>        friend void call_once(once_flag& flag, Function f);        template<typename Function, typename T0>        friend void call_once(once_flag& flag, Function f, T0 p0);        template<typename Function, typename T0, typename T0>        friend void call_once(once_flag& flag, Function f, T0 p0, T0 p0);        template<typename Function, typename T0, typename T0, typename T0>        friend void call_once(once_flag& flag, Function f, T0 p0, T0 p0, T0 p0);  