https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80265
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> --- That doesn't work in this case: constexpr bool str() { char s[] = "str"; return ce_char_traits<char>::length(s) == 3; } static_assert( str() ); i.e. it works OK for C++11-style constant expressions, but not C++14 ones.