ios - Protocol extension methods don't get called when protocol extends existing Apple API protocol

Question

I want a protocol to inherit from an Apple protocol UIViewControllerTransitioningDelegate, add additional protocol requirements and provide a default implementation for some methods in that protocol. When I do that, the methods do not get called. When I implement the methods in the class itself, the methods