Allow `self = x` in class convenience initializers

[Moximillian (Mox)]

@DevAndArtist

// After:
extension UIView {
  static func instantiateFromXib() -> Self {
    let metatype = Self.self
    let bundle = Bundle(for: metatype)
    let nib = UINib(nibName: "\(metatype)", bundle: bundle)
    guard let view = nib.instantiate(withOwner: nil, options: nil).first as? Self else {
      fatalError("Could not load view from nib file.")
    }
    return view
  }
}

As pointed out by @ahti in Workarounds for Self in classes, for static methods self is equivalent to self.Type. So your example above can be simpler:

// After:
extension UIView {
  static func instantiateFromXib() -> Self {
    let bundle = Bundle(for: self)
    let nib = UINib(nibName: "\(self)", bundle: bundle)
    guard let view = nib.instantiate(withOwner: nil, options: nil).first as? Self else {
      fatalError("Could not load view from nib file.")
    }
    return view
  }
}

However, what is missing is coercing to Self. Neither as? self nor as? Self is ok in classes, but latter works fine in protocols. On the other hand, is there a way to workaround this if one could assign to self? I guess there would still be type mismatch?