diff --git a/lib/Headers/stddef.h b/lib/Headers/stddef.h
index 0c5c961..6a64d6d 100644
--- a/lib/Headers/stddef.h
+++ b/lib/Headers/stddef.h
@@ -58,6 +58,9 @@
 #if !defined(_WCHAR_T) || __has_feature(modules)
 #if !__has_feature(modules)
 #define _WCHAR_T
+#if defined(_MSC_EXTENSIONS)
+#define _WCHAR_T_DEFINED
+#endif
 #endif
 typedef __WCHAR_TYPE__ wchar_t;
 #endif
diff --git a/test/Headers/ms-wchar.c b/test/Headers/ms-wchar.c
new file mode 100644
index 0000000..f015fc7
--- /dev/null
+++ b/test/Headers/ms-wchar.c
@@ -0,0 +1,15 @@
+// RUN: %clang -fsyntax-only -target i386-pc-win32 %s
+
+#if defined(_WCHAR_T_DEFINED)
+#error "_WCHAR_T_DEFINED should not be defined in C99"
+#endif
+
+#include <stddef.h>
+
+#if !defined(_WCHAR_T_DEFINED)
+#error "_WCHAR_T_DEFINED should have been set by stddef.h"
+#endif
+
+#if defined(_NATIVE_WCHAR_T_DEFINED)
+#error "_NATIVE_WCHAR_T_DEFINED should not be defined"
+#endif
diff --git a/test/Preprocessor/init.c b/test/Preprocessor/init.c
index c1519a8..b69c76c 100644
--- a/test/Preprocessor/init.c
+++ b/test/Preprocessor/init.c
@@ -88,10 +88,24 @@
 // C94:#define __STDC_VERSION__ 199409L
 //
 // 
-// RUN: %clang_cc1 -fms-extensions -triple i686-pc-win32 -fobjc-runtime=gcc -E -dM < /dev/null | FileCheck -check-prefix MSEXT %s
+// RUN: %clang_cc1 -fms-extensions -triple i686-pc-win32 -E -dM < /dev/null | FileCheck -check-prefix MSEXT %s
 //
 // MSEXT-NOT:#define __STDC__
 // MSEXT:#define _INTEGRAL_MAX_BITS 64
+// MSEXT-NOT:#define _NATIVE_WCHAR_T_DEFINED 1
+// MSEXT-NOT:#define _WCHAR_T_DEFINED 1
+//
+//
+// RUN: %clang_cc1 -x c++ -fms-extensions -triple i686-pc-win32 -E -dM < /dev/null | FileCheck -check-prefix MSEXT-CXX %s
+//
+// MSEXT-CXX:#define _NATIVE_WCHAR_T_DEFINED 1
+// MSEXT-CXX:#define _WCHAR_T_DEFINED 1
+//
+//
+// RUN: %clang_cc1 -x c++ -fno-wchar -fms-extensions -triple i686-pc-win32 -E -dM < /dev/null | FileCheck -check-prefix MSEXT-CXX-NOWCHAR %s
+//
+// MSEXT-CXX-NOWCHAR-NOT:#define _NATIVE_WCHAR_T_DEFINED 1
+// MSEXT-CXX-NOWCHAR-NOT:#define _WCHAR_T_DEFINED 1
 //
 // 
 // RUN: %clang_cc1 -x objective-c -E -dM < /dev/null | FileCheck -check-prefix OBJC %s