Generics with Self on class type failing compiler

@propertyWrapper
public final class Wrapper {
    
    public static subscript<EnclosingSelf>(
      _enclosingInstance observed: EnclosingSelf,
      wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Int?>,
      storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
    ) -> Int? {
      get {
        return observed[keyPath: storageKeyPath].stored
      }
      set {
        let oldValue = observed[keyPath: storageKeyPath].stored
        if newValue != oldValue {
            // TODO: call wrapper instance with enclosing self
        }
        observed[keyPath: storageKeyPath].stored = newValue
      }
    }
    
    public var wrappedValue: Int? {
      get { fatalError("called wrappedValue getter") }
      set { fatalError("called wrappedValue setter") }
    }
    
    public init(str: String) {
        self.str = str
    }
    
    // MARK: - Private
    
    private let str: String
    private var stored: Int?
}

open class TestView: UIView {
    @Wrapper(str: "HelloWorld") public var testProp: Int?
}

/** 
    Swift Compiler Error
    Type '_' has no member 'testProp'
*/