diff --git a/include/clang/Basic/Diagnostic.h b/include/clang/Basic/Diagnostic.h
index 0327636..2d9d288 100644
--- a/include/clang/Basic/Diagnostic.h
+++ b/include/clang/Basic/Diagnostic.h
@@ -1319,6 +1319,26 @@
   }
 };
 
+/// \brief Diagnostic consumer that forwards diagnostics along to an
+/// existing, already-initialized diagnostic consumer.
+///
+class ForwardingDiagnosticConsumer : public DiagnosticConsumer {
+  DiagnosticConsumer &Target;
+
+public:
+  ForwardingDiagnosticConsumer(DiagnosticConsumer &Target) : Target(Target) {}
+
+  virtual ~ForwardingDiagnosticConsumer();
+
+  virtual void HandleDiagnostic(DiagnosticsEngine::Level DiagLevel,
+                                const Diagnostic &Info);
+  virtual void clear();
+
+  virtual bool IncludeInDiagnosticCounts() const;
+
+  virtual DiagnosticConsumer *clone(DiagnosticsEngine &Diags) const;
+};
+
 // Struct used for sending info about how a type should be printed.
 struct TemplateDiffTypes {
   intptr_t FromType;
diff --git a/include/clang/Lex/ModuleMap.h b/include/clang/Lex/ModuleMap.h
index 33c92f5..dc75f18 100644
--- a/include/clang/Lex/ModuleMap.h
+++ b/include/clang/Lex/ModuleMap.h
@@ -161,7 +161,7 @@
   /// \param LangOpts Language options for this translation unit.
   ///
   /// \param Target The target for this translation unit.
-  ModuleMap(FileManager &FileMgr, const DiagnosticConsumer &DC,
+  ModuleMap(FileManager &FileMgr, DiagnosticConsumer &DC,
             const LangOptions &LangOpts, const TargetInfo *Target,
             HeaderSearch &HeaderInfo);
 
diff --git a/lib/Basic/Diagnostic.cpp b/lib/Basic/Diagnostic.cpp
index 4a5b191..5eff86c 100644
--- a/lib/Basic/Diagnostic.cpp
+++ b/lib/Basic/Diagnostic.cpp
@@ -972,6 +972,28 @@
 
 void IgnoringDiagConsumer::anchor() { }
 
+ForwardingDiagnosticConsumer::~ForwardingDiagnosticConsumer() {}
+
+void ForwardingDiagnosticConsumer::HandleDiagnostic(
+       DiagnosticsEngine::Level DiagLevel,
+       const Diagnostic &Info) {
+  Target.HandleDiagnostic(DiagLevel, Info);
+}
+
+void ForwardingDiagnosticConsumer::clear() {
+  DiagnosticConsumer::clear();
+  Target.clear();
+}
+
+bool ForwardingDiagnosticConsumer::IncludeInDiagnosticCounts() const {
+  return Target.IncludeInDiagnosticCounts();
+}
+
+DiagnosticConsumer *
+ForwardingDiagnosticConsumer::clone(DiagnosticsEngine &Diags) const {
+  return new ForwardingDiagnosticConsumer(Target);
+}
+
 PartialDiagnostic::StorageAllocator::StorageAllocator() {
   for (unsigned I = 0; I != NumCached; ++I)
     FreeList[I] = Cached + I;
diff --git a/lib/Frontend/ASTMerge.cpp b/lib/Frontend/ASTMerge.cpp
index bfb3083..b6c644e 100644
--- a/lib/Frontend/ASTMerge.cpp
+++ b/lib/Frontend/ASTMerge.cpp
@@ -34,7 +34,7 @@
 void ASTMergeAction::ExecuteAction() {
   CompilerInstance &CI = getCompilerInstance();
   CI.getDiagnostics().getClient()->BeginSourceFile(
-                                         CI.getASTContext().getLangOpts());
+                                             CI.getASTContext().getLangOpts());
   CI.getDiagnostics().SetArgToStringFn(&FormatASTNodeDiagnosticArgument,
                                        &CI.getASTContext());
   IntrusiveRefCntPtr<DiagnosticIDs>
@@ -42,8 +42,9 @@
   for (unsigned I = 0, N = ASTFiles.size(); I != N; ++I) {
     IntrusiveRefCntPtr<DiagnosticsEngine>
         Diags(new DiagnosticsEngine(DiagIDs, &CI.getDiagnosticOpts(),
-                                    CI.getDiagnostics().getClient(),
-                                    /*ShouldOwnClient=*/false));
+                                    new ForwardingDiagnosticConsumer(
+                                          *CI.getDiagnostics().getClient()),
+                                    /*ShouldOwnClient=*/true));
     ASTUnit *Unit = ASTUnit::LoadFromASTFile(ASTFiles[I], Diags,
                                              CI.getFileSystemOpts(), false);
     if (!Unit)
diff --git a/lib/Frontend/ASTUnit.cpp b/lib/Frontend/ASTUnit.cpp
index c1115ae..1b4e5a0 100644
--- a/lib/Frontend/ASTUnit.cpp
+++ b/lib/Frontend/ASTUnit.cpp
@@ -236,6 +236,11 @@
 }
 
