message: Add forward definition of variant to fwd.hpp
diff: @@ -48,6 +48,7 @@ template <class, class, class> class random_topic_scatterer;
  // -- variadic templates -------------------------------------------------------
  template <class...> class result;
  +template <class...> class variant;
  template <class...> class delegated;
  template <class...> class typed_actor;
  template <class...> class typed_actor_pointer;
  