https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111351
--- Comment #7 from James Y Knight <foom at fuhm dot net> --- On the libc++ side, a suggestion was given that instead of making this an _error_, we could instead emit a warning if "a constexpr or constinit object is a basic_string or contains a basic_string subobject, or the definition of a constexpr or constinit variable extends the lifetime of a temporary object that meets the previous condition." I think that was a really great suggestion -- diagnosing via a warning is a nicer solution than putting is_constant_evaluated hacks into the library (as MSVC had, and libc++ currently has but will likely remove). One could either hardcode std::basic_string for the diagnostic, or add a new type-attribute to permit any type to opt-in to such a warning. You'd want to use it if you have a type where you don't _intend_ to support constant-initialization, but where it may sometimes be possible as an implementation detail, and you want to tell users not to rely on that implementation detail.
