[swift-evolution] [Pitch] Tweak `Self` and introduce `Current`

[Braeden Profile]

> 
> Of course, I would love being able to use an initializer setup, but there are serious bugs in the implementation.
> 
> protocol Clonable
> {
> 	init(other: Self)
> }
> 
> extension Clonable
> {
> 	func clone() -> Self
> 		{ return type(of: self).init(other: self) }
> }
> 
> 
> class Base: Clonable
> {
> 	var x: Int
> 	
> 	init(x: Int)
> 		{ self.x = x }
> 	
> 	required init(other: Base)
> 		{ self.x = other.x }
> }
> 
> class Derived: Base
> {
> 	var y: String
> 	
> 	init(x: Int, y: String)
> 	{
> 		self.y = y
> 		super.init(x: x)
> 	}
> 	
> 	// Should be required by the Clonable protocol, but it isn't.
> 	required init(other: Derived)
> 	{
> 		self.y = other.y
> 		super.init(other: other)
> 	}
> 	
> 	// Required because it was `required` in Base.  Even a `Derived` calls this initializer to clone, which is wrong.  Bugs abound.
> 	required init(other: Base)
> 		{ fatalError("init(other:) is wrong.") }
> }
> 
> 
> 
> let me = Derived(x: 1, y: "food")
> let alienClone = me.clone() // "init(other:) is wrong."
> 
> 
> Agree. That seems wrong. Great example.

So, is this odd behavior intentional, a bug, or a design deficiency?  I would think that when a protocol has a method or initializer has `Self` parameters—like in Clonable—every subclass would be required to implement its own specialized version (much like a required initializer).  That would be a special case of the protocol system, though.

As it sits, even fixing the calling behavior of my example leaves us with the problem of subclasses inheriting inapplicable required initializers from superclasses that actually don’t make any sense.

Does this deserve its own thread?

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