* c-common.h (cxx_dialect): Add cxx2a as a dialect.
* opt.c: Add options for -std=c++2a and -std=gnu++2a.
* c-opts.c (set_std_cxx2a): New.
(c_common_handle_option): Set options when -std=c++2a is enabled.
(c_common_post_options): Adjust comments.
(set_std_cxx14, set_std_cxx17): Likewise.
* doc/cpp.texi (__cplusplus): Document value for -std=c++2a
or -std=gnu+2a.
* doc/invoke.texi: Document -std=c++2a and -std=gnu++2a.
* lib/target-supports.exp (check_effective_target_c++17): Return
1 also if check_effective_target_c++2a.
(check_effective_target_c++17_down): New.
(check_effective_target_c++2a_only): New.
(check_effective_target_c++2a): New.
* g++.dg/cpp2a/cplusplus.C: New.
* include/cpplib.h (c_lang): Add CXX2A and GNUCXX2A.
* init.c (lang_defaults): Add rows for CXX2A and GNUCXX2A.
(cpp_init_builtins): Set __cplusplus to 201709L for C++2a.
Co-Authored-By: Jakub Jelinek <jakub@redhat.com>
From-SVN: r252850
