From 36f06b07d9cfb3974c160ecc22def3cb1b117106 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Thu, 9 Feb 2023 17:09:23 -0800
Subject: [PATCH] nullptr: work around Apple clang 14 issue

Problem reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2023-02/msg00098.html
* doc/gnulib.texi (nullptr): Document limitations better.
* m4/nullptr.m4 (gl_NULLPTR): Check for incompatibility of macOS
clang 14.0.0 (clang-1400.0.29.202), where <stddef.h> defines a
non-working nullptr macro.
---
 ChangeLog       | 10 ++++++++++
 doc/gnulib.texi |  7 ++++---
 m4/nullptr.m4   | 30 ++++++++++++++++++++----------
 3 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 867107bb14..ce5ce330e6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,13 @@
+2023-02-09  Paul Eggert  <eggert@cs.ucla.edu>
+
+	nullptr: work around Apple clang 14 issue
+	Problem reported by Bruno Haible in:
+	https://lists.gnu.org/r/bug-gnulib/2023-02/msg00098.html
+	* doc/gnulib.texi (nullptr): Document limitations better.
+	* m4/nullptr.m4 (gl_NULLPTR): Check for incompatibility of macOS
+	clang 14.0.0 (clang-1400.0.29.202), where <stddef.h> defines a
+	non-working nullptr macro.
+
 2023-02-09  Bruno Haible  <bruno@clisp.org>
 
 	math: Fix compilation error in C++ mode on macOS 12.5.
diff --git a/doc/gnulib.texi b/doc/gnulib.texi
index 6fe2678834..28d8c20d8c 100644
--- a/doc/gnulib.texi
+++ b/doc/gnulib.texi
@@ -954,9 +954,10 @@ In C, it has type @code{void *}; in C++ it has an integer type.
 
 @item
 On older platforms Gnulib cannot easily emulate @code{nullptr_t}, so
-null pointer type checking is more error prone, and @code{_Generic}
-expressions cannot reliably distinguish @code{nullptr}'s type from
-integer or @code{void *} types.
+null pointer type checking is more error prone.  In C, @code{_Generic}
+expressions cannot reliably distinguish the type of @code{nullptr}
+from integer or @code{void *} types.  C++ overloading has similar
+limitations.
 @end itemize
 
 @node static_assert
diff --git a/m4/nullptr.m4 b/m4/nullptr.m4
index a0b438be94..1f8ab51b85 100644
--- a/m4/nullptr.m4
+++ b/m4/nullptr.m4
@@ -28,20 +28,30 @@ AC_DEFUN([gl_NULLPTR],
        [AC_COMPILE_IFELSE(
           [AC_LANG_SOURCE([[int *p = nullptr;]])],
           [gl_cv_cxx_nullptr=yes],
-          [gl_cv_cxx_nullptr=no])])
-      gl_cxx_nullptr=$gl_cv_cxx_nullptr
-      AC_LANG_POP([C++])],
-     [gl_cxx_nullptr=no])
-  if test "$gl_cxx_nullptr" = yes; then
-    AC_DEFINE([HAVE_CXX_NULLPTR], [1], [Define to 1 if C++ nullptr works.])
-  fi
+          [AC_COMPILE_IFELSE(
+             [AC_LANG_SOURCE([[#include <stddef.h>
+                               int *p = nullptr;]])],
+             [gl_cv_cxx_nullptr="yes, but it is a <stddef.h> macro"],
+             [gl_cv_cxx_nullptr=no])])])
+     AS_CASE([$gl_cv_cxx_nullptr],
+       [yes],  [gl_have_cxx_nullptr=1],
+       [yes*], [gl_have_cxx_nullptr="(-1)"],
+               [gl_have_cxx_nullptr=0])
+     AC_DEFINE_UNQUOTED([HAVE_CXX_NULLPTR], [$gl_have_cxx_nullptr],
+                        [Define to 1 if C++ nullptr works, 0 if not,
+                         (-1) if it is a <stddef.h> macro.])
+     AC_LANG_POP([C++])])
 ])
 
-  AH_VERBATIM([nullptr],
-[#ifndef nullptr /* keep config.h idempotent */
+  AH_VERBATIM([zznullptr],
+[#if defined __cplusplus && HAVE_CXX_NULLPTR < 0
+# include <stddef.h>
+# undef/**/nullptr
+#endif
+#ifndef nullptr
 # if !defined __cplusplus && !defined HAVE_C_NULLPTR
 #  define nullptr ((void *) 0)
-# elif defined __cplusplus && !defined HAVE_CXX_NULLPTR
+# elif defined __cplusplus && HAVE_CXX_NULLPTR <= 0
 #  if 3 <= __GNUG__
 #   define nullptr __null
 #  else
-- 
2.39.1