());

    // If the parent was unchanged, return the original pointer.
    return resultType->isEqual(type) ? type : resultType;
  }

  // Form the bound generic type with the type parameters provided.
  SmallVector<Type, 2> genericArgs;
  for (auto gp : *genericParams) {
    genericArgs.push_back(gp->getDeclaredInterfaceType());
  }

  Type resultType = BoundGenericType::get(nominal, parentType, genericArgs);
  return resultType->isEqual(type) ? type : resultType;
}

/// Visit the given generic parameter lists from the outermost to the innermost,
/// calling the visitor function for each list.
static void visitOuterToInner(
                      GenericParamList *genericParams,
                      llvm::function_ref<void(GenericParamList *)> visitor) {
  if (auto outerGenericParams = genericParams->getOuterParameters())
    visitOuterToInner(outerGenericParams, visitor);

  visitor(genericParams);
}

/// Check the generic parameters of an extension, recursively handling all of
/// the parameter lists within the extension.
static std::pair<GenericEnvironment *, Type>
checkExtensionGenericParams(TypeChecker &tc, ExtensionDecl *ext, Type type,
                            GenericParamList *genericParams) {
  // Form the interface type of the extension.
  Type extInterfaceType = formExtensionInterfaceType(type, genericParams);

  // Prepare all of the generic parameter lists for generic signature
  // validation.
  visitOuterToInner(genericParams, [&](GenericParamList *gpList) {
    tc.prepareGenericParamList(gpList, ext);
  });

  // Local function used to infer requirements from the extended type.
  auto inferExtendedTypeReqs = [&](ArchetypeBuilder &builder) {
    builder.inferRequirements(TypeLoc::withoutLoc(extInterfaceType),
                              /*minDepth=*/0,
                              /*maxDepth=*/genericParams->getDepth());
  };

  // Validate the generic type signature.
  auto *env = tc.checkGenericEnvironment(genericParams,
                                         ext->getDeclContext(), nullptr,
                                         /*allowConcreteGenericParams=*/true,
                                         inferExtendedTypeReqs);

  // Validate the generic parameters for the last time, to splat down
  // actual archetypes.
  visitOuterToInner(genericParams, [&](GenericParamList *gpList) {
    tc.revertGenericParamList(gpList);
  });
  GenericTypeToArchetypeResolver archetypeResolver(env);
  visitOuterToInner(genericParams, [&](GenericParamList *gpList) {
    tc.checkGenericParamList(nullptr, gpList, nullptr, &archetypeResolver);
  });

  Type extContextType =
    env->mapTypeIntoContext(ext->getModuleContext(), extInterfaceType);
  return { env, extContextType };
}

// FIXME: In TypeChecker.cpp; only needed because LLDB creates
// extensions of typealiases to unbound generic types, which is
// ill-formed but convenient.
namespace swift {
GenericParamList *cloneGenericParams(ASTContext &ctx,
                                     DeclContext *dc,
                                     GenericParamList *fromParams);
} // namespace swift

void TypeChecker::validateExtension(ExtensionDecl *ext) {
  // If we already validated this extension, there's nothing more to do.
  if (ext->validated())
    return;

  ext->setValidated();

  ext->setIsBeingValidated();
  SWIFT_DEFER { ext->setIsBeingValidated(false); };

  // If the extension is already known to be invalid, we're done.
  if (ext->isInvalid())
    return;

  // FIXME: We need to check whether anything is specialized, because
  // the innermost extended type might itself be a non-generic type
  // within a generic type.
  auto extendedType = ext->getExtendedType();

  if (extendedType.isNull() || extendedType->hasError())
    return;

  if (extendedType->hasUnboundGenericType()) {
    // Validate the nominal type declaration being extended.
    auto nominal = extendedType->getAnyNominal();
    validateDecl(nominal);

    auto genericParams = ext->getGenericParams();

    // The debugger synthesizes typealiases of unbound generic types
    // to produce its extensions, which subverts bindExtensionDecl's
    // ability to create the generic parameter lists. Create the list now.
    if (!genericParams && Context.LangOpts.DebuggerSupport) {
      genericParams = cloneGenericParams(Context, ext,
                                         nominal->getGenericParams());
      ext->setGenericParams(genericParams);
    }
    assert(genericParams && "bindExtensionDecl didn't set generic params?");

