import UIKit

protocol SomeProtocol {
    var navController: UINavigationController? { get }
}

public class BaseViewController: UIViewController {
    var navController: UINavigationController? {
        return navigationController
    }
}

class ViewController: BaseViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
    }
}

extension SomeProtocol where Self: ViewController {
    func doStuff() {
        let x: () -> Void = { [weak self] in
            // Ambiguous use of 'navController'
            // Used to build fine on Xcode 9.4 GM, doesn't build since
            // Xcode 10b1 (and not on the latest b4).
            guard let nav = self?.navController else { return }
        }
    }
}