diff --git a/include/clang/AST/Redeclarable.h b/include/clang/AST/Redeclarable.h
index cd5f186..89a9d3c 100644
--- a/include/clang/AST/Redeclarable.h
+++ b/include/clang/AST/Redeclarable.h
@@ -21,6 +21,60 @@
 namespace clang {
 class ASTContext;
 
+// Some notes on redeclarables:
+//
+//  - Every redeclarable is on a circular linked list.
+//
+//  - Every decl has a pointer to the first element of the chain _and_ a
+//    DeclLink that may point to one of 3 possible states:
+//      - the "previous" (temporal) element in the chain
+//      - the "latest" (temporal) element in the chain
+//      - the an "uninitialized-latest" value (when newly-constructed)
+//
+//  - The first element is also often called the canonical element. Every
+//    element has a pointer to it so that "getCanonical" can be fast.
+//
+//  - Most links in the chain point to previous, except the link out of
+//    the first; it points to latest.
+//
+//  - Elements are called "first", "previous", "latest" or
+//    "most-recent" when referring to temporal order: order of addition
+//    to the chain.
+//
+//  - To make matters confusing, the DeclLink type uses the term "next"
+//    for its pointer-storage internally (thus functions like
+//    NextIsPrevious). It's easiest to just ignore the implementation of
+//    DeclLink when making sense of the redeclaration chain.
+//
+//  - There's also a "definition" link for several types of
+//    redeclarable, where only one definition should exist at any given
+//    time (and the defn pointer is stored in the decl's "data" which
+//    is copied to every element on the chain when it's changed).
+//
+//    Here is some ASCII art:
+//
+//      "first"                                     "latest"
+//      "canonical"                                 "most recent"
+//      +------------+         first                +--------------+
+//      |            | <--------------------------- |              |
+//      |            |                              |              |
+//      |            |                              |              |
+//      |            |       +--------------+       |              |
+//      |            | first |              |       |              |
+//      |            | <---- |              |       |              |
+//      |            |       |              |       |              |
+//      | @class A   |  link | @interface A |  link | @class A     |
+//      | seen first | <---- | seen second  | <---- | seen third   |
+//      |            |       |              |       |              |
+//      +------------+       +--------------+       +--------------+
+//      | data       | defn  | data         |  defn | data         |
+//      |            | ----> |              | <---- |              |
+//      +------------+       +--------------+       +--------------+
+//        |                     |     ^                  ^
+//        |                     |defn |                  |
+//        | link                +-----+                  |
+//        +-->-------------------------------------------+
+
 /// \brief Provides common interface for the Decls that can be redeclared.
 template<typename decl_type>
 class Redeclarable {
diff --git a/lib/Serialization/ASTReaderDecl.cpp b/lib/Serialization/ASTReaderDecl.cpp
index cbf54de..ca8ded7 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -125,6 +125,9 @@
     void ReadObjCDefinitionData(struct ObjCInterfaceDecl::DefinitionData &Data);
     void MergeDefinitionData(ObjCInterfaceDecl *D,
                              struct ObjCInterfaceDecl::DefinitionData &&NewDD);
+    void ReadObjCDefinitionData(struct ObjCProtocolDecl::DefinitionData &Data);
+    void MergeDefinitionData(ObjCProtocolDecl *D,
+                             struct ObjCProtocolDecl::DefinitionData &&NewDD);
 
     static NamedDecl *getAnonymousDeclForMerging(ASTReader &Reader,
                                                  DeclContext *DC,
@@ -1032,18 +1035,8 @@
   IVD->setSynthesize(synth);
 }
 
-void ASTDeclReader::VisitObjCProtocolDecl(ObjCProtocolDecl *PD) {
-  RedeclarableResult Redecl = VisitRedeclarable(PD);
-  VisitObjCContainerDecl(PD);
-  mergeRedeclarable(PD, Redecl);
-
-  if (Record.readInt()) {
-    // Read the definition.
-    PD->allocateDefinitionData();
-
-    // Set the definition data of the canonical declaration, so other
-    // redeclarations will see it.
-    PD->getCanonicalDecl()->Data = PD->Data;
+void ASTDeclReader::ReadObjCDefinitionData(
+         struct ObjCProtocolDecl::DefinitionData &Data) {
 
