template< typename base, typename v >
	constexpr bool is_passable_fn( ... ) { static_assert ( sizeof (base) > 0, "Pass to incomplete type." ); return false; }

	template< typename base, typename v >
	constexpr typename std::enable_if< std::is_void< decltype ( next_stage( std::declval< base & >() ).template pass< pass_policy::enable_if >( std::declval< v >() ) ) >::value, bool >::type
	is_passable_fn( int ) { return true; }

	template< typename base, typename v, typename q = void >
	struct is_passable : std::integral_constant< bool, is_passable_fn< base, v >(0) > {};

	template< typename base, typename v >
	struct is_passable< base, v, decltype( next_stage( std::declval< base & >() ).template pass< pass_policy::enable_if >( std::declval< v && >() ) ) > : std::true_type {};