For now, I think these two are incompatible. There are more features in -fconcepts than in C++20 (so far). There are also some changes in syntax and semantics that would be nice to diagnose. A good example would be 'concept' as a decl-specifier (TS) vs. 'concept' as a declaration introducer (WD).
I was going to submit a followup that emits a warning when both -std=c++2a and -fconcepts are both enabled and then disables -fconcepts. Andrew Sutton On Thu, Jul 20, 2017 at 1:04 PM, Markus Trippelsdorf <mar...@trippelsdorf.de> wrote: > On 2017.07.20 at 09:33 -0400, Andrew Sutton wrote: >> This adds a new C++ dialect, enabled by -std=c++2a. >> >> libcpp/ >> Add support for C++2a. >> * 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 201707L for C++2x. >> >> gcc/c-family/ >> Add support for -std=c++2a. >> * 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. >> >> gcc/testsuite/ >> New test for -std=c++2a. >> * g++.dg/cpp2a/cplusplus.C: New. > > Perhaps you should enable -fconcepts by default? > > -- > Markus
