[clang][headers] Including stddef.h always redefines NULL

stddef.h always includes __stddef_null.h. This is fine in modules because it's not possible to re-include the pcm, and it's necessary to export the _Builtin_stddef.null submodule. However, without modules it causes NULL to always get redefined which disrupts some C++ code. Rework the inclusion of __stddef_null.h so that with not building with modules it's only included if __need_NULL is set by the includer, or it's the first time stddef.h is being included.

clang/lib/Headers/stdarg.h

@@ -20,19 +20,18 @@
  * modules.
  */
 #if defined(__MVS__) && __has_include_next(<stdarg.h>)
-#include <__stdarg_header_macro.h>
 #undef __need___va_list
 #undef __need_va_list
 #undef __need_va_arg
 #undef __need___va_copy
 #undef __need_va_copy
+#include <__stdarg_header_macro.h>
 #include_next <stdarg.h>
 
 #else
 #if !defined(__need___va_list) && !defined(__need_va_list) &&                  \
     !defined(__need_va_arg) && !defined(__need___va_copy) &&                   \
     !defined(__need_va_copy)
-#include <__stdarg_header_macro.h>
 #define __need___va_list
 #define __need_va_list
 #define __need_va_arg
@@ -45,6 +44,7 @@
     !defined(__STRICT_ANSI__)
 #define __need_va_copy
 #endif
+#include <__stdarg_header_macro.h>
 #endif
 
 #ifdef __need___va_list

clang/lib/Headers/stddef.h

@@ -20,7 +20,6 @@
  * modules.
  */
 #if defined(__MVS__) && __has_include_next(<stddef.h>)
-#include <__stddef_header_macro.h>
 #undef __need_ptrdiff_t
 #undef __need_size_t
 #undef __need_rsize_t
@@ -31,6 +30,7 @@
 #undef __need_max_align_t
 #undef __need_offsetof
 #undef __need_wint_t
+#include <__stddef_header_macro.h>
 #include_next <stddef.h>
 
 #else
@@ -40,7 +40,6 @@
     !defined(__need_NULL) && !defined(__need_nullptr_t) &&                     \
     !defined(__need_unreachable) && !defined(__need_max_align_t) &&            \
     !defined(__need_offsetof) && !defined(__need_wint_t)
-#include <__stddef_header_macro.h>
 #define __need_ptrdiff_t
 #define __need_size_t
 /* ISO9899:2011 7.20 (C11 Annex K): Define rsize_t if __STDC_WANT_LIB_EXT1__ is
@@ -49,7 +48,24 @@
 #define __need_rsize_t
 #endif
 #define __need_wchar_t
+#if !defined(__STDDEF_H) || __has_feature(modules)
+/*
+ * __stddef_null.h is special when building without modules: if __need_NULL is
+ * set, then it will unconditionally redefine NULL. To avoid stepping on client
+ * definitions of NULL, __need_NULL should only be set the first time this
+ * header is included, that is when __STDDEF_H is not defined. However, when
+ * building with modules, this header is a textual header and needs to
+ * unconditionally include __stdef_null.h to support multiple submodules
+ * exporting _Builtin_stddef.null. Take module SM with submodules A and B, whose
+ * headers both include stddef.h When SM.A builds, __STDDEF_H will be defined.
+ * When SM.B builds, the definition from SM.A will leak when building without
+ * local submodule visibility. stddef.h wouldn't include __stddef_null.h, and
+ * SM.B wouldn't import _Builtin_stddef.null, and SM.B's `export *` wouldn't
+ * export NULL as expected. When building with modules, always include
+ * __stddef_null.h so that everything works as expected.
+ */
 #define __need_NULL
+#endif
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) ||              \
     defined(__cplusplus)
 #define __need_nullptr_t
@@ -65,6 +81,7 @@
 /* wint_t is provided by <wchar.h> and not <stddef.h>. It's here
  * for compatibility, but must be explicitly requested. Therefore
  * __need_wint_t is intentionally not defined here. */
+#include <__stddef_header_macro.h>
 #endif
 
 #if defined(__need_ptrdiff_t)

clang/test/Headers/stddefneeds.cpp

@@ -56,14 +56,21 @@ max_align_t m5;
 #undef NULL
 #define NULL 0
 
-// glibc (and other) headers then define __need_NULL and rely on stddef.h
-// to redefine NULL to the correct value again.
-#define __need_NULL
+// Including stddef.h again shouldn't redefine NULL
 #include <stddef.h>
 
 // gtk headers then use __attribute__((sentinel)), which doesn't work if NULL
 // is 0.
-void f(const char* c, ...) __attribute__((sentinel));
+void f(const char* c, ...) __attribute__((sentinel)); // expected-note{{function has been explicitly marked sentinel here}}
 void g() {
+  f("", NULL); // expected-warning{{missing sentinel in function call}}
+}
+
+// glibc (and other) headers then define __need_NULL and rely on stddef.h
+// to redefine NULL to the correct value again.
+#define __need_NULL
+#include <stddef.h>
+
+void h() {
   f("", NULL);  // Shouldn't warn.
 }