https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69261
--- Comment #4 from Jens Auer <jens.auer at cgi dot com> ---
It produces the correct results if you change foo to not use constexpr:
void foo()
{
auto const s1 = s( "bla" );
auto const s2 = s( "blu" );
string_constexpr<7> const s1s2 = concat(s1,s2);
auto constexpr c = concat("bla", "blu");
std::cout << s1.data << std::endl << s2.data << std::endl << s1s2.data <<
std::endl << c << std::endl;
}
$ ./a.out
bla
blu
blablu
blablu
