mrdocs::has_nullable_traits_v

template<class T>
concept has_nullable_traits_v = requires
    {
        { nullable_traits<T>::is_null(std::declval<const T&>()) } ‐> std::convertible_to<bool>;
        { nullable_traits<T>::null() } ‐> std::same_as<T>;
        { nullable_traits<T>::make_null(std::declval<T&>()) } ‐> std::same_as<void>;
    };