@wadetregaskis wrote a summary of the issue we discovered in this thread, caused by an Objective-C superclass implementing -copyWithZone: by calling NSCopyObject.
Wade’s final point is key:
Compounding the problem is that
NSCopyingdoesn’t work, by default, on subclasses . You have to overridecopy(with:)in every subclass […]
We’ve seen this exact pattern before with NSCoding. But because NSCoding uses an initializer, we have a tool to enforce its implementation in subclasses: required. Alas, NSCopying can’t be modeled with a required initializer, because then you could do something like let base = Base(); let copy = Derived.init(copying: base).
Does it make sense to deprecate NSCopying entirely as part of transitioning Foundation to Swift? Alternatively, if arbitrary protocol methods could be required, would that be enough to make NSCopying both useful and safe?