message: Fix build on Clang 4
diff: @@ -76,20 +76,20 @@ template <class T>
  struct default_actor_traits<T, true> {
  /// Denotes whether `T` is dynamically typed.
  static constexpr bool is_dynamically_typed
  - = std::is_base_of_v<dynamically_typed_actor_base, T>;
  + = std::is_base_of<dynamically_typed_actor_base, T>::value;
  /// Denotes whether `T` is statically typed.
  static constexpr bool is_statically_typed
  - = std::is_base_of_v<statically_typed_actor_base, T>;
  + = std::is_base_of<statically_typed_actor_base, T>::value;
  /// Denotes whether `T` is a blocking actor type.
  static constexpr bool is_blocking
  - = std::is_base_of_v<blocking_actor_base, T> //
  + = std::is_base_of<blocking_actor_base, T>::value
  || mixin::is_blocking_requester<T>::value;
  /// Denotes whether `T` is a non-blocking actor type.
  static constexpr bool is_non_blocking
  - = std::is_base_of_v<non_blocking_actor_base, T>;
  + = std::is_base_of<non_blocking_actor_base, T>::value;
  /// Denotes whether `T` is an incomplete actor type that misses one or more
  /// markers.
  