Allow implicit `self` for `weak self` captures, after `self` is unwrapped

Evolution Pitches
2

With all the special handling of self in captures, I wonder whether it should also be given additional restrictions. This code, for example, would be legal but confusing:

class C {
  static var global: C? = C(name: "global C")
  let name: String
  func identify() { print("hello I am \(name)!") }
  func identifyLater() {
    Task { [weak self] in
      guard let self = self ?? C.global else { return }
      identify() // What does this print?
    }
}

Perhaps it should be illegal to re-bind self to anything but the result of successfully unwrapping weak self in a closure that captures self.

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