Reconsider the type of `self` of an actor to actually be Self

[ktoso]

Description

In classes the type of Self is dynamic like this:

class C {
  func f() { self /* self is a C */ }
  func f() -> Self { self /* self is a Self */ }
}

this is because of inheritance.

Inside an actor this causes perhaps surprising errors like this:

dy.swift:3:22: error: cannot convert value of type 'C' to expected argument type 'Self'
    { (x: Self) in }(self)
                     ^
                          as! Self

Actors do not allow inheritance, and therefore we could say that self is always Self.

This would set in stone that actors cannot have inheritance though.

Additional information

Radar: rdar://121091417