 ASTUnit::~ASTUnit() {
+  // If we loaded from an AST file, balance out the BeginSourceFile call.
+  if (MainFileIsAST && getDiagnostics().getClient()) {
+    getDiagnostics().getClient()->EndSourceFile();
+  }
+
   clearFileLevelDecls();
 
   // Clean up the temporary files and the preamble file.
@@ -581,14 +586,22 @@
   }
 };
 
+  /// \brief Diagnostic consumer that saves each diagnostic it is given.
 class StoredDiagnosticConsumer : public DiagnosticConsumer {
   SmallVectorImpl<StoredDiagnostic> &StoredDiags;
-  
+  SourceManager *SourceMgr;
+
 public:
   explicit StoredDiagnosticConsumer(
                           SmallVectorImpl<StoredDiagnostic> &StoredDiags)
-    : StoredDiags(StoredDiags) { }
-  
+    : StoredDiags(StoredDiags), SourceMgr(0) { }
+
+  virtual void BeginSourceFile(const LangOptions &LangOpts,
+                               const Preprocessor *PP = 0) {
+    if (PP)
+      SourceMgr = &PP->getSourceManager();
+  }
+
   virtual void HandleDiagnostic(DiagnosticsEngine::Level Level,
                                 const Diagnostic &Info);
   
@@ -635,7 +648,11 @@
   // Default implementation (Warnings/errors count).
   DiagnosticConsumer::HandleDiagnostic(Level, Info);
 
-  StoredDiags.push_back(StoredDiagnostic(Level, Info));
+  // Only record the diagnostic if it's part of the source manager we know
+  // about. This effectively drops diagnostics from modules we're building.
+  // FIXME: In the long run, ee don't want to drop source managers from modules.
+  if (!Info.hasSourceManager() || &Info.getSourceManager() == SourceMgr)
+    StoredDiags.push_back(StoredDiagnostic(Level, Info));
 }
 
 ASTDeserializationListener *ASTUnit::getDeserializationListener() {
@@ -835,6 +852,9 @@
   ReaderPtr->InitializeSema(*AST->TheSema);
   AST->Reader = ReaderPtr;
 
+  // Tell the diagnostic client that we have started a source file.
+  AST->getDiagnostics().getClient()->BeginSourceFile(Context.getLangOpts(),&PP);
+
   return AST.take();
 }
 
diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index df06a81..027e99a 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -868,9 +868,11 @@
   // module.
   CompilerInstance Instance;
   Instance.setInvocation(&*Invocation);
-  Instance.createDiagnostics(&ImportingInstance.getDiagnosticClient(),
+
+  Instance.createDiagnostics(new ForwardingDiagnosticConsumer(
+                                   ImportingInstance.getDiagnosticClient()),
                              /*ShouldOwnClient=*/true,
-                             /*ShouldCloneClient=*/true);
+                             /*ShouldCloneClient=*/false);
 