    // Check generic parameters.
    GenericEnvironment *env;
    std::tie(env, extendedType) = checkExtensionGenericParams(
        *this, ext, ext->getExtendedType(), ext->getGenericParams());

    ext->getExtendedTypeLoc().setType(extendedType);
    ext->setGenericEnvironment(env);
    return;
  }
  
  // If we're extending a protocol, check the generic parameters.
  //
  // Canonicalize the type to work around the fact that getAs<> cannot
  // "look through" protocol<X, Y> where X and Y both desugar to the same
  // thing.
  //
  // FIXME: Probably the above comes up elsewhere, perhaps getAs<>()
  // should be fixed.
  if (auto proto = extendedType->getCanonicalType()->getAs<ProtocolType>()) {
    if (!isa<ProtocolType>(extendedType.getPointer()) &&
        proto->getDecl()->getParentModule() == ext->getParentModule()) {
      // Protocols in the same module cannot be extended via a typealias;
      // we could end up being unable to resolve the generic signature.
      diagnose(ext->getLoc(), diag::extension_protocol_via_typealias, proto)
        .fixItReplace(ext->getExtendedTypeLoc().getSourceRange(),
                      proto->getDecl()->getName().str());
      ext->setInvalid();
      ext->getExtendedTypeLoc().setInvalidType(Context);
      return;
    }

    GenericEnvironment *env;
    std::tie(env, extendedType) =
        checkExtensionGenericParams(*this, ext, proto, ext->getGenericParams());

    ext->getExtendedTypeLoc().setType(extendedType);
    ext->setGenericEnvironment(env);

    // Speculatively ban extension of AnyObject; it won't be a
    // protocol forever, and we don't want to allow code that we know
    // we'll break later.
    if (proto->getDecl()->isSpecificProtocol(
          KnownProtocolKind::AnyObject)) {
      diagnose(ext, diag::extension_anyobject)
        .highlight(ext->getExtendedTypeLoc().getSourceRange());
    }
    return;
  }

  assert(extendedType->is<NominalType>());
}

ArrayRef<ProtocolDecl *>
TypeChecker::getDirectConformsTo(ProtocolDecl *proto) {
  resolveInheritedProtocols(proto);
  return proto->getInheritedProtocols(nullptr);
}

/// Build a default initializer string for the given pattern.
///
/// This string is suitable for display in diagnostics.
static Optional<std::string> buildDefaultInitializerString(TypeChecker &tc,
                                                           DeclContext *dc,
                                                           Pattern *pattern) {
  switch (pattern->getKind()) {
#define REFUTABLE_PATTERN(Id, Parent) case PatternKind::Id:
#define PATTERN(Id, Parent)
#include "swift/AST/PatternNodes.def"
    return None;
  case PatternKind::Any:
    return None;

  case PatternKind::Named: {
    if (!pattern->hasType())
      return None;

    // Special-case the various types we might see here.
    auto type = pattern->getType();

    // For literal-convertible types, form the corresponding literal.
#define CHECK_LITERAL_PROTOCOL(Kind, String) \
    if (auto proto = tc.getProtocol(SourceLoc(), KnownProtocolKind::Kind)) { \
      if (tc.conformsToProtocol(type, proto, dc, \
                                ConformanceCheckFlags::InExpression)) \
        return std::string(String); \
    }
    CHECK_LITERAL_PROTOCOL(ExpressibleByArrayLiteral, "[]")
    CHECK_LITERAL_PROTOCOL(ExpressibleByDictionaryLiteral, "[:]")
    CHECK_LITERAL_PROTOCOL(ExpressibleByUnicodeScalarLiteral, "\"\"")
    CHECK_LITERAL_PROTOCOL(ExpressibleByExtendedGraphemeClusterLiteral, "\"\"")
    CHECK_LITERAL_PROTOCOL(ExpressibleByFloatLiteral, "0.0")
    CHECK_LITERAL_PROTOCOL(ExpressibleByIntegerLiteral, "0")
    CHECK_LITERAL_PROTOCOL(ExpressibleByStringLiteral, "\"\"")
#undef CHECK_LITERAL_PROTOCOL

