[openwrt/openwrt] autoconf-archive: backport patch for C++23 support

[LEDE Commits]

ansuel pushed a commit to openwrt/openwrt.git, branch main:
https://git.openwrt.org/8ad5f35a9013ca7c65e86e7aef92617903afeab0

commit 8ad5f35a9013ca7c65e86e7aef92617903afeab0
Author: Christian Marangi <ansuelsmth at gmail.com>
AuthorDate: Wed Dec 3 19:58:00 2025 +0100

    autoconf-archive: backport patch for C++23 support
    
    Backport upstream patch for C++23 support on ax_cxx_compile_stdcxx.m4.
    
    This is needed by Asterisk package to correctly configure.
    
    It's currently not possible to bump to a more recent autoconf-archive
    release 2024.10.16 as it's currently affected by multiple BUGs ([1])
    
    [1] https://github.com/openwrt/openwrt/commit/e1acc7a67b92fa964710afe0a3ae2dad5c1f00c7
    
    Signed-off-by: Christian Marangi <ansuelsmth at gmail.com>
---
 ...01-AX_CXX_COMPILE_STDCXX-Add-C-23-support.patch | 90 ++++++++++++++++++++++
 1 file changed, 90 insertions(+)

diff --git a/tools/autoconf-archive/patches/001-AX_CXX_COMPILE_STDCXX-Add-C-23-support.patch b/tools/autoconf-archive/patches/001-AX_CXX_COMPILE_STDCXX-Add-C-23-support.patch
new file mode 100644
index 0000000000..c44409cc15
--- /dev/null
+++ b/tools/autoconf-archive/patches/001-AX_CXX_COMPILE_STDCXX-Add-C-23-support.patch
@@ -0,0 +1,90 @@
+From 833c9c2e5b65383c8c1353b6beb88f52dede640f Mon Sep 17 00:00:00 2001
+From: Olly Betts <olly at survex.com>
+Date: Wed, 29 May 2024 07:54:24 +1200
+Subject: [PATCH] AX_CXX_COMPILE_STDCXX: Add C++23 support
+
+---
+ m4/ax_cxx_compile_stdcxx.m4 | 47 ++++++++++++++++++++++++++++++++++---
+ 1 file changed, 44 insertions(+), 3 deletions(-)
+
+--- a/m4/ax_cxx_compile_stdcxx.m4
++++ b/m4/ax_cxx_compile_stdcxx.m4
+@@ -10,8 +10,8 @@
+ #
+ #   Check for baseline language coverage in the compiler for the specified
+ #   version of the C++ standard.  If necessary, add switches to CXX and
+-#   CXXCPP to enable support.  VERSION may be '11', '14', '17', or '20' for
+-#   the respective C++ standard version.
++#   CXXCPP to enable support.  VERSION may be '11', '14', '17', '20', or
++#   '23' for the respective C++ standard version.
+ #
+ #   The second argument, if specified, indicates whether you insist on an
+ #   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
+@@ -43,7 +43,7 @@
+ #   and this notice are preserved.  This file is offered as-is, without any
+ #   warranty.
+ 
+-#serial 18
++#serial 19
+ 
+ dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
+ dnl  (serial version number 13).
+@@ -53,6 +53,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
+         [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
+         [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
+         [$1], [20], [ax_cxx_compile_alternatives="20"],
++        [$1], [23], [ax_cxx_compile_alternatives="23"],
+         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
+   m4_if([$2], [], [],
+         [$2], [ext], [],
+@@ -186,6 +187,16 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbo
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
+ )
+ 
++dnl  Test body for checking C++23 support
++
++m4_define([_AX_CXX_COMPILE_STDCXX_testbody_23],
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
++  _AX_CXX_COMPILE_STDCXX_testbody_new_in_23
++)
++
+ 
+ dnl  Tests for new features in C++11
+ 
+@@ -1016,3 +1027,33 @@ namespace cxx20
+ #endif  // __cplusplus < 202002L && !defined _MSC_VER
+ 
+ ]])
++
++
++dnl  Tests for new features in C++23
++
++m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_23], [[
++
++#ifndef __cplusplus
++
++#error "This is not a C++ compiler"
++
++#elif (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202302L
++
++#error "This is not a C++23 compiler"
++
++#else
++
++#include <version>
++
++namespace cxx23
++{
++
++// As C++23 supports feature test macros in the standard, there is no
++// immediate need to actually test for feature availability on the
++// Autoconf side.
++
++}  // namespace cxx23
++
++#endif  // (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202302L
++
++]])