diff --git a/clang/include/clang/Config/config.h.cmake b/clang/include/clang/Config/config.h.cmake
--- a/clang/include/clang/Config/config.h.cmake
+++ b/clang/include/clang/Config/config.h.cmake
@@ -16,9 +16,19 @@
 
 /* Default C/ObjC standard to use. */
 #cmakedefine CLANG_DEFAULT_STD_C LangStandard::lang_${CLANG_DEFAULT_STD_C}
+// Always #define something so that missing the config.h #include at use sites
+// becomes a compile error.
+#ifndef CLANG_DEFAULT_STD_C
+#define CLANG_DEFAULT_STD_C LangStandard::lang_unspecified
+#endif
 
 /* Default C++/ObjC++ standard to use. */
 #cmakedefine CLANG_DEFAULT_STD_CXX LangStandard::lang_${CLANG_DEFAULT_STD_CXX}
+// Always #define something so that missing the config.h #include at use sites
+// becomes a compile error.
+#ifndef CLANG_DEFAULT_STD_CXX
+#define CLANG_DEFAULT_STD_CXX LangStandard::lang_unspecified
+#endif
 
 /* Default C++ stdlib to use. */
 #define CLANG_DEFAULT_CXX_STDLIB "${CLANG_DEFAULT_CXX_STDLIB}"
diff --git a/clang/lib/Basic/LangStandards.cpp b/clang/lib/Basic/LangStandards.cpp
--- a/clang/lib/Basic/LangStandards.cpp
+++ b/clang/lib/Basic/LangStandards.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Basic/LangStandard.h"
+#include "clang/Config/config.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -57,27 +58,24 @@
     return LangStandard::lang_cuda;
   case Language::Asm:
   case Language::C:
-#if defined(CLANG_DEFAULT_STD_C)
-    return CLANG_DEFAULT_STD_C;
-#else
+    if (CLANG_DEFAULT_STD_C != LangStandard::lang_unspecified)
+      return CLANG_DEFAULT_STD_C;
+
     // The PS4 uses C99 as the default C standard.
     if (T.isPS4())
       return LangStandard::lang_gnu99;
     return LangStandard::lang_gnu17;
-#endif
   case Language::ObjC:
-#if defined(CLANG_DEFAULT_STD_C)
-    return CLANG_DEFAULT_STD_C;
-#else
+    if (CLANG_DEFAULT_STD_C != LangStandard::lang_unspecified)
+      return CLANG_DEFAULT_STD_C;
+
     return LangStandard::lang_gnu11;
-#endif
   case Language::CXX:
   case Language::ObjCXX:
-#if defined(CLANG_DEFAULT_STD_CXX)
-    return CLANG_DEFAULT_STD_CXX;
-#else
+    if (CLANG_DEFAULT_STD_CXX != LangStandard::lang_unspecified)
+      return CLANG_DEFAULT_STD_CXX;
+
     return LangStandard::lang_gnucxx14;
-#endif
   case Language::RenderScript:
     return LangStandard::lang_c99;
   case Language::HIP: