TypeInference.swift

// Find the first ViewController of an arbitrary type anywhere in the hierarchy of a base ViewController
private func findParent<V: UIViewController>(ofViewController viewController: UIViewController?) -> V? {
    if let typedViewController = viewController as? V {
        return typedViewController
    }

    guard let parentViewController = viewController?.parentViewController else {
        return nil
    }

    return findParent(ofViewController: parentViewController)
}

class MyViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        // These two calls will return different things, because the type V is inferred by
        // the annotation on the left hand side of the assignment
        let navController: UINavigationController? = findParent(ofViewController: self)
        let tabController: UITabBarController? = findParent(ofViewController: self)
    }
}