I understand, but it isn't enough. You have to have a selective receive primitive to write something like:

    for(;;) { // The main state.
      receive(
        when([](M1 msg1){ // Dive into another state.
          while(some_condition) {
            receive(when([](M2 msg2){...},
              when([](M7 msg7){...},
              ...);
          }
        }),
        when([](M3 msg3){...}),
        when([](M5 msg5){...}),
        ...);
    }

In that case, if you receive M2 before M1 the instance of M2 will be kept in the queue.

But if you have to write something like that:

    for(;;) { // The main state.
      auto * m = receive();
      if(m->type == M1::msg_typeid) { // Dive into another state.
        while(some_condition) {
          auto * m2 = receive();
          if(m2->type == M2::msg_typeid) {...}
          else if(m2->type == M7::msg_typeid){...}
          else if...
        }
      }
      else if(m->type == M3::msg_typeid){...}
      else if(m->type == M5::msg_typeid){...}
      else if...
    }

then you can easily lose a message by a mistake.