namespace std {
    template<typename T> struct hash_traits {
        template<typename A, typename B> struct heterogenous {
            bool operator()(const T& t) { return false; }
        };
    };
    template<> struct hash_traits<std::hash<>> {
        template<typename A, typename B> struct heterogenous;
    };
    template<typename T> struct hash_traits<std::hash<>>::heterogenous<std::unique_ptr<T>, T*> {
        bool operator()(std::hash<>) { return true; }
    };
}