[functional] member_pointer_class_t

[GuillaumeDua]

see https://godbolt.org/z/d9dnq1MW4

template <concepts::member_pointer T>
class invoke_result<T> {
   using class_t = type_traits::member_pointer_class_type_t<T>;
public:
    using type = std::invoke_result_t<T, class_t>;
}