diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index b059965..07812bd 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -329,14 +329,7 @@
 
   PP->setPreprocessedOutput(getPreprocessorOutputOpts().ShowCPP);
 
-  // Set up the module path, including the hash for the
-  // module-creation options.
-  SmallString<256> SpecificModuleCache(
-                           getHeaderSearchOpts().ModuleCachePath);
-  if (!getHeaderSearchOpts().DisableModuleHash)
-    llvm::sys::path::append(SpecificModuleCache,
-                            getInvocation().getModuleHash());
-  PP->getHeaderSearchInfo().setModuleCachePath(SpecificModuleCache);
+  PP->getHeaderSearchInfo().setModuleCachePath(getSpecificModuleCachePath());
 
   // Handle generating dependencies, if requested.
   const DependencyOutputOptions &DepOpts = getDependencyOutputOpts();
@@ -373,6 +366,17 @@
   }
 }
 
+std::string CompilerInstance::getSpecificModuleCachePath() {
+  // Set up the module path, including the hash for the
+  // module-creation options.
+  SmallString<256> SpecificModuleCache(
+                           getHeaderSearchOpts().ModuleCachePath);
+  if (!getHeaderSearchOpts().DisableModuleHash)
+    llvm::sys::path::append(SpecificModuleCache,
+                            getInvocation().getModuleHash());
+  return SpecificModuleCache.str();
+}
+
 // ASTContext
 
 void CompilerInstance::createASTContext() {
@@ -388,32 +392,30 @@
 void CompilerInstance::createPCHExternalASTSource(
     StringRef Path, bool DisablePCHValidation, bool AllowPCHWithCompilerErrors,
     void *DeserializationListener, bool OwnDeserializationListener) {
-  IntrusiveRefCntPtr<ExternalASTSource> Source;
   bool Preamble = getPreprocessorOpts().PrecompiledPreambleBytes.first != 0;
-  Source = createPCHExternalASTSource(
+  ModuleManager = createPCHExternalASTSource(
       Path, getHeaderSearchOpts().Sysroot, DisablePCHValidation,
       AllowPCHWithCompilerErrors, getPreprocessor(), getASTContext(),
       DeserializationListener, OwnDeserializationListener, Preamble,
       getFrontendOpts().UseGlobalModuleIndex);
-  ModuleManager = static_cast<ASTReader*>(Source.get());
-  getASTContext().setExternalSource(Source);
 }
 
-ExternalASTSource *CompilerInstance::createPCHExternalASTSource(
+IntrusiveRefCntPtr<ASTReader> CompilerInstance::createPCHExternalASTSource(
     StringRef Path, const std::string &Sysroot, bool DisablePCHValidation,
     bool AllowPCHWithCompilerErrors, Preprocessor &PP, ASTContext &Context,
     void *DeserializationListener, bool OwnDeserializationListener,
     bool Preamble, bool UseGlobalModuleIndex) {
   HeaderSearchOptions &HSOpts = PP.getHeaderSearchInfo().getHeaderSearchOpts();
 
-  std::unique_ptr<ASTReader> Reader;
-  Reader.reset(new ASTReader(PP, Context,
-                             Sysroot.empty() ? "" : Sysroot.c_str(),
-                             DisablePCHValidation,
-                             AllowPCHWithCompilerErrors,
-                             /*AllowConfigurationMismatch*/false,
-                             HSOpts.ModulesValidateSystemHeaders,
-                             UseGlobalModuleIndex));
+  IntrusiveRefCntPtr<ASTReader> Reader(
+      new ASTReader(PP, Context, Sysroot.empty() ? "" : Sysroot.c_str(),
+                    DisablePCHValidation, AllowPCHWithCompilerErrors,
+                    /*AllowConfigurationMismatch*/ false,
+                    HSOpts.ModulesValidateSystemHeaders, UseGlobalModuleIndex));
+
+  // We need the external source to be set up before we read the AST, because
+  // eagerly-deserialized declarations may use it.
+  Context.setExternalSource(Reader.get());
 
   Reader->setDeserializationListener(
       static_cast<ASTDeserializationListener *>(DeserializationListener),
@@ -427,7 +429,7 @@
     // Set the predefines buffer as suggested by the PCH reader. Typically, the
     // predefines buffer will be empty.
     PP.setPredefines(Reader->getSuggestedPredefines());
-    return Reader.release();
+    return Reader;
 
   case ASTReader::Failure:
     // Unrecoverable failure: don't even try to process the input file.
@@ -442,6 +444,7 @@
     break;
   }
 
