[Var] build: Autotools: Update ax_cxx_compile_stdcxx.m4 to version 21.

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@21833 56274372-70c3-4bfc-bfc3-4c3a0b034d27

build/autotools/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.
@@ -37,13 +37,14 @@
 # Copyright (c) 2019 Enji Cooper <yaneurabeya@gmail.com>
 # Copyright (c) 2020 Jason Merrill <jason@redhat.com>
 # Copyright (c) 2021 Jörn Heusipp <osmanx@problemloesungsmaschine.de>
+# Copyright (c) 2015, 2022, 2023, 2024 Olly Betts
 #
 # Copying and distribution of this file, with or without modification, are
 # permitted in any medium without royalty provided the copyright notice
 # and this notice are preserved. This file is offered as-is, without any
 # warranty.
 
-#serial 14
+#serial 21
 
 dnl This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
 dnl (serial version number 13).
@@ -53,6 +54,7 @@ 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], [],
@@ -75,7 +77,7 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
  ac_success=yes
  fi])
 
- m4_if([$2], [noext], [], [dnl
+ m4_if([$2], [noext], [dnl
  if test x$ac_success = xno; then
  for alternative in ${ax_cxx_compile_alternatives}; do
  switch="-std=gnu++${alternative}"
@@ -99,14 +101,23 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
  done
  fi])
 
- m4_if([$2], [ext], [], [dnl
+ m4_if([$2], [ext], [dnl
  if test x$ac_success = xno; then
  dnl HP's aCC needs +std=c++11 according to:
  dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
  dnl Cray's crayCC needs "-h std=c++11"
+ dnl MSVC needs -std:c++NN for C++17 and later (default is C++14)
  for alternative in ${ax_cxx_compile_alternatives}; do
- for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}"; do
- cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
+ for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}" MSVC; do
+ if test x"$switch" = xMSVC; then
+ dnl AS_TR_SH maps both `:` and `=` to `_` so -std:c++17 would collide
+ dnl with -std=c++17. We suffix the cache variable name with _MSVC to
+ dnl avoid this.
+ switch=-std:c++${alternative}
+ cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_${switch}_MSVC])
+ else
+ cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
+ fi
  AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
  $cachevar,
  [ac_save_CXX="$CXX"
@@ -150,31 +161,41 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
 dnl Test body for checking C++11 support
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_11],
- _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+ [_AX_CXX_COMPILE_STDCXX_testbody_new_in_11]
 )
 
 dnl Test body for checking C++14 support
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
- _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
- _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+ [_AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14]
 )
 
 dnl Test body for checking C++17 support
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
- _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_11
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17]
 )
 
 dnl Test body for checking C++20 support
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_20],
+ [_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]
+)
+
+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
 )
 
 
@@ -189,7 +210,21 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201103L
+// MSVC always sets __cplusplus to 199711L in older versions; newer versions
+// only set it correctly if /Zc:__cplusplus is specified as well as a
+// /std:c++NN switch:
+//
+// https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
+//
+// The value __cplusplus ought to have is available in _MSVC_LANG since
+// Visual Studio 2015 Update 3:
+//
+// https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+//
+// This was also the first MSVC version to support C++14 so we can't use the
+// value of either __cplusplus or _MSVC_LANG to quickly rule out MSVC having
+// C++11 or C++14 support, but we can check _MSVC_LANG for C++17 and later.
+#elif __cplusplus < 201103L && !defined _MSC_VER
 
 #error "This is not a C++11 compiler"
 
@@ -480,7 +515,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_14], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201402L
+#elif __cplusplus < 201402L && !defined _MSC_VER
 
 #error "This is not a C++14 compiler"
 
@@ -604,7 +639,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201703L
+#elif (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 201703L
 
 #error "This is not a C++17 compiler"
 
@@ -970,7 +1005,7 @@ namespace cxx17
 
 } // namespace cxx17
 
-#endif // __cplusplus < 201703L
+#endif // (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 201703L
 
 ]])
 
@@ -983,7 +1018,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_20], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 202002L
+#elif (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202002L
 
 #error "This is not a C++20 compiler"
 
@@ -1000,6 +1035,36 @@ namespace cxx20
 
 } // namespace cxx20
 
-#endif // __cplusplus < 202002L
+#endif // (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202002L
+
+]])
+
+
+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
 
 ]])