Index: test/Headers/ms-null-ms-header-vs-stddef.cpp
===================================================================
--- test/Headers/ms-null-ms-header-vs-stddef.cpp	(revision 0)
+++ test/Headers/ms-null-ms-header-vs-stddef.cpp	(revision 0)
@@ -0,0 +1,16 @@
+// RUN: %clang -fsyntax-only -target i686-pc-win32 %s
+// RUN: %clang -fsyntax-only -target i386-mingw32 %s
+
+// Something in MSVC's headers (pulled in e.g. by <crtdefs.h>) defines __null
+// to something, mimick that.
+#define __null
+
+#include <stddef.h>
+
+// __null is used as a type annotation in MS headers, with __null defined to
+// nothing in regular builds. This should continue to work even with stddef.h
+// included.
+void f(__null void* p) { }
+
+// NULL should work fine even with __null defined to nothing.
+void* p = NULL;
Index: lib/Headers/stddef.h
===================================================================
--- lib/Headers/stddef.h	(revision 155478)
+++ lib/Headers/stddef.h	(working copy)
@@ -43,10 +43,13 @@
 
 #undef NULL
 #ifdef __cplusplus
-#undef __null  // VC++ hack.
-#define NULL __null
+#  if !defined(__MINGW32__) && !defined(_MSC_VER)
+#    define NULL __null
+#  else
+#    define NULL 0
+#  endif
 #else
-#define NULL ((void*)0)
+#  define NULL ((void*)0)
 #endif
 
 #define offsetof(t, d) __builtin_offsetof(t, d)