diff --git a/lib/Frontend/ASTUnit.cpp b/lib/Frontend/ASTUnit.cpp
index 241a2c3..5a9f077 100644
--- a/lib/Frontend/ASTUnit.cpp
+++ b/lib/Frontend/ASTUnit.cpp
@@ -667,7 +667,8 @@
   AST->SourceMgr = new SourceManager(AST->getDiagnostics(),
                                      AST->getFileManager());
   AST->HeaderInfo.reset(new HeaderSearch(AST->getFileManager(),
-                                         AST->getDiagnostics()));
+                                         AST->getDiagnostics(),
+                                         AST->ASTFileLangOpts));
   
   for (unsigned I = 0; I != NumRemappedFiles; ++I) {
     FilenameOrMemBuf fileOrBuf = RemappedFiles[I].second;
diff --git a/lib/Frontend/CompilerInstance.cpp b/lib/Frontend/CompilerInstance.cpp
index 12d3685..498f6fb 100644
--- a/lib/Frontend/CompilerInstance.cpp
+++ b/lib/Frontend/CompilerInstance.cpp
@@ -252,7 +252,8 @@
 
   // Create the Preprocessor.
   HeaderSearch *HeaderInfo = new HeaderSearch(getFileManager(), 
-                                              getDiagnostics());
+                                              getDiagnostics(),
+                                              getLangOpts());
   PP = new Preprocessor(getDiagnostics(), getLangOpts(), &getTarget(),
                         getSourceManager(), *HeaderInfo, *this, PTHMgr,
                         /*OwnsHeaderSearch=*/true);
@@ -1284,6 +1285,19 @@
       
       return 0;
     }
+
+    // Check whether this module is available.
+    StringRef Feature;
+    if (!Module->isAvailable(getLangOpts(), Feature)) {
+      getDiagnostics().Report(ImportLoc, diag::err_module_unavailable)
+        << Module->getFullModuleName()
+        << Feature
+        << SourceRange(Path.front().second, Path.back().second);
+      LastModuleImportLoc = ImportLoc;
+      LastModuleImportResult = 0;
+      return 0;
+    }
+
     ModuleManager->makeModuleVisible(Module, Visibility);
   }
   
diff --git a/lib/Frontend/FrontendActions.cpp b/lib/Frontend/FrontendActions.cpp
index a97a32b..dd98fcf 100644
--- a/lib/Frontend/FrontendActions.cpp
+++ b/lib/Frontend/FrontendActions.cpp
@@ -136,6 +136,10 @@
 static void collectModuleHeaderIncludes(const LangOptions &LangOpts,
                                         clang::Module *Module,
                                         llvm::SmallString<256> &Includes) {
+  // Don't collect any headers for unavailable modules.
+  if (!Module->isAvailable())
+    return;
+
   // Add includes for each of these headers.
   for (unsigned I = 0, N = Module->Headers.size(); I != N; ++I) {
     if (LangOpts.ObjC1)
@@ -222,7 +226,17 @@
     
     return false;
   }
-  
+
+  // Check whether we can build this module at all.
+  StringRef Feature;
+  if (!Module->isAvailable(CI.getLangOpts(), Feature)) {
+    CI.getDiagnostics().Report(diag::err_module_unavailable)
+      << Module->getFullModuleName()
+      << Feature;
+
+    return false;
+  }
+
   // Do we have an umbrella header for this module?
   const FileEntry *UmbrellaHeader = Module->getUmbrellaHeader();