Let NULL and MSVC headers coexist better.

Fixes the two issues mentioned in PR12146.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155490 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Headers/stddef.h

@@ -43,10 +43,13 @@ typedef __WCHAR_TYPE__ wchar_t;
 
 #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)

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

+// 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;