Index: cfe/trunk/lib/Sema/SemaDeclObjC.cpp
===================================================================
--- cfe/trunk/lib/Sema/SemaDeclObjC.cpp
+++ cfe/trunk/lib/Sema/SemaDeclObjC.cpp
@@ -1210,6 +1210,11 @@
     PDecl = ObjCProtocolDecl::Create(Context, CurContext, ProtocolName,
                                      ProtocolLoc, AtProtoInterfaceLoc,
                                      /*PrevDecl=*/nullptr);
+
+    // If we are using modules, add the decl to the context in order to
+    // serialize something meaningful.
+    if (getLangOpts().Modules)
+      PushOnScopeChains(PDecl, TUScope);
     PDecl->startDefinition();
   } else {
     if (PrevDecl) {
Index: cfe/trunk/test/Modules/Inputs/protocol-redefinition/Base.framework/Headers/Base.h
===================================================================
--- cfe/trunk/test/Modules/Inputs/protocol-redefinition/Base.framework/Headers/Base.h
+++ cfe/trunk/test/Modules/Inputs/protocol-redefinition/Base.framework/Headers/Base.h
@@ -0,0 +1,3 @@
+@protocol Foo
+- (void)someMethodOnFoo;
+@end
Index: cfe/trunk/test/Modules/Inputs/protocol-redefinition/Base.framework/Modules/module.modulemap
===================================================================
--- cfe/trunk/test/Modules/Inputs/protocol-redefinition/Base.framework/Modules/module.modulemap
+++ cfe/trunk/test/Modules/Inputs/protocol-redefinition/Base.framework/Modules/module.modulemap
@@ -0,0 +1,4 @@
+framework module Base {
+  header "Base.h"
+  export *
+}
\ No newline at end of file
Index: cfe/trunk/test/Modules/Inputs/protocol-redefinition/Kit.framework/Headers/Kit.h
===================================================================
--- cfe/trunk/test/Modules/Inputs/protocol-redefinition/Kit.framework/Headers/Kit.h
+++ cfe/trunk/test/Modules/Inputs/protocol-redefinition/Kit.framework/Headers/Kit.h
@@ -0,0 +1,6 @@
+#import <Base/Base.h>
+
+// REDECLARATION
+@protocol Foo
+- (void)someMethodOnFoo;
+@end
Index: cfe/trunk/test/Modules/Inputs/protocol-redefinition/Kit.framework/Modules/module.modulemap
===================================================================
--- cfe/trunk/test/Modules/Inputs/protocol-redefinition/Kit.framework/Modules/module.modulemap
+++ cfe/trunk/test/Modules/Inputs/protocol-redefinition/Kit.framework/Modules/module.modulemap
@@ -0,0 +1,4 @@
+framework module Kit {
+  header "Kit.h"
+  export *
+}
\ No newline at end of file
Index: cfe/trunk/test/Modules/protocol-redefinition.m
===================================================================
--- cfe/trunk/test/Modules/protocol-redefinition.m
+++ cfe/trunk/test/Modules/protocol-redefinition.m
@@ -0,0 +1,6 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -F%S/Inputs/protocol-redefinition -fsyntax-only %s -Wno-private-module -verify
+
+// expected-no-diagnostics
+
+@import Kit;