NSCopying's copy() returns Any by default, which is terribly annoying. Here I force it to be of the same type as Self.

NSCopying.swift

import Foundation

extension NSCopying {

    func copy() -> Self {

        // Use copy(with zone:) here to disambiguate between this
        // overload and the one we actually want to call.
        let c = copy(with: nil)

        // Copies will be of type self, right??
        return c as! Self
    }
}