Accessing self/newValue within willSet on class marked @Observable

Using Swift
,
1

I'm playing around with the new Xcode 15.0 beta (15A5160n) trying to implement an observable model that holds a weak reference to a UIScrollView

@Observable
final class BackingStore {
    var initialOffset: CGPoint? = nil
    weak var scrollView: UIScrollView? = nil {
        willSet {
            print(#function, self.scrollView, newValue, self.initialOffset)
            // do something with newValue...
        }
    }
}

However I'm getting weird errors:

Cannot find 'newValue' in scope
Cannot find 'self' in scope; did you mean to use it in a type or extension context?

I even tried reaching for the _scrollView and _$observationRegistrar values instead but self isn't in scope apparently so instance variables aren't available even if they're added by a macro.

Is it not possible to use willSet of didSet in @Observable marked classes? I read in the proposal that it was possible, though synthesised into the Registrar mutation methods.

Am I missing something?