diff --git a/clang/lib/Headers/stdbool.h b/clang/lib/Headers/stdbool.h
--- a/clang/lib/Headers/stdbool.h
+++ b/clang/lib/Headers/stdbool.h
@@ -23,7 +23,7 @@
 #elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
 /* Define _Bool as a GNU extension. */
 #define _Bool bool
-#if defined(__cplusplus) && __cplusplus < 201103L
+#if !defined(__cplusplus) || __cplusplus < 201103L
 /* For C++98, define bool, false, true as a GNU extension. */
 #define bool bool
 #define false false