   // Note that this module is part of the module build stack, so that we
   // can detect cycles in the module graph.
@@ -892,6 +894,7 @@
   llvm::CrashRecoveryContext CRC;
   CompileModuleMapData Data = { Instance, CreateModuleAction };
   CRC.RunSafelyOnThread(&doCompileMapModule, &Data, ThreadStackSize);
+
   
   // Delete the temporary module map file.
   // FIXME: Even though we're executing under crash protection, it would still
diff --git a/lib/Frontend/VerifyDiagnosticConsumer.cpp b/lib/Frontend/VerifyDiagnosticConsumer.cpp
index 4f05aef..91aa52b 100644
--- a/lib/Frontend/VerifyDiagnosticConsumer.cpp
+++ b/lib/Frontend/VerifyDiagnosticConsumer.cpp
@@ -111,8 +111,13 @@
 
 void VerifyDiagnosticConsumer::HandleDiagnostic(
       DiagnosticsEngine::Level DiagLevel, const Diagnostic &Info) {
-  if (Info.hasSourceManager())
+  if (Info.hasSourceManager()) {
+    // If this diagnostic is for a different source manager, ignore it.
+    if (SrcManager && &Info.getSourceManager() != SrcManager)
+      return;
+
     setSourceManager(Info.getSourceManager());
+  }
 
 #ifndef NDEBUG
   // Debug build tracks unparsed files for possible
@@ -476,6 +481,11 @@
 bool VerifyDiagnosticConsumer::HandleComment(Preprocessor &PP,
                                              SourceRange Comment) {
   SourceManager &SM = PP.getSourceManager();
+
+  // If this comment is for a different source manager, ignore it.
+  if (SrcManager && &SM != SrcManager)
+    return false;
+
   SourceLocation CommentBegin = Comment.getBegin();
 
   const char *CommentRaw = SM.getCharacterData(CommentBegin);
diff --git a/lib/Lex/ModuleMap.cpp b/lib/Lex/ModuleMap.cpp
index 1458a73..3e7a44c 100644
--- a/lib/Lex/ModuleMap.cpp
+++ b/lib/Lex/ModuleMap.cpp
@@ -83,7 +83,7 @@
   return Context;
 }
 
-ModuleMap::ModuleMap(FileManager &FileMgr, const DiagnosticConsumer &DC,
+ModuleMap::ModuleMap(FileManager &FileMgr, DiagnosticConsumer &DC,
                      const LangOptions &LangOpts, const TargetInfo *Target,
                      HeaderSearch &HeaderInfo)
   : LangOpts(LangOpts), Target(Target), HeaderInfo(HeaderInfo),
@@ -92,7 +92,8 @@
   IntrusiveRefCntPtr<DiagnosticIDs> DiagIDs(new DiagnosticIDs);
   Diags = IntrusiveRefCntPtr<DiagnosticsEngine>(
             new DiagnosticsEngine(DiagIDs, new DiagnosticOptions));
-  Diags->setClient(DC.clone(*Diags), /*ShouldOwnClient=*/true);
+  Diags->setClient(new ForwardingDiagnosticConsumer(DC),
+                   /*ShouldOwnClient=*/true);
   SourceMgr = new SourceManager(*Diags, FileMgr);
 }
 
