protocol Boxable {
  func boxed() -> Self
}

class List: Boxable {
  let next: List?
  init(next: List?) { self.next = next }
  func boxed() -> Self { List(next: self) as! Self }
}

class DerivedList: List {
  let foo: String
  init(foo: String, next: List?) { self.foo = foo; super.init(next: next) }
}

let derived = DerivedList(foo: "bar", next: nil).boxed()
print(derived.foo)