に std::common_type というメタ関数が入る予定 template <class ...T> struct common_type; template <class T> struct common_type<T> { typedef T type; }; template <class T, class U> struct common_type<T, U> { private: static T&& t(); static U&& u(); public: typedef decltype(true ? t() : u())</t,></class></t></class></class>…