In my opinion, `__builtin_constant_p()` is not that obscure of a feature. In C, it is used in macros to imitate constant functions, and in C++ it is useful for

In my opinion, `__builtin_constant_p()` is not that obscure of a feature. In C, it is used in macros to imitate constant functions, and in C++ it is useful for determining the current alternative that has lifetime in a `union` within a constant function. Granted that `__builtin_is_constant_evaluated()` has obsoleted its primary purpose, but there are enough ways it's still useful that I see it from time to time.