diff --git a/include/clang/Sema/ExternalSemaSource.h b/include/clang/Sema/ExternalSemaSource.h
index cbce757..bb18500 100644
--- a/include/clang/Sema/ExternalSemaSource.h
+++ b/include/clang/Sema/ExternalSemaSource.h
@@ -14,6 +14,7 @@
 #define LLVM_CLANG_SEMA_EXTERNAL_SEMA_SOURCE_H
 
 #include "clang/AST/ExternalASTSource.h"
+#include "clang/AST/Type.h"
 #include "clang/Sema/Weak.h"
 #include "llvm/ADT/MapVector.h"
 #include <utility>
@@ -177,6 +178,17 @@ public:
                  SmallVectorImpl<std::pair<ValueDecl *, 
                                            SourceLocation> > &Pending) {}
 
+
+  /// \brief Maybe produce a diagnostic note if this class can find a
+  /// way to get at the complete definition for T.
+  ///
+  /// \param Loc the location at which a complete type was required but not
+  /// provided
+  ///
+  /// \param T the QualType that should have been complete at Loc
+  virtual void MaybeDiagnoseMissingCompleteType(SourceLocation Loc, QualType T)
+    { }
+
   // isa/cast/dyn_cast support
   static bool classof(const ExternalASTSource *Source) {
     return Source->SemaSource;
diff --git a/include/clang/Sema/MultiplexExternalSemaSource.h b/include/clang/Sema/MultiplexExternalSemaSource.h
index ff87d05..3dbb59c 100644
--- a/include/clang/Sema/MultiplexExternalSemaSource.h
+++ b/include/clang/Sema/MultiplexExternalSemaSource.h
@@ -322,6 +322,8 @@ public:
   virtual void ReadPendingInstantiations(
               SmallVectorImpl<std::pair<ValueDecl*, SourceLocation> >& Pending);
 
+  virtual void MaybeDiagnoseMissingCompleteType(SourceLocation Loc, QualType T);
+
   // isa/cast/dyn_cast support
   static bool classof(const MultiplexExternalSemaSource*) { return true; }
   //static bool classof(const ExternalSemaSource*) { return true; }
diff --git a/lib/Sema/MultiplexExternalSemaSource.cpp b/lib/Sema/MultiplexExternalSemaSource.cpp
index d85624b..06bcf4e 100644
--- a/lib/Sema/MultiplexExternalSemaSource.cpp
+++ b/lib/Sema/MultiplexExternalSemaSource.cpp
@@ -267,3 +267,9 @@ void MultiplexExternalSemaSource::ReadPendingInstantiations(
   for(size_t i = 0; i < Sources.size(); ++i)
     Sources[i]->ReadPendingInstantiations(Pending);
 }
+
+void MultiplexExternalSemaSource::MaybeDiagnoseMissingCompleteType(
+                                  SourceLocation Loc, QualType T) {
+  for(size_t i = 0; i < Sources.size(); ++i)
+    Sources[i]->MaybeDiagnoseMissingCompleteType(Loc, T);
+}
\ No newline at end of file
diff --git a/lib/Sema/SemaType.cpp b/lib/Sema/SemaType.cpp
index 72a6d1a..70255be 100644
--- a/lib/Sema/SemaType.cpp
+++ b/lib/Sema/SemaType.cpp
@@ -4958,6 +4958,11 @@ bool Sema::RequireCompleteType(SourceLocation Loc, QualType T,
   if (IFace && !IFace->getDecl()->isInvalidDecl())
     Diag(IFace->getDecl()->getLocation(), diag::note_forward_class);
 
+  // If we have external information that we can use to suggest a fix,
+  // produce a note.
+  if (ExternalSource)
+    ExternalSource->MaybeDiagnoseMissingCompleteType(Loc, T);
+
   return true;
 }