[Pitch] Weak let

Evolution Pitches
15

That's very close, yes! I would suggest a few very subtle refinements:

struct Container {
    // Note that this is optional: the weak property can be nil
    // because its referent has been destroyed, but it can also just
    // be nil because you stored nil into it.
    var c_storage: WeakReference<C>?
    var c: C? {
        get {
            guard let ref = c.storage else { return nil }
            return ref.isDeallocated ? nil : ref.object
        }
        set {
            if let newObject = newValue {
              // Note that we make a new reference instead of
              // modifying the existing one. `Container` still has
              // value semantics: properties of copies of the same
              // value have to be independent.
              c_storage = WeakReference(object: newObject)
            } else {
              c_storage = nil
            }
        }
    }
}
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