relates to:

• SR-5475 Can't set property declared optional in protocol

Issue Description:

Public bug for <rdar://problem/15233922> "Cannot assign to a property through AnyObject"

I'm attempting to implement NSOpenSavePanelDelegate in a Sandboxed Mac app.

The `sender: Any` parameter is a private system class that I cannot cast to. The private classes do not inherit from the public alternatives, so I cannot cast to those either.

I'm forced to use AnyObject. But doing so results in the error `Cannot assign to immutable expression`.

Sample Code:

extension PanelController: NSOpenSavePanelDelegate {
    
    func panel(_ sender: Any, didChangeToDirectoryURL url: URL?) {
        switch sender {
        case is NSSavePanel: () // false
        case is NSOpenPanel: () // false
        default:
            print("Sandboxed app. Uses private PowerBox open/save panel classes such as NSVBOpenPanel")
        }
        
        var panel = sender as AnyObject
        
        // Error: Cannot assign to immutable expression of type 'URL?'
        panel.directoryURL? = URL(fileURLWithPath: "/")!
    }

}

Also posted in the Swift Forums here:
https://forums.swift.org/t/anyobject-parameter-cannot-assign-to-immutable-expression/16642