There has been a push to declare as many standard library functions constexpr as possible. However, declaring a function constexpr does not guarantee that a call to that function will be a constant expression. If we don't know whether a call will be a constant expression, then the fact that it is marked constexpr is useless.

In most cases, the standard does not specify the way in which standard library functions are implemented, as this is considered an implementation detail. However, in the case of a constexpr function, these implementation details actually become part of the function's interface, because they determine whether or not the function will be usable in a constant expression.

Obviously the call cannot be a constant expression if the library function is specified to invoke some other function that is not constexpr (e.g., std::sort invokes the user's comparator). However, it's not entirely clear to me what happens in all other cases.
Brian Bi