Struct std::SignalSub

struct SignalSub {
    fn void(?&Erased) listener;
    ?&Erased userdata;
}

One subscriber. Listeners on the same signal carry unrelated context types, so each is stored type-erased; signal_subscribe() only ever pairs a fn void(?&U) with a ?&U, so the erased call is always well-typed (see std/erased.h).