https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80955
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> --- Testing this: --- a/libcpp/lex.c +++ b/libcpp/lex.c @@ -1903,7 +1903,7 @@ lex_raw_string (cpp_reader *pfile, cpp_token *token, const uchar *base, literal thus breaking the program. Try to identify macros with is_macro. A warning is issued. The macro name should not start with '_' for this warning. */ - if ((*cur != '_') && is_macro (pfile, cur)) + if ((*cur != '_' || cur[1] == '_') && is_macro (pfile, cur)) { /* Raise a warning, but do not consume subsequent tokens. */ if (CPP_OPTION (pfile, warn_literal_suffix) && !pfile->state.skipping) @@ -2034,7 +2034,7 @@ lex_string (cpp_reader *pfile, cpp_token *token, const uchar *base) literal thus breaking the program. Try to identify macros with is_macro. A warning is issued. The macro name should not start with '_' for this warning. */ - if ((*cur != '_') && is_macro (pfile, cur)) + if ((*cur != '_' || cur[1] == '_') && is_macro (pfile, cur)) { /* Raise a warning, but do not consume subsequent tokens. */ if (CPP_OPTION (pfile, warn_literal_suffix) && !pfile->state.skipping)