Index: clang-tools-extra/trunk/clangd/ClangdServer.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/ClangdServer.cpp
+++ clang-tools-extra/trunk/clangd/ClangdServer.cpp
@@ -200,10 +200,12 @@
   };
 
   // We use a potentially-stale preamble because latency is critical here.
-  WorkScheduler.runWithPreamble("CodeComplete", File,
-                                Opts.AllowFallback ? TUScheduler::StaleOrAbsent
-                                                   : TUScheduler::Stale,
-                                Bind(Task, File.str(), std::move(CB)));
+  WorkScheduler.runWithPreamble(
+      "CodeComplete", File,
+      (Opts.RunParser == CodeCompleteOptions::AlwaysParse)
+          ? TUScheduler::Stale
+          : TUScheduler::StaleOrAbsent,
+      Bind(Task, File.str(), std::move(CB)));
 }
 
 void ClangdServer::signatureHelp(PathRef File, Position Pos,
Index: clang-tools-extra/trunk/clangd/CodeComplete.h
===================================================================
--- clang-tools-extra/trunk/clangd/CodeComplete.h
+++ clang-tools-extra/trunk/clangd/CodeComplete.h
@@ -115,11 +115,18 @@
   /// Such completions can insert scope qualifiers.
   bool AllScopes = false;
 
-  /// Whether to allow falling back to code completion without compiling files
-  /// (using identifiers in the current file and symbol indexes), when file
-  /// cannot be built (e.g. missing compile command), or the build is not ready
-  /// (e.g. preamble is still being built).
-  bool AllowFallback = false;
+  /// Whether to use the clang parser, or fallback to text-based completion
+  /// (using identifiers in the current file and symbol indexes).
+  enum CodeCompletionParse {
+    /// Block until we can run the parser (e.g. preamble is built).
+    /// Return an error if this fails.
+    AlwaysParse,
+    /// Run the parser if inputs (preamble) are ready.
+    /// Otherwise, use text-based completion.
+    ParseIfReady,
+    /// Always use text-based completion.
+    NeverParse,
+  } RunParser = ParseIfReady;
 };
 
 // Semi-structured representation of a code-complete suggestion for our C++ API.
Index: clang-tools-extra/trunk/clangd/CodeComplete.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/CodeComplete.cpp
+++ clang-tools-extra/trunk/clangd/CodeComplete.cpp
@@ -1738,9 +1738,10 @@
   auto Flow = CodeCompleteFlow(
       FileName, Preamble ? Preamble->Includes : IncludeStructure(),
       SpecFuzzyFind, Opts);
-  return Preamble ? std::move(Flow).run(
-                        {FileName, Command, Preamble, Contents, *Offset, VFS})
-                  : std::move(Flow).runWithoutSema(Contents, *Offset, VFS);
+  return (!Preamble || Opts.RunParser == CodeCompleteOptions::NeverParse)
+             ? std::move(Flow).runWithoutSema(Contents, *Offset, VFS)
+             : std::move(Flow).run(
+                   {FileName, Command, Preamble, Contents, *Offset, VFS});
 }
 
 SignatureHelp signatureHelp(PathRef FileName,
Index: clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
+++ clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
@@ -256,13 +256,18 @@
                                 "Offsets are in UTF-16 code units")),
     llvm::cl::init(OffsetEncoding::UnsupportedEncoding));
 
