message: Add is_actor_handle type trait
diff: @@ -626,6 +626,16 @@ struct is_specialization : std::false_type { };
  template <template <class...> class T, class... Ts>
  struct is_specialization<T, T<Ts...>> : std::true_type { };
  +/// Checks whether `T` is an `actor` or a `typed_actor<...>`.
  +template <class T>
  +struct is_actor_handle : std::false_type {};
  +
  +template <>
  +struct is_actor_handle<actor> : std::true_type {};
  +
  +template <class... Ts>
  +struct is_actor_handle<typed_actor<Ts...>> : std::true_type {};
  +
  } // namespace detail
  } // namespace caf
  