Class-constrained protocol extension has incorrect mutatingness

[DevAndArtist (Adrian Meister)]

What exactly does P where Self mean?
Is P the protocol or the existential here?
If the latter, what does Self mean in context of the existential?

---

Just trying to understand these tiny things and complete my knowledge about the existential type:

Is the 'existential' a value or ref type?

There is hidden achievement to unlock in this setup. First, we know that the "existential" type itself is a value type which has no value semantics unless it wraps another type with value semantics. struct Existential<Value> { // invisible to the user var _value: Value // forwards all members of Value } Now let's see our protocols: protocol P0 { // this is a `mutating set` var x: Int { get set } } // protocol is class bound but it still has a `mutating set` protocol P1: AnyObject,…