-static llvm::cl::opt<bool> AllowFallbackCompletion(
-    "allow-fallback-completion",
-    llvm::cl::desc(
-        "Allow falling back to code completion without compiling files (using "
-        "identifiers and symbol indexes), when file cannot be built or the "
-        "build is not ready."),
-    llvm::cl::init(false));
+static llvm::cl::opt<CodeCompleteOptions::CodeCompletionParse>
+    CodeCompletionParse(
+        "completion-parse",
+        llvm::cl::desc("Whether the clang-parser is used for code-completion"),
+        llvm::cl::values(clEnumValN(CodeCompleteOptions::AlwaysParse, "always",
+                                    "Block until the parser can be used"),
+                         clEnumValN(CodeCompleteOptions::ParseIfReady, "auto",
+                                    "Use text-based completion if the parser "
+                                    "is not ready"),
+                         clEnumValN(CodeCompleteOptions::NeverParse, "never",
+                                    "Always used text-based completion")),
+        llvm::cl::init(CodeCompleteOptions().RunParser), llvm::cl::Hidden);
 
 namespace {
 
@@ -475,7 +480,7 @@
   CCOpts.SpeculativeIndexRequest = Opts.StaticIndex;
   CCOpts.EnableFunctionArgSnippets = EnableFunctionArgSnippets;
   CCOpts.AllScopes = AllScopesCompletion;
-  CCOpts.AllowFallback = AllowFallbackCompletion;
+  CCOpts.RunParser = CodeCompletionParse;
 
   RealFileSystemProvider FSProvider;
   // Initialize and run ClangdLSPServer.
Index: clang-tools-extra/trunk/clangd/unittests/ClangdTests.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/unittests/ClangdTests.cpp
+++ clang-tools-extra/trunk/clangd/unittests/ClangdTests.cpp
@@ -9,6 +9,7 @@
 #include "Annotations.h"
 #include "ClangdLSPServer.h"
 #include "ClangdServer.h"
+#include "CodeComplete.h"
 #include "GlobalCompilationDatabase.h"
 #include "Matchers.h"
 #include "SyncAPI.h"
@@ -1072,7 +1073,7 @@
   FS.Files[FooCpp] = FooCpp;
 
   auto Opts = clangd::CodeCompleteOptions();
-  Opts.AllowFallback = true;
+  Opts.RunParser = CodeCompleteOptions::ParseIfReady;
 
   // This will make compile command broken and preamble absent.
   CDB.ExtraClangFlags = {"yolo.cc"};
@@ -1089,11 +1090,17 @@
   CDB.ExtraClangFlags = {"-std=c++11"};
   Server.addDocument(FooCpp, Code.code());
   ASSERT_TRUE(Server.blockUntilIdleForTest());
-  EXPECT_THAT(cantFail(runCodeComplete(Server, FooCpp, Code.point(),
-                                       clangd::CodeCompleteOptions()))
-                  .Completions,
-              ElementsAre(AllOf(Field(&CodeCompletion::Name, "xyz"),
-                                Field(&CodeCompletion::Scope, "ns::"))));
+  EXPECT_THAT(
+      cantFail(runCodeComplete(Server, FooCpp, Code.point(), Opts)).Completions,
+      ElementsAre(AllOf(Field(&CodeCompletion::Name, "xyz"),
+                        Field(&CodeCompletion::Scope, "ns::"))));
+
+  // Now force identifier-based completion.
+  Opts.RunParser = CodeCompleteOptions::NeverParse;
+  EXPECT_THAT(
+      cantFail(runCodeComplete(Server, FooCpp, Code.point(), Opts)).Completions,
+      ElementsAre(AllOf(Field(&CodeCompletion::Name, "xyz"),
+                        Field(&CodeCompletion::Scope, ""))));
 }
 
 TEST_F(ClangdVFSTest, FallbackWhenWaitingForCompileCommand) {
@@ -1140,7 +1147,7 @@
   // hasn't been scheduled.
   std::this_thread::sleep_for(std::chrono::milliseconds(10));
   auto Opts = clangd::CodeCompleteOptions();
-  Opts.AllowFallback = true;
+  Opts.RunParser = CodeCompleteOptions::ParseIfReady;
 
   auto Res = cantFail(runCodeComplete(Server, FooCpp, Code.point(), Opts));
   EXPECT_EQ(Res.Context, CodeCompletionContext::CCC_Recovery);