[clang] [clang][headers] Including stddef.h always redefines NULL (PR #99727)

[Ian Anderson via cfe-commits]

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@@ -49,7 +48,25 @@
 #define __need_rsize_t
 #endif
 #define __need_wchar_t
+#if !defined(__STDDEF_H) && !__building_module(_Builtin_stddef)
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ian-twilightcoder wrote:

We do still need everything when we're building with modules for the reasons outlined in the comment, but I think you're right that `__building_module(_Builtin_stddef)` doesn't do anything and that needs to be `__has_feature(modules)`

https://github.com/llvm/llvm-project/pull/99727