Index: include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- include/clang/Basic/DiagnosticCommonKinds.td
+++ include/clang/Basic/DiagnosticCommonKinds.td
@@ -96,6 +96,8 @@
   DefaultFatal;
 def err_module_prebuilt : Error<
   "error in loading module '%0' from prebuilt module path">, DefaultFatal;
+def err_module_invalidated_by_child : Error<
+  "module '%0' is invalidated by a child thread">, DefaultFatal;
 def note_pragma_entered_here : Note<"#pragma entered here">;  
 def note_decl_hiding_tag_type : Note<
   "%1 %0 is hidden by a non-type declaration of %0 here">;
Index: include/clang/Serialization/ModuleManager.h
===================================================================
--- include/clang/Serialization/ModuleManager.h
+++ include/clang/Serialization/ModuleManager.h
@@ -46,6 +46,9 @@
   /// \brief All loaded modules, indexed by name.
   llvm::DenseMap<const FileEntry *, ModuleFile *> Modules;
 
+  /// \brief Modules that are invalidated by the child threads.
+  llvm::DenseMap<const FileEntry *, std::string> InvalidatedModules;
+
   /// \brief FileManager that handles translating between filenames and
   /// FileEntry *.
   FileManager &FileMgr;
@@ -280,6 +283,16 @@
                         time_t ExpectedModTime,
                         const FileEntry *&File);
 
+  /// In the context of a parent thread spawn a child thread to build a module,
+  /// the child thread's module manager checks to see if any successfully loaded
+  /// module in the parent thread is invalidated here. It returns the name of
+  /// the module if such module exists.
+  std::string checkForInvalidateEntries(const ModuleMap *modMapInParent,
+                                        ModuleManager *parentManager);
+
+  /// Pull invalidated entries from the child manager.
+  void mergeInvalidatedEntries(ModuleManager *childManager);
+
   /// \brief View the graphviz representation of the module graph.
   void viewGraph();
 };
Index: lib/Frontend/CompilerInstance.cpp
===================================================================
--- lib/Frontend/CompilerInstance.cpp
+++ lib/Frontend/CompilerInstance.cpp
@@ -1060,6 +1060,21 @@
                                             diag::remark_module_build_done)
     << Module->Name;
 
+  if (ImportingInstance.getModuleManager() && Instance.getModuleManager()) {
+    serialization::ModuleManager &ParentManager =
+        ImportingInstance.getModuleManager()->getModuleManager();
+    serialization::ModuleManager &ChildManager =
+        Instance.getModuleManager()->getModuleManager();
+
+    // Copy the invalidated modules to the parent thread.
+    ParentManager.mergeInvalidatedEntries(&ChildManager);
+    std::string Invalid = ChildManager.checkForInvalidateEntries(
+        &ModMap, &ParentManager);
+    if (!Invalid.empty())
+      Instance.getDiagnostics().Report(ImportLoc,
+          diag::err_module_invalidated_by_child) << Invalid;
+  }
+
   // Delete the temporary module map file.
   // FIXME: Even though we're executing under crash protection, it would still
   // be nice to do this with RemoveFileOnSignal when we can. However, that
Index: lib/Serialization/ModuleManager.cpp
===================================================================
--- lib/Serialization/ModuleManager.cpp
+++ lib/Serialization/ModuleManager.cpp
@@ -182,6 +182,29 @@
   return NewlyLoaded;
 }
 
+// Check to see if the parent thread is holding on to an entry that the child
+// thread invalidated.
+std::string ModuleManager::checkForInvalidateEntries(
+    const ModuleMap *modMapInParent, ModuleManager *parentManager) {
+  for (auto E : InvalidatedModules) {
+    if (parentManager->lookup(E.first))
+      return E.second;
+
+    if (modMapInParent) {
+      if (Module *mod = modMapInParent->findModule(E.second)) {
+        if (mod->getASTFile() == E.first)
+          return E.second;
+      }
+    }
+  }
+  return "";
+}
+
+void ModuleManager::mergeInvalidatedEntries(ModuleManager *child) {
+  for (auto E : child->InvalidatedModules)
+    InvalidatedModules[E.first] = E.second;
+}
+
 void ModuleManager::removeModules(
     ModuleIterator first, ModuleIterator last,
     llvm::SmallPtrSetImpl<ModuleFile *> &LoadedSuccessfully,
@@ -228,8 +251,10 @@
     // Files that didn't make it through ReadASTCore successfully will be
     // rebuilt (or there was an error). Invalidate them so that we can load the
     // new files that will be renamed over the old ones.
-    if (LoadedSuccessfully.count(*victim) == 0)
+    if (LoadedSuccessfully.count(*victim) == 0) {
+      InvalidatedModules[(*victim)->File] = (*victim)->ModuleName;
       FileMgr.invalidateCache((*victim)->File);
+    }
 
     delete *victim;
   }
Index: test/Modules/Inputs/system-out-of-date/X.h
===================================================================
--- test/Modules/Inputs/system-out-of-date/X.h
+++ test/Modules/Inputs/system-out-of-date/X.h
@@ -0,0 +1 @@
+#import <Y.h>
Index: test/Modules/Inputs/system-out-of-date/Y.h
===================================================================
--- test/Modules/Inputs/system-out-of-date/Y.h
+++ test/Modules/Inputs/system-out-of-date/Y.h
@@ -0,0 +1 @@
+//empty
Index: test/Modules/Inputs/system-out-of-date/Z.h
===================================================================
--- test/Modules/Inputs/system-out-of-date/Z.h
+++ test/Modules/Inputs/system-out-of-date/Z.h
@@ -0,0 +1 @@
+#import <Y.h>
Index: test/Modules/Inputs/system-out-of-date/module.map
===================================================================
--- test/Modules/Inputs/system-out-of-date/module.map
+++ test/Modules/Inputs/system-out-of-date/module.map
@@ -0,0 +1,12 @@
+module X [system] {
+  header "X.h" // imports Y
+  export *
+}
+module Y {
+  header "Y.h"
+  export *
+}
+module Z {
+  header "Z.h" // imports Y
+  export *
+}
Index: test/Modules/system-out-of-date-test.m
===================================================================
--- test/Modules/system-out-of-date-test.m
+++ test/Modules/system-out-of-date-test.m
@@ -0,0 +1,14 @@
+// RUN: rm -rf %t
+// RUN: echo '@import X;' | \
+// RUN:   %clang_cc1 -fmodules -fimplicit-module-maps \
+// RUN:     -fmodules-cache-path=%t -I %S/Inputs/system-out-of-date \
+// RUN:     -fsyntax-only -x objective-c -
+
+// We have an version built with different diagnostic options.
+// RUN: not %clang_cc1 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/system-out-of-date -Wnon-modular-include-in-framework-module -Werror=non-modular-include-in-framework-module %s -fsyntax-only 2>&1 | FileCheck %s
+
+@import X;
+
+#import <Z.h>
+// CHECK: fatal error: module 'Y' is invalidated by its child
+// CHECK: fatal error: could not build module 'Z'