https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111351
--- Comment #4 from James Y Knight <foom at fuhm dot net> --- vector and string are different in one key way: a zero-sized vector has no accessible storage, while a zero-sized string has 1 byte of readable storage -- the NUL terminator. Because of that, I don't think it's unreasonable for a zero-length vector to be constinit'able, while a zero-length string is not. But, certainly the _more_ concerning issue is with non-zero-sized strings where the validity of the program depends on what exact SSO size was chosen by the implementation. > libc++'s choice is confusing even for the experts, who have to maintain its > split-brain SSO logic forever because Hyrum's Law "Hyrum's Law" is exactly why I think it's a mistake to permit SSO-allocated strings. It makes the SSO length a critical part of the interface. People will start writing code which assumes that a constexpr global string of size "N" works, and that will cause problems for other standard libraries which use a different SSO size "M", if M < N. E.g. if libc++ starts allowing this, then people who first target libc++ will find that strings up to 22 characters work, and be surprised/annoyed by libstdc++ failing to build their program. It is much simpler to say to users "you cannot make a constexpr std::string unless it lives fully within constant-evaluation-time." then to also add "...OR unless the size is short enough, where that size depends on your implementation."
