message: Fix serialization of std::optional
diff: @@ -436,7 +436,8 @@ struct inspector_access<optional<T>> : optional_inspector_access<optional<T>> {
  #ifdef CAF_HAS_STD_OPTIONAL
  template <class T>
  -struct optional_inspector_traits<std::optional<T>> {
  +struct optional_inspector_traits<std::optional<T>>
  + : optional_inspector_traits_base {
  using container_type = std::optional<T>;
  using value_type = T;
  