auto | requires_ (T &&t, U &&u) -> decltype( concepts::valid_expr( concepts::model_of< WeaklyOrdered, T >(), concepts::model_of< WeaklyOrdered, U >(), concepts::model_of< CommonReference, T const &, U const & >(), concepts::model_of< WeaklyOrdered, C >(), concepts::convertible_to< bool >(t< u), concepts::convertible_to< bool >(u< t), concepts::convertible_to< bool >(t > u), concepts::convertible_to< bool >(u > t), concepts::convertible_to< bool >(t<=u), concepts::convertible_to< bool >(u<=t), concepts::convertible_to< bool >(t >=u), concepts::convertible_to< bool >(u >=t) )) |