message: Fix static assertion
diff: #pragma once
  +#include <vector>
  +
  #include "caf/default_downstream_manager.hpp"
  #include "caf/detail/stream_stage_driver_impl.hpp"
  #include "caf/detail/stream_stage_impl.hpp"
  @@ -74,10 +76,14 @@ attach_stream_stage(scheduled_actor* self, const stream<In>& in,
  void(state_type&),
  typename detail::get_callable_trait<Init>::fun_sig>::value,
  "Expected signature `void (State&)` for init function");
  - static_assert(std::is_same<
  - void(state_type&, downstream<output_type>&, In),
  - typename detail::get_callable_trait<Fun>::fun_sig>::value,
  + using consume_one = void(state_type&, downstream<output_type>&, In);
  + using consume_all = void(state_type&, downstream<output_type>&,
  + std::vector<In>&);
  + using fun_sig = typename detail::get_callable_trait<Fun>::fun_sig;
  + static_assert(std::is_same<fun_sig, consume_one>::value
  + || std::is_same<fun_sig, consume_all>::value,
  "Expected signature `void (State&, downstream<Out>&, In)` "
  + "or `void (State&, downstream<Out>&, std::vector<In>&)` "
  "for consume function");
  using driver = detail::stream_stage_driver_impl<
  typename Trait::input, DownstreamManager, Fun, Finalize>;
  