Make protocols conforming to protocols as only struct/enum/class types can conform to protocols now

[Nickolas_Pohilets (Nickolas Pokhylets)]

What you are facing here is a combination of the two known problems -

• Existential subtyping as generic constraint
• Should weak be a type? - #22 by John_McCall

@John_McCall

However, there is currently no generic constraint that can fully express "is any kind of class reference type";

So, in the ideal world, generic signature of your class would look like this:

class Listeners<ListenerType> where ListenerType: any ListenerProtocol, ListenerType: Referencable {
...
}