https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110968
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- Oh dear, I must have broken this function at some point while implementing std::format: // Locale-specific format. basic_string<_CharT> _M_localize(basic_string_view<_CharT> __str, char __expc, const locale& __loc) const { basic_string<_CharT> __lstr; if (__loc == locale::classic()) return __lstr; // Nothing to do. That just returns an empty string, which the caller then uses assuming it's valid.