where is xvariant? #286

        template <class... T>
        struct xdynamic_traits_impl<xtl::variant<T...>>
        {
            using value_type = xtl::variant<xtl::xoptional<T, bool>...>;
            using reference = xtl::variant<xtl::xoptional<T&, bool&>...>;
            using const_reference = xtl::variant<xtl::xoptional<const T&, const bool&>...>;
            using pointer = xtl::xclosure_pointer<reference>;
            using const_pointer = xtl::xclosure_pointer<const_reference>;
        };