message: Add variant trait helper
diff: @@ -75,6 +75,9 @@ struct is_variant<const variant<Ts...>&> : std::true_type {};
  template <class... Ts>
  struct is_variant<const variant<Ts...>&&> : std::true_type {};
  +template <class... Ts>
  +using is_variant_t = typename is_variant<Ts...>::type;
  +
  template <class T>
  struct variant_assign_helper {
  using result_type = void;
  