Let NULL and MSVC headers coexist better.

Fixes the two issues mentioned in PR12146.

llvm-svn: 155490

clang/lib/Headers/stddef.h

@@ -43,9 +43,12 @@ typedef __WCHAR_TYPE__ wchar_t;
 
 #undef NULL
 #ifdef __cplusplus
-#undef __null  // VC++ hack.
+#  if !defined(__MINGW32__) && !defined(_MSC_VER)
 #    define NULL __null
 #  else
+#    define NULL 0
+#  endif
+#else
 #  define NULL ((void*)0)
 #endif
 

clang/test/Headers/ms-null-ms-header-vs-stddef.cpp

@@ -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;