    // For optional types, use 'nil'.
    if (type->getAnyOptionalObjectType())
      return std::string("nil");

    return None;
  }

  case PatternKind::Paren: {
    if (auto sub = buildDefaultInitializerString(
                     tc, dc, cast<ParenPattern>(pattern)->getSubPattern())) {
      return "(" + *sub + ")";
    }

    return None;
  }

  case PatternKind::Tuple: {
    std::string result = "(";
    bool first = true;
    for (auto elt : cast<TuplePattern>(pattern)->getElements()) {
      if (auto sub = buildDefaultInitializerString(tc, dc, elt.getPattern())) {
        if (first) {
          first = false;
        } else {
          result += ", ";
        }

        result += *sub;
      } else {
        return None;
      }
    }
    result += ")";
    return result;
  }

  case PatternKind::Typed:
    return buildDefaultInitializerString(
             tc, dc, cast<TypedPattern>(pattern)->getSubPattern());

  case PatternKind::Var:
    return buildDefaultInitializerString(
             tc, dc, cast<VarPattern>(pattern)->getSubPattern());
  }

  llvm_unreachable("Unhandled PatternKind in switch.");
}

/// Diagnose a class that does not have any initializers.
static void diagnoseClassWithoutInitializers(TypeChecker &tc,
                                             ClassDecl *classDecl) {
  tc.diagnose(classDecl, diag::class_without_init,
              classDecl->getDeclaredType());

  for (auto member : classDecl->getMembers()) {
    auto pbd = dyn_cast<PatternBindingDecl>(member);
    if (!pbd)
      continue;

    if (pbd->isStatic() || !pbd->hasStorage() || isDefaultInitializable(pbd) ||
        pbd->isInvalid())
      continue;
   
    for (auto entry : pbd->getPatternList()) {
      if (entry.getInit()) continue;
      
      SmallVector<VarDecl *, 4> vars;
      entry.getPattern()->collectVariables(vars);
      if (vars.empty()) continue;

      auto varLoc = vars[0]->getLoc();
      
      Optional<InFlightDiagnostic> diag;
      switch (vars.size()) {
      case 1:
        diag.emplace(tc.diagnose(varLoc, diag::note_no_in_class_init_1,
                                 vars[0]->getName()));
        break;
      case 2:
        diag.emplace(tc.diagnose(varLoc, diag::note_no_in_class_init_2,
                                 vars[0]->getName(), vars[1]->getName()));
        break;
      case 3:
        diag.emplace(tc.diagnose(varLoc, diag::note_no_in_class_init_3plus,
                                 vars[0]->getName(), vars[1]->getName(), 
                                 vars[2]->getName(), false));
        break;
      default:
        diag.emplace(tc.diagnose(varLoc, diag::note_no_in_class_init_3plus,
                                 vars[0]->getName(), vars[1]->getName(), 
                                 vars[2]->getName(), true));
        break;
      }

      if (auto defaultValueSuggestion
             = buildDefaultInitializerString(tc, classDecl, entry.getPattern()))
        diag->fixItInsertAfter(entry.getPattern()->getEndLoc(),
                               " = " + *defaultValueSuggestion);
    }
  }
}

/// Diagnose a missing required initializer.
static void diagnoseMissingRequiredInitializer(
              TypeChecker &TC,
              ClassDecl *classDecl,
              ConstructorDecl *superInitializer) {
  // Find the location at which we should insert the new initializer.
  SourceLoc insertionLoc;
  SourceLoc indentationLoc;
  for (auto member : classDecl->getMembers()) {
    // If we don't have an indentation location yet, grab one from this
    // member.
    if (indentationLoc.isInvalid()) {
      indentationLoc = member->getLoc();
    }

    // We only want to look at explicit constructors.
    auto ctor = dyn_cast<ConstructorDecl>(member);
    if (!ctor)
      continue;

    if (ctor->isImplicit())
      continue;

    insertionLoc = ctor->getEndLoc();
    indentationLoc = ctor->getLoc();
  }

  // If no initializers were listed, start at the opening '{' for the class.
  if (insertionLoc.isInvalid()) {
    insertionLoc = classDecl->getBraces().Start;
  }
  if (indentationLoc.isInvalid()) {
    indentationLoc = classDecl->getBraces().End;
  }

  // Adjust the insertion location to point at the end of this line (i.e.,
  // the start of the next line).
  insertionLoc = Lexer::getLocForEndOfLine(TC.Context.SourceMgr,
                                           insertionLoc);

