so_5::details::make_message_instance_impl metafunction doesn't set message mutability flag properly

[eao197]

It seems that make_message_instance_impl should call mark_as_mutable_if_necessary:

sobjectizer/dev/so_5/message.hpp

Lines 862 to 876 in 5035f99

 template< bool is_signal, typename Msg > 
 struct make_message_instance_impl 
 	{ 
 		using E = typename message_payload_type< Msg >::envelope_type; 
  
 		template< typename... Args > 
 		[[nodiscard]] 
 		static std::unique_ptr< E > 
 		make( Args &&... args ) 
 			{ 
 				ensure_not_signal< Msg >(); 
  
 				return std::unique_ptr< E >( new E( std::forward< Args >(args)... ) ); 
 			} 
 	}; 

Without this call limit_then_transform can make a mutable message and send it into MPMC mbox.

[eao197]

The fix will be a part of upcoming 5.8.1 release.