diff --git a/clang/include/clang/Basic/Stack.h b/clang/include/clang/Basic/Stack.h
--- a/clang/include/clang/Basic/Stack.h
+++ b/clang/include/clang/Basic/Stack.h
@@ -39,7 +39,7 @@
   /// is insufficient, calls Diag to emit a diagnostic before calling Fn.
   inline void runWithSufficientStackSpace(llvm::function_ref<void()> Diag,
                                           llvm::function_ref<void()> Fn) {
-#ifdef LLVM_ENABLE_THREADS
+#if LLVM_ENABLE_THREADS
     if (LLVM_UNLIKELY(isStackNearlyExhausted()))
       runWithSufficientStackSpaceSlow(Diag, Fn);
     else