  // Find the indentation used on the indentation line.
  StringRef indentation = Lexer::getIndentationForLine(TC.Context.SourceMgr,
                                                       indentationLoc);

  // Pretty-print the superclass initializer into a string.
  // FIXME: Form a new initializer by performing the appropriate
  // substitutions of subclass types into the superclass types, so that
  // we get the right generic parameters.
  std::string initializerText;
  {
    PrintOptions options;
    options.PrintDefaultParameterPlaceholder = false;
    options.PrintImplicitAttrs = false;

    // Render the text.
    llvm::raw_string_ostream out(initializerText);
    {
      ExtraIndentStreamPrinter printer(out, indentation);
      printer.printNewline();

      // If there is no explicit 'required', print one.
      bool hasExplicitRequiredAttr = false;
      if (auto requiredAttr
            = superInitializer->getAttrs().getAttribute<RequiredAttr>())
          hasExplicitRequiredAttr = !requiredAttr->isImplicit();

      if (!hasExplicitRequiredAttr)
        printer << "required ";

      superInitializer->print(printer, options);
    }

    // FIXME: Infer body indentation from the source rather than hard-coding
    // 4 spaces.

    // Add a dummy body.
    out << " {\n";
    out << indentation << "    fatalError(\"";
    superInitializer->getFullName().printPretty(out);
    out << " has not been implemented\")\n";
    out << indentation << "}\n";
  }

  // Complain.
  TC.diagnose(insertionLoc, diag::required_initializer_missing,
              superInitializer->getFullName(),
              superInitializer->getDeclContext()->getDeclaredTypeOfContext())
    .fixItInsert(insertionLoc, initializerText);

  TC.diagnose(findNonImplicitRequiredInit(superInitializer),
              diag::required_initializer_here);
}

void TypeChecker::addImplicitConstructors(NominalTypeDecl *decl) {
  // We can only synthesize implicit constructors for classes and structs.
 if (!isa<ClassDecl>(decl) && !isa<StructDecl>(decl))
   return;

  // If we already added implicit initializers, we're done.
  if (decl->addedImplicitInitializers())
    return;
  
  // Don't add implicit constructors for an invalid declaration
  if (decl->isInvalid())
    return;

  // Local function that produces the canonical parameter type of the given
  // initializer.
  // FIXME: Doesn't work properly for generics.
  auto getInitializerParamType = [](ConstructorDecl *ctor) -> CanType {
    auto interfaceTy = ctor->getInterfaceType();

    // Skip the 'self' parameter.
    auto uncurriedInitTy = interfaceTy->castTo<AnyFunctionType>()->getResult();

    // Grab the parameter type;
    auto paramTy = uncurriedInitTy->castTo<AnyFunctionType>()->getInput();

    return paramTy->getCanonicalType();
  };

  // Check whether there is a user-declared constructor or an instance
  // variable.
  bool FoundMemberwiseInitializedProperty = false;
  bool SuppressDefaultInitializer = false;
  bool SuppressMemberwiseInitializer = false;
  bool FoundDesignatedInit = false;
  decl->setAddedImplicitInitializers();
  SmallPtrSet<CanType, 4> initializerParamTypes;
  llvm::SmallPtrSet<ConstructorDecl *, 4> overriddenInits;
  for (auto member : decl->getMembers()) {
    if (auto ctor = dyn_cast<ConstructorDecl>(member)) {
      validateDecl(ctor);

      if (ctor->isDesignatedInit())
        FoundDesignatedInit = true;

      if (!ctor->isInvalid())
        initializerParamTypes.insert(getInitializerParamType(ctor));

      if (auto overridden = ctor->getOverriddenDecl())
        overriddenInits.insert(overridden);

      continue;
    }

    if (auto var = dyn_cast<VarDecl>(member)) {
      if (var->hasStorage() && !var->isStatic() && !var->isInvalid()) {
        // Initialized 'let' properties have storage, but don't get an argument
        // to the memberwise initializer since they already have an initial
        // value that cannot be overridden.
        if (var->isLet() && var->getParentInitializer()) {
          
