https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69865
--- Comment #2 from Bernd Edlinger <bernd.edlinger at hotmail dot de> --- in gcc/c-family/c-opts.c: following code in line 805: /* Set C++ standard to C++14 if not specified on the command line. */ if (c_dialect_cxx () && cxx_dialect == cxx_unset) set_std_cxx14 (/*ISO*/false); ... defeats this in line 667 above: case OPT_trigraphs: cpp_opts->trigraphs = 1; break;