Derived from the changes that added C++2a support in 2017.
r8-3237-g026a79f70cf33f836ea5275eda72d4870a3041e5
No C++23 features are added here.
Use of -std=c++23 sets __cplusplus to 202100L.
$ g++ -std=c++23 -dM -E -x c++ - < /dev/null | grep cplusplus
#define __cplusplus 202100L
gcc/
* doc/cpp.texi (__cplusplus): Document value for -std=c++23
or -std=gnu++23.
* doc/invoke.texi: Document -std=c++23 and -std=gnu++23.
* dwarf2out.c (highest_c_language): Recognise C++20 and C++23.
(gen_compile_unit_die): Recognise C++23.
gcc/c-family/
* c-common.h (cxx_dialect): Add cxx23 as a dialect.
* c.opt: Add options for -std=c++23, std=c++2b, -std=gnu++23
and -std=gnu++2b
* c-opts.c (set_std_cxx23): New.
(c_common_handle_option): Set options when -std=c++23 is enabled.
(c_common_post_options): Adjust comments.
(set_std_cxx20): Likewise.
gcc/testsuite/
* lib/target-supports.exp (check_effective_target_c++2a):
Check for C++2a or C++23.
(check_effective_target_c++20_down): New.
(check_effective_target_c++23_only): New.
(check_effective_target_c++23): New.
* g++.dg/cpp23/cplusplus.C: New.
libcpp/
* include/cpplib.h (c_lang): Add CXX23 and GNUCXX23.
* init.c (lang_defaults): Add rows for CXX23 and GNUCXX23.
(cpp_init_builtins): Set __cplusplus to 202100L for C++23.
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