          // We cannot handle properties like:
          //   let (a,b) = (1,2)
          // for now, just disable implicit init synthesization in structs in
          // this case.
          auto SP = var->getParentPattern();
          if (auto *TP = dyn_cast<TypedPattern>(SP))
            SP = TP->getSubPattern();
          if (!isa<NamedPattern>(SP) && isa<StructDecl>(decl))
            return;
          
          continue;
        }
        
        FoundMemberwiseInitializedProperty = true;
      }
      
      // FIXME: Disable memberwise initializer if a property uses a behavior.
      // Behaviors should be able to control whether they interact with
      // memberwise initialization.
      if (var->hasBehavior())
        SuppressMemberwiseInitializer = true;
      continue;
    }

    // If a stored property lacks an initial value and if there is no way to
    // synthesize an initial value (e.g. for an optional) then we suppress
    // generation of the default initializer.
    if (auto pbd = dyn_cast<PatternBindingDecl>(member)) {
      if (pbd->hasStorage() && !pbd->isStatic() && !pbd->isImplicit())
        for (auto entry : pbd->getPatternList()) {
          if (entry.getInit()) continue;

          // If one of the bound variables is @NSManaged, go ahead no matter
          // what.
          bool CheckDefaultInitializer = true;
          entry.getPattern()->forEachVariable([&](VarDecl *vd) {
            if (vd->getAttrs().hasAttribute<NSManagedAttr>())
              CheckDefaultInitializer = false;
          });
          
          // If we cannot default initialize the property, we cannot
          // synthesize a default initializer for the class.
          if (CheckDefaultInitializer && !isDefaultInitializable(pbd))
            SuppressDefaultInitializer = true;
        }
      continue;
    }
  }

  if (auto structDecl = dyn_cast<StructDecl>(decl)) {
    if (!FoundDesignatedInit && !SuppressMemberwiseInitializer
        && !structDecl->hasUnreferenceableStorage()) {
      // For a struct with memberwise initialized properties, we add a
      // memberwise init.
      if (FoundMemberwiseInitializedProperty) {
        // Create the implicit memberwise constructor.
        auto ctor = createImplicitConstructor(
                      *this, decl, ImplicitConstructorKind::Memberwise);
        decl->addMember(ctor);
      }

      // If we found a stored property, add a default constructor.
      if (!SuppressDefaultInitializer)
        defineDefaultConstructor(decl);
    }
    return;
  }
 
  // For a class with a superclass, automatically define overrides
  // for all of the superclass's designated initializers.
  // FIXME: Currently skipping generic classes.
  auto classDecl = cast<ClassDecl>(decl);
  if (classDecl->hasSuperclass()) {
    bool canInheritInitializers = !FoundDesignatedInit;

    // We can't define these overrides if we have any uninitialized
    // stored properties.
    if (SuppressDefaultInitializer && !FoundDesignatedInit) {
      diagnoseClassWithoutInitializers(*this, classDecl);
      return;
    }

    auto superclassTy = classDecl->getSuperclass();
    auto *superclassDecl = superclassTy->getClassOrBoundGenericClass();
    assert(superclassDecl && "Superclass of class is not a class?");
    if (!superclassDecl->addedImplicitInitializers())
      addImplicitConstructors(superclassDecl);

    auto ctors = lookupConstructors(classDecl, superclassTy,
                                    NameLookupFlags::IgnoreAccessibility);

    for (auto memberResult : ctors) {
      auto member = memberResult.Decl;

      // Skip unavailable superclass initializers.
      if (AvailableAttr::isUnavailable(member))
        continue;

      // Skip invalid superclass initializers.
      auto superclassCtor = dyn_cast<ConstructorDecl>(member);
      if (superclassCtor->isInvalid())
        continue;

      // We only care about required or designated initializers.
      if (!superclassCtor->isRequired() &&
          !superclassCtor->isDesignatedInit())
        continue;

      // If we have an override for this constructor, it's okay.
      if (overriddenInits.count(superclassCtor) > 0)
        continue;

      // If the superclass constructor is a convenience initializer
      // that is inherited into the current class, it's okay.
      if (superclassCtor->isInheritable() &&
          classDecl->inheritsSuperclassInitializers(this)) {
        assert(superclassCtor->isRequired());
        continue;
      }

      // Diagnose a missing override of a required initializer.
      if (superclassCtor->isRequired() && FoundDesignatedInit) {
        diagnoseMissingRequiredInitializer(*this, classDecl, superclassCtor);
        continue;
      }

      // A designated or required initializer has not been overridden.