     unsigned NumProtoRefs = Record.readInt();
     SmallVector<ObjCProtocolDecl *, 16> ProtoRefs;
@@ -1054,9 +1047,37 @@
     ProtoLocs.reserve(NumProtoRefs);
     for (unsigned I = 0; I != NumProtoRefs; ++I)
       ProtoLocs.push_back(ReadSourceLocation());
-    PD->setProtocolList(ProtoRefs.data(), NumProtoRefs, ProtoLocs.data(),
-                        Reader.getContext());
+    Data.ReferencedProtocols.set(ProtoRefs.data(), NumProtoRefs,
+                                 ProtoLocs.data(), Reader.getContext());
+}
 
+void ASTDeclReader::MergeDefinitionData(ObjCProtocolDecl *D,
+         struct ObjCProtocolDecl::DefinitionData &&NewDD) {
+  // FIXME: odr checking?
+}
+
+void ASTDeclReader::VisitObjCProtocolDecl(ObjCProtocolDecl *PD) {
+  RedeclarableResult Redecl = VisitRedeclarable(PD);
+  VisitObjCContainerDecl(PD);
+  mergeRedeclarable(PD, Redecl);
+
+  if (Record.readInt()) {
+    // Read the definition.
+    PD->allocateDefinitionData();
+
+    ReadObjCDefinitionData(PD->data());
+
+    ObjCProtocolDecl *Canon = PD->getCanonicalDecl();
+    if (Canon->Data.getPointer()) {
+      // If we already have a definition, keep the definition invariant and
+      // merge the data.
+      MergeDefinitionData(Canon, std::move(PD->data()));
+      PD->Data = Canon->Data;
+    } else {
+      // Set the definition data of the canonical declaration, so other
+      // redeclarations will see it.
+      PD->getCanonicalDecl()->Data = PD->Data;
+    }
     // Note that we have deserialized a definition.
     Reader.PendingDefinitions.insert(PD);
   } else {
diff --git a/test/Modules/Inputs/lookup-assert-protocol/Base.h b/test/Modules/Inputs/lookup-assert-protocol/Base.h
new file mode 100644
index 0000000..5c9506f
--- /dev/null
+++ b/test/Modules/Inputs/lookup-assert-protocol/Base.h
@@ -0,0 +1,3 @@
+@protocol BaseProtocol
+- (void) test;
+@end
diff --git a/test/Modules/Inputs/lookup-assert-protocol/Derive.h b/test/Modules/Inputs/lookup-assert-protocol/Derive.h
new file mode 100644
index 0000000..fdcde61
--- /dev/null
+++ b/test/Modules/Inputs/lookup-assert-protocol/Derive.h
@@ -0,0 +1,4 @@
+#include "Base.h"
+@protocol DerivedProtocol<BaseProtocol>
+- (void) test2;
+@end
diff --git a/test/Modules/Inputs/lookup-assert-protocol/H3.h b/test/Modules/Inputs/lookup-assert-protocol/H3.h
new file mode 100644
index 0000000..3d8f878
--- /dev/null
+++ b/test/Modules/Inputs/lookup-assert-protocol/H3.h
@@ -0,0 +1 @@
+#include "Base.h"
diff --git a/test/Modules/Inputs/lookup-assert-protocol/module.map b/test/Modules/Inputs/lookup-assert-protocol/module.map
new file mode 100644
index 0000000..e8a89eb
--- /dev/null
+++ b/test/Modules/Inputs/lookup-assert-protocol/module.map
@@ -0,0 +1,4 @@
+module X {
+  header "H3.h"
+  export *
+}
diff --git a/test/Modules/lookup-assert-protocol.m b/test/Modules/lookup-assert-protocol.m
new file mode 100644
index 0000000..3c093f1
--- /dev/null
+++ b/test/Modules/lookup-assert-protocol.m
@@ -0,0 +1,17 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs/lookup-assert-protocol %s -verify
+// expected-no-diagnostics
+
+#include "Derive.h"
+#import <H3.h>
+
+__attribute__((objc_root_class))
+@interface Thing<DerivedProtocol>
+@end
+
+@implementation Thing
+- (void)test {
+}
+- (void)test2 {
+}
+@end