Should weak be a type?

[Nickolas_Pohilets (Nickolas Pokhylets)]

@John_McCall

Weak references are nullified when the object is destroyed. What do you want to happen when the object is destroyed?

I want weak references to maintain reference-counted pointer to a side table, even after object is destroyed. This would enable to use address of the side table as a stable unique comparison key.

Stable - meaning it does not change when object is destroyed
Unique - it cannot be reused when new object is allocated if there are remaining weak references to the old one.

Currently when copying weak reference that contains non-nil pointer to the side table of the destroyed object, copy contains nil pointer to the side table. This leads to various equality paradoxes - references that used to be distinct became equal; reference and its copy may be non-equal.

@John_McCall

With all of that, you could make a property wrapper that duplicated most of the functionality of weak and unowned without compiler support. However, there is currently no generic constraint that can fully express "is any kind of class reference type"; the AnyObject constraint requires the type to have a simple object pointer representation, which precludes non- @objc class-bounded protocol and protocol-composition types. Therefore, that wrapper type — and the proposed Weak<T> and Unowned<T> , for that matter — would not be able to support the full range of types that you can use with weak and unowned today.

That's a custom case of not being able to express existential subtyping as a generic constraint. I hope to include this to the GenericsManifesto by this MR.