      // Skip this designated initializer if it's in an extension.
      // FIXME: We shouldn't allow this.
      if (isa<ExtensionDecl>(superclassCtor->getDeclContext()))
        continue;

      // If we have already introduced an initializer with this parameter type,
      // don't add one now.
      if (!initializerParamTypes.insert(
             getInitializerParamType(superclassCtor)).second)
        continue;

      // If we're inheriting initializers, create an override delegating
      // to 'super.init'. Otherwise, create a stub which traps at runtime.
      auto kind = canInheritInitializers
                    ? DesignatedInitKind::Chaining
                    : DesignatedInitKind::Stub;

      // If the superclass initializer is not accessible from the derived
      // class, we cannot chain to 'super.init' either -- create a stub.
      if (!superclassCtor->isAccessibleFrom(classDecl)) {
        assert(!superclassCtor->isRequired() &&
               "required initializer less visible than the class?");
        kind = DesignatedInitKind::Stub;
      }

      // We have a designated initializer. Create an override of it.
      if (auto ctor = createDesignatedInitOverride(
                        *this, classDecl, superclassCtor, kind)) {
        classDecl->addMember(ctor);
      }
    }

    return;
  }

  if (!FoundDesignatedInit) {
    // For a class with no superclass, automatically define a default
    // constructor.

    // ... unless there are uninitialized stored properties.
    if (SuppressDefaultInitializer) {
      diagnoseClassWithoutInitializers(*this, classDecl);
      return;
    }

    defineDefaultConstructor(decl);
  }
}

void TypeChecker::addImplicitStructConformances(StructDecl *SD) {
  // Type-check the protocol conformances of the struct decl to instantiate its
  // derived conformances.
  checkConformancesInContext(SD, SD);
}

void TypeChecker::addImplicitEnumConformances(EnumDecl *ED) {
  // Type-check the raw values of the enum.
  for (auto elt : ED->getAllElements()) {
    assert(elt->hasRawValueExpr());
    if (elt->getTypeCheckedRawValueExpr()) continue;
    Expr *typeChecked = elt->getRawValueExpr();
    Type rawTy = ED->mapTypeIntoContext(ED->getRawType());
    bool error = typeCheckExpression(typeChecked, ED, 
                                     TypeLoc::withoutLoc(rawTy),
                                     CTP_EnumCaseRawValue);
    assert(!error); (void)error;
    elt->setTypeCheckedRawValueExpr(typeChecked);
    checkEnumElementErrorHandling(elt);
  }
  
  // Type-check the protocol conformances of the enum decl to instantiate its
  // derived conformances.
  checkConformancesInContext(ED, ED);
}

void TypeChecker::defineDefaultConstructor(NominalTypeDecl *decl) {
  PrettyStackTraceDecl stackTrace("defining default constructor for",
                                  decl);

  // Clang-imported types should never get a default constructor, just a
  // memberwise one.
  if (decl->hasClangNode())
    return;

  // For a class, check whether the superclass (if it exists) is
  // default-initializable.
  if (isa<ClassDecl>(decl)) {
    // We need to look for a default constructor.
    if (auto superTy = getSuperClassOf(decl->getDeclaredInterfaceType())) {
      // If there are no default ctors for our supertype, we can't do anything.
      auto ctors = lookupConstructors(decl, superTy);
      if (!ctors)
        return;

      // Check whether we have a constructor that can be called with an empty
      // tuple.
      bool foundDefaultConstructor = false;
      for (auto memberResult : ctors) {
        auto member = memberResult.Decl;

        // Dig out the parameter tuple for this constructor.
        auto ctor = dyn_cast<ConstructorDecl>(member);
        if (!ctor || ctor->isInvalid())
          continue;

        // Check to see if this ctor has zero arguments, or if they all have
        // default values.
        auto params = ctor->getParameters();
        
        bool missingInit = false;
        for (auto param : *params) {
          if (!param->isDefaultArgument()) {
            missingInit = true;
            break;
          }
        }

        // Check to see if this is an impossible candidate.
        if (missingInit) {
          // If we found an impossible designated initializer, then we cannot
          // call super.init(), even if there is a match.
          if (ctor->isDesignatedInit())
            return;