diff --git a/test/Modules/Inputs/ModuleDiags/has_errors.h b/test/Modules/Inputs/ModuleDiags/has_errors.h
new file mode 100644
index 0000000..2c0929a
--- /dev/null
+++ b/test/Modules/Inputs/ModuleDiags/has_errors.h
@@ -0,0 +1,2 @@
+static void foo(void) { }
+static void foo(void) { }
diff --git a/test/Modules/Inputs/ModuleDiags/has_warnings.h b/test/Modules/Inputs/ModuleDiags/has_warnings.h
new file mode 100644
index 0000000..87112be
--- /dev/null
+++ b/test/Modules/Inputs/ModuleDiags/has_warnings.h
@@ -0,0 +1,3 @@
+
+int int_val;
+float *float_ptr = &int_val;
diff --git a/test/Modules/Inputs/ModuleDiags/module.map b/test/Modules/Inputs/ModuleDiags/module.map
new file mode 100644
index 0000000..09b2508
--- /dev/null
+++ b/test/Modules/Inputs/ModuleDiags/module.map
@@ -0,0 +1,7 @@
+module HasWarnings {
+  header "has_warnings.h"
+}
+
+module HasErrors {
+  header "has_errors.h"
+}
diff --git a/test/Modules/cycles.c b/test/Modules/cycles.c
index 4326e76..5f83092 100644
--- a/test/Modules/cycles.c
+++ b/test/Modules/cycles.c
@@ -6,8 +6,8 @@
 // CHECK: While building module 'MutuallyRecursive1' imported from
 // CHECK: While building module 'MutuallyRecursive2' imported from
 // CHECK: MutuallyRecursive2.h:3:9: fatal error: cyclic dependency in module 'MutuallyRecursive1': MutuallyRecursive1 -> MutuallyRecursive2 -> MutuallyRecursive1
-// CHECK: While building module 'MutuallyRecursive1' imported from
 // CHECK: MutuallyRecursive1.h:2:9: fatal error: could not build module 'MutuallyRecursive2'
 // CHECK: cycles.c:4:9: fatal error: could not build module 'MutuallyRecursive1'
-// CHECK-NOT: error:
+// CHECK: 3 errors generated
+
 
diff --git a/test/Modules/serialized-diags.m b/test/Modules/serialized-diags.m
new file mode 100644
index 0000000..99ba80a
--- /dev/null
+++ b/test/Modules/serialized-diags.m
@@ -0,0 +1,32 @@
+@import HasWarnings;
+
+#ifdef WITH_ERRORS
+@import HasErrors;
+#endif
+
+float float_val;
+double *double_ptr = &float_val;
+
+// RUN: rm -rf %t %t.diag %t.out
+// RUN: %clang -fmodules -fmodules-cache-path=%t/ModuleCache -I %S/Inputs/ModuleDiags -fsyntax-only %s --serialize-diagnostics %t.diag > /dev/null 2>&1
+// RUN: c-index-test -read-diagnostics %t.diag > %t.out 2>&1
+// RUN: FileCheck --input-file=%t.out %s
+
+// CHECK: Inputs/ModuleDiags/has_warnings.h:3:8: warning: incompatible pointer types initializing 'float *'
+// CHECK: serialized-diags.m:1:9: note: while building module 'HasWarnings' imported from
+// CHECK: serialized-diags.m:8:9: warning: incompatible pointer types initializing 'double *'
+// CHECK: Number of diagnostics: 2
+
+// RUN: rm -rf %t %t.diag_errors %t.out_errors
+// RUN: not %clang -fmodules -fmodules-cache-path=%t/ModuleCache -I %S/Inputs/ModuleDiags -fsyntax-only -DWITH_ERRORS %s --serialize-diagnostics %t.diag_errors > /dev/null 2>&1
+// RUN: c-index-test -read-diagnostics %t.diag_errors > %t.out_errors 2>&1
+// RUN: FileCheck -check-prefix=CHECK-WITH-ERRORS --input-file=%t.out_errors %s
+
+// CHECK-WITH-ERRORS: has_warnings.h:3:8: warning: incompatible pointer types initializing 'float *'
+// CHECK-WITH-ERRORS: serialized-diags.m:1:9: note: while building module 'HasWarnings'
+// CHECK-WITH-ERRORS: has_errors.h:2:13: error: redefinition of 'foo'
+// CHECK-WITH-ERRORS: serialized-diags.m:4:9: note: while building module 'HasErrors'
+// CHECK-WITH-ERRORS: has_errors.h:1:13: note: previous definition is here
+// CHECK-WITH-ERRORS: serialized-diags.m:4:9: fatal: could not build module 'HasErrors'
+// CHECK-WITH-ERRORS: Number of diagnostics: 3
+