diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 9f69e9c..923881c 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -990,9 +990,10 @@
                                  SourceLocation ImportLoc,
                                  SourceLocation ModuleNameLoc, Module *Module,
                                  StringRef ModuleFileName) {
+  DiagnosticsEngine &Diags = ImportingInstance.getDiagnostics();
+
   auto diagnoseBuildFailure = [&] {
-    ImportingInstance.getDiagnostics().Report(ModuleNameLoc,
-                                              diag::err_module_not_built)
+    Diags.Report(ModuleNameLoc, diag::err_module_not_built)
         << Module->Name << SourceRange(ImportLoc, ModuleNameLoc);
   };
 
@@ -1006,6 +1007,8 @@
     llvm::LockFileManager Locked(ModuleFileName);
     switch (Locked) {
     case llvm::LockFileManager::LFS_Error:
+      Diags.Report(ModuleNameLoc, diag::err_module_lock_failure)
+          << Module->Name;
       return false;
 
     case llvm::LockFileManager::LFS_Owned:
@@ -1040,6 +1043,10 @@
       continue;
     } else if (ReadResult == ASTReader::Missing) {
       diagnoseBuildFailure();
+    } else if (ReadResult != ASTReader::Success &&
+               !Diags.hasErrorOccurred()) {
+      // The ASTReader didn't diagnose the error, so conservatively report it.
+      diagnoseBuildFailure();
     }
     return ReadResult == ASTReader::Success;
   }
@@ -1362,6 +1369,8 @@
       // Try to compile and then load the module.
       if (!compileAndLoadModule(*this, ImportLoc, ModuleNameLoc, Module,
                                 ModuleFileName)) {
+        assert(getDiagnostics().hasErrorOccurred() &&
+               "undiagnosed error in compileAndLoadModule");
         if (getPreprocessorOpts().FailedModules)
           getPreprocessorOpts().FailedModules->addFailed(ModuleName);
         KnownModules[Path[0].first] = nullptr;