+  Context.setExternalSource(nullptr);
   return nullptr;
 }
 
@@ -1023,9 +1026,19 @@
     case llvm::LockFileManager::LFS_Shared:
       // Someone else is responsible for building the module. Wait for them to
       // finish.
-      if (Locked.waitForUnlock() == llvm::LockFileManager::Res_OwnerDied)
+      switch (Locked.waitForUnlock()) {
+      case llvm::LockFileManager::Res_Success:
+        ModuleLoadCapabilities |= ASTReader::ARR_OutOfDate;
+        break;
+      case llvm::LockFileManager::Res_OwnerDied:
         continue; // try again to get the lock.
-      ModuleLoadCapabilities |= ASTReader::ARR_OutOfDate;
+      case llvm::LockFileManager::Res_Timeout:
+        Diags.Report(ModuleNameLoc, diag::err_module_lock_timeout)
+            << Module->Name;
+        // Clear the lock file so that future invokations can make progress.
+        Locked.unsafeRemoveLockFile();
+        return false;
+      }
       break;
     }
 
@@ -1275,6 +1288,12 @@
     bool needsImportVisitation() const override { return true; }
 
     void visitImport(StringRef FileName) override {
+      if (!CI.ExplicitlyLoadedModuleFiles.insert(FileName).second) {
+        if (ModuleFileStack.size() == 0)
+          TopFileIsModule = true;
+        return;
+      }
+
       ModuleFileStack.push_back(FileName);
       if (ASTReader::readASTFileControlBlock(FileName, CI.getFileManager(),
                                              *this)) {
@@ -1292,7 +1311,9 @@
         TopFileIsModule = true;
 
       auto &ModuleFile = CI.ModuleFileOverrides[ModuleName];
-      if (!ModuleFile.empty() && ModuleFile != ModuleFileStack.back())
+      if (!ModuleFile.empty() &&
+          CI.getFileManager().getFile(ModuleFile) !=
+              CI.getFileManager().getFile(ModuleFileStack.back()))
         CI.getDiagnostics().Report(SourceLocation(),
                                    diag::err_conflicting_module_files)
             << ModuleName << ModuleFile << ModuleFileStack.back();
@@ -1363,6 +1384,12 @@
 
     auto Override = ModuleFileOverrides.find(ModuleName);
     bool Explicit = Override != ModuleFileOverrides.end();
+    if (!Explicit && !getLangOpts().ImplicitModules) {
+      getDiagnostics().Report(ModuleNameLoc, diag::err_module_build_disabled)
+          << ModuleName;
+      ModuleBuildFailed = true;
+      return ModuleLoadResult();
+    }
 
     std::string ModuleFileName =
         Explicit ? Override->second
@@ -1552,7 +1579,7 @@
 
     // Check whether this module is available.
     clang::Module::Requirement Requirement;
-    clang::Module::HeaderDirective MissingHeader;
+    clang::Module::UnresolvedHeaderDirective MissingHeader;
     if (!Module->isAvailable(getLangOpts(), getTarget(), Requirement,
                              MissingHeader)) {
       if (MissingHeader.FileNameLoc.isValid()) {
@@ -1581,9 +1608,16 @@
                      Module, ImportLoc);
   }
 
+  // Determine whether we're in the #include buffer for a module. The #includes
+  // in that buffer do not qualify as module imports; they're just an
+  // implementation detail of us building the module.
+  bool IsInModuleIncludes = !getLangOpts().CurrentModule.empty() &&
+                            getSourceManager().getFileID(ImportLoc) ==
+                                getSourceManager().getMainFileID();
+
   // If this module import was due to an inclusion directive, create an 
   // implicit import declaration to capture it in the AST.
-  if (IsInclusionDirective && hasASTContext()) {
+  if (IsInclusionDirective && hasASTContext() && !IsInModuleIncludes) {
     TranslationUnitDecl *TU = getASTContext().getTranslationUnitDecl();
     ImportDecl *ImportD = ImportDecl::CreateImplicit(getASTContext(), TU,
                                                      ImportLoc, Module,