          // Otherwise, keep looking.
          continue;
        }

        // Ok, we found a constructor that can be invoked with an empty tuple.
        // If this is our second, then we bail out, because we don't want to
        // pick one arbitrarily.
        if (foundDefaultConstructor)
          return;

        foundDefaultConstructor = true;
      }

      // If our superclass isn't default constructible, we aren't either.
      if (!foundDefaultConstructor) return;
    }
  }

  // Create the default constructor.
  auto ctor = createImplicitConstructor(*this, decl,
                                        ImplicitConstructorKind::Default);

  // Add the constructor.
  decl->addMember(ctor);

  // Create an empty body for the default constructor. The type-check of the
  // constructor body will introduce default initializations of the members.
  ctor->setBody(BraceStmt::create(Context, SourceLoc(), { }, SourceLoc()));
}

static void validateAttributes(TypeChecker &TC, Decl *D) {
  DeclAttributes &Attrs = D->getAttrs();

  auto checkObjCDeclContext = [](Decl *D) {
    DeclContext *DC = D->getDeclContext();
    if (DC->getAsClassOrClassExtensionContext())
      return true;
    if (auto *PD = dyn_cast<ProtocolDecl>(DC))
      if (PD->isObjC())
        return true;
    return false;
  };

  if (auto objcAttr = Attrs.getAttribute<ObjCAttr>()) {
    // Only certain decls can be ObjC.
    Optional<Diag<>> error;
    if (isa<ClassDecl>(D) ||
        isa<ProtocolDecl>(D)) {
      /* ok */
    } else if (auto ED = dyn_cast<EnumDecl>(D)) {
      if (ED->isGenericContext())
        error = diag::objc_enum_generic;
    } else if (auto EED = dyn_cast<EnumElementDecl>(D)) {
      auto ED = EED->getParentEnum();
      if (!ED->getAttrs().hasAttribute<ObjCAttr>())
        error = diag::objc_enum_case_req_objc_enum;
      else if (objcAttr->hasName() && EED->getParentCase()->getElements().size() > 1)
        error = diag::objc_enum_case_multi;
    } else if (auto *func = dyn_cast<FuncDecl>(D)) {
      if (!checkObjCDeclContext(D))
        error = diag::invalid_objc_decl_context;
      else if (func->isAccessor() && !func->isGetterOrSetter())
        error = diag::objc_observing_accessor;
    } else if (isa<ConstructorDecl>(D) ||
               isa<DestructorDecl>(D) ||
               isa<SubscriptDecl>(D) ||
               isa<VarDecl>(D)) {
      if (!checkObjCDeclContext(D))
        error = diag::invalid_objc_decl_context;
      /* ok */
    } else {
      error = diag::invalid_objc_decl;
    }

    if (error) {
      TC.diagnose(D->getStartLoc(), *error)
        .fixItRemove(objcAttr->getRangeWithAt());
      objcAttr->setInvalid();
      return;
    }

    // If there is a name, check whether the kind of name is
    // appropriate.
    if (auto objcName = objcAttr->getName()) {
      if (isa<ClassDecl>(D) || isa<ProtocolDecl>(D) || isa<VarDecl>(D)
          || isa<EnumDecl>(D) || isa<EnumElementDecl>(D)) {
        // Types and properties can only have nullary
        // names. Complain and recover by chopping off everything
        // after the first name.
        if (objcName->getNumArgs() > 0) {
          int which = isa<ClassDecl>(D)? 0
                    : isa<ProtocolDecl>(D)? 1
                    : isa<EnumDecl>(D)? 2
                    : isa<EnumElementDecl>(D)? 3
                    : 4;
          SourceLoc firstNameLoc = objcAttr->getNameLocs().front();
          SourceLoc afterFirstNameLoc = 
            Lexer::getLocForEndOfToken(TC.Context.SourceMgr, firstNameLoc);
          TC.diagnose(firstNameLoc, diag::objc_name_req_nullary, which)
            .fixItRemoveChars(afterFirstNameLoc, objcAttr->getRParenLoc());
          const_cast<ObjCAttr *>(objcAttr)->setName(
            ObjCSelector(TC.Context, 0, objcName->getSelectorPieces()[0]),
            /*implicit=*/false);
        }
      } else if (isa<SubscriptDecl>(D)) {
        // Subscripts can never have names.
        TC.diagnose(objcAttr->getLParenLoc(), diag::objc_name_subscript);
        const_cast<ObjCAttr *>(objcAttr)->clearName();
      } else {
        // We have a function. Make sure that the number of parameters
        // matches the "number of colons" in the name.
        auto func = cast<AbstractFunctionDecl>(D);
        auto params = func->getParameterList(1);
        unsigned numParameters = params->size();
        if (auto CD = dyn_cast<ConstructorDecl>(func))
          if (CD->isObjCZeroParameterWithLongSelector())
            numParameters = 0;  // Something like "init(foo: ())"

        // A throwing method has an error parameter.
        if (func->hasThrows())
          ++numParameters;

        unsigned numArgumentNames = objcName->getNumArgs();
        if (numArgumentNames != numParameters) {
          TC.diagnose(objcAttr->getNameLocs().front(), 
                      diag::objc_name_func_mismatch,
                      isa<FuncDecl>(func), 
                      numArgumentNames, 
                      numArgumentNames != 1,
                      numParameters,
                      numParameters != 1,
                      func->hasThrows());
          D->getAttrs().add(
            ObjCAttr::createUnnamed(TC.Context,
                                    objcAttr->AtLoc,
                                    objcAttr->Range.Start));
          D->getAttrs().removeAttribute(objcAttr);
        }
      }
    } else if (isa<EnumElementDecl>(D)) {
      // Enum elements require names.
      TC.diagnose(objcAttr->getLocation(), diag::objc_enum_case_req_name)
        .fixItRemove(objcAttr->getRangeWithAt());
      objcAttr->setInvalid();
    }
  }

  if (auto nonObjcAttr = Attrs.getAttribute<NonObjCAttr>()) {
    // Only methods, properties, subscripts and constructors can be NonObjC.
    // The last three are handled automatically by generic attribute
    // validation -- for the first one, we have to check FuncDecls
    // ourselves.
    Optional<Diag<>> error;

    auto func = dyn_cast<FuncDecl>(D);
    if (func &&
        (isa<DestructorDecl>(func) ||
         !checkObjCDeclContext(func) ||
         (func->isAccessor() && !func->isGetterOrSetter()))) {
      error = diag::invalid_nonobjc_decl;
    }

    if (error) {
      TC.diagnose(D->getStartLoc(), *error)
        .fixItRemove(nonObjcAttr->getRangeWithAt());
      nonObjcAttr->setInvalid();
      return;
    }
  }

  // Only protocol members can be optional.
  if (auto *OA = Attrs.getAttribute<OptionalAttr>()) {
    if (!isa<ProtocolDecl>(D->getDeclContext())) {
      TC.diagnose(OA->getLocation(), diag::optional_attribute_non_protocol)
        .fixItRemove(OA->getRange());
      D->getAttrs().removeAttribute(OA);
    } else if (!cast<ProtocolDecl>(D->getDeclContext())->isObjC()) {
      TC.diagnose(OA->getLocation(),
                  diag::optional_attribute_non_objc_protocol);
      D->getAttrs().removeAttribute(OA);
    } else if (isa<ConstructorDecl>(D)) {
      TC.diagnose(OA->getLocation(),
                  diag::optional_attribute_initializer);
      D->getAttrs().removeAttribute(OA);
    } else {
      auto objcAttr = D->getAttrs().getAttribute<ObjCAttr>();
      if (!objcAttr || objcAttr->isImplicit()) {
        auto diag = TC.diagnose(OA->getLocation(),
                                diag::optional_attribute_missing_explicit_objc);
        if (auto VD = dyn_cast<ValueDecl>(D))
          diag.fixItInsert(VD->getAttributeInsertionLoc(false), "@objc ");
      }
    }
  }

  // Only protocols that are @objc can have "unavailable" methods.
  if (auto AvAttr = Attrs.getUnavailable(TC.Context)) {
    if (auto PD = dyn_cast<ProtocolDecl>(D->getDeclContext())) {
      if (!PD->isObjC()) {
        TC.diagnose(AvAttr->getLocation(),
                    diag::unavailable_method_non_objc_protocol);
        D->getAttrs().removeAttribute(AvAttr);
      }
    }
  }
}