Copyable

Copyable.swift

protocol Copyable {
    typealias CopyType
    func copy() -> CopyType
}

class A: Copyable {
    var x: Int
    init(x: Int) { self.x = x }
  
    func copy() -> A {
        return A(x: self.x)
    }
}

class B: A  {
    var y : Int
    
    init(x: Int, y: Int) {
        self.y = y
        super.init(x: x)
    }

    override func copy() -> B {
        return B(x: self.x, y: self.y)
    }
}


let a = A(x: 1)
let b = a.copy()

let c = B(x: 1, y: 2)
let d = c.copy()

Trouble is that CopyType can be anything. So to use Copyable in a useful way, we have to put it in a generic with a constraint of "C: Copyable where C.CopyType == C". Since it has an associated type, you can't accept a parameter of type "Copyable."

Your current code would work just as well if Copyable didn't exist; it's really just methods that happen to be on the classes. I suspect in practice this will make it not worth the trouble. Still, the closest to useful I've seen so far.

Of course I've been spending a lot of time thinking about what I'd use Copyable for. I keep feeling like I need it, but I don't think it's ever shown up in pure Swift code, and when bridging to ObjC I have to use NSCopying in practice anyway.

For sure. It's not useful in practice. A type requirement of Self in a protocol should travel down the class inheritance chain. I don't think it makes any practical sense the way it behaves now.