template <int idx,
typename S >
struct visit_struct::type_at_s;

using visit_struct::type_at_s< idx, S >::type_c =  decltype(traits::visitable<traits::clean_t<S> >::type_at(std::integral_constant<int, idx>{}));

using visit_struct::type_at_s< idx, S >::type =  typename type_c::type;