Index: clang-tools-extra/trunk/clangd/index/Background.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/index/Background.cpp
+++ clang-tools-extra/trunk/clangd/index/Background.cpp
@@ -382,6 +382,14 @@
   if (!Action->Execute())
     return createStringError(inconvertibleErrorCode(), "Execute() failed");
   Action->EndSourceFile();
+  if (Clang->hasDiagnostics() &&
+      Clang->getDiagnostics().hasUncompilableErrorOccurred()) {
+    return createStringError(inconvertibleErrorCode(),
+                             "IndexingAction failed: has uncompilable errors");
+  }
+
+  assert(Index.Symbols && Index.Refs && Index.Sources
+     && "Symbols, Refs and Sources must be set.");
 
   log("Indexed {0} ({1} symbols, {2} refs, {3} files)",
       Inputs.CompileCommand.Filename, Index.Symbols->size(),
Index: clang-tools-extra/trunk/unittests/clangd/BackgroundIndexTests.cpp
===================================================================
--- clang-tools-extra/trunk/unittests/clangd/BackgroundIndexTests.cpp
+++ clang-tools-extra/trunk/unittests/clangd/BackgroundIndexTests.cpp
@@ -66,6 +66,25 @@
   BackgroundIndexTest() { preventThreadStarvationInTests(); }
 };
 
+TEST_F(BackgroundIndexTest, NoCrashOnErrorFile) {
+  MockFSProvider FS;
+  FS.Files[testPath("root/A.cc")] = "error file";
+  llvm::StringMap<std::string> Storage;
+  size_t CacheHits = 0;
+  MemoryShardStorage MSS(Storage, CacheHits);
+  OverlayCDB CDB(/*Base=*/nullptr);
+  BackgroundIndex Idx(Context::empty(), "", FS, CDB,
+                      [&](llvm::StringRef) { return &MSS; });
+
+  tooling::CompileCommand Cmd;
+  Cmd.Filename = testPath("root/A.cc");
+  Cmd.Directory = testPath("root");
+  Cmd.CommandLine = {"clang++", "-DA=1", testPath("root/A.cc")};
+  CDB.setCompileCommand(testPath("root/A.cc"), Cmd);
+
+  ASSERT_TRUE(Idx.blockUntilIdleForTest());
+}
+
 TEST_F(BackgroundIndexTest, IndexTwoFiles) {
   MockFSProvider FS;
   // a.h yields different symbols when included by A.cc vs B.cc.