[llvm-branch-commits] [libcxx] release/18.x: [libc++] Simplify the implementation of <stddef.h> (#86843) (PR #87374)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/87374

Backport 2950283dddab03c183c1be2d7de9d4999cc86131

Requested by: @ian-twilightcoder

>From 20cb23caea75d1d94e930ce0593eb92770f3c94d Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 2 Apr 2024 08:14:04 -0400
Subject: [PATCH] [libc++] Simplify the implementation of <stddef.h> (#86843)

Libc++'s own <stddef.h> is complicated by the need to handle various
platform-specific macros and to support duplicate inclusion. In reality,
we only need to add a declaration of nullptr_t to it, so we can simply
include the underlying <stddef.h> outside of our guards to let it handle
re-inclusion itself.

(cherry picked from commit 2950283dddab03c183c1be2d7de9d4999cc86131)
---
 libcxx/include/stddef.h | 25 ++++++++-----------------
 1 file changed, 8 insertions(+), 17 deletions(-)

diff --git a/libcxx/include/stddef.h b/libcxx/include/stddef.h
index 887776b150e49d..470b5408336c6d 100644
--- a/libcxx/include/stddef.h
+++ b/libcxx/include/stddef.h
@@ -7,18 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if defined(__need_ptrdiff_t) || defined(__need_size_t) || defined(__need_wchar_t) || defined(__need_NULL) ||          \
-    defined(__need_wint_t)
-
-#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
-#    pragma GCC system_header
-#  endif
-
-#  include_next <stddef.h>
-
-#elif !defined(_LIBCPP_STDDEF_H)
-#  define _LIBCPP_STDDEF_H
-
 /*
     stddef.h synopsis
 
@@ -36,16 +24,19 @@
 
 */
 
-#  include <__config>
+#include <__config>
+
+// Note: This include is outside of header guards because we sometimes get included multiple times
+//       with different defines and the underlying <stddef.h> will know how to deal with that.
+#include_next <stddef.h>
+
+#ifndef _LIBCPP_STDDEF_H
+#  define _LIBCPP_STDDEF_H
 
 #  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #    pragma GCC system_header
 #  endif
 
-#  if __has_include_next(<stddef.h>)
-#    include_next <stddef.h>
-#  endif
-
 #  ifdef __cplusplus
 typedef decltype(nullptr) nullptr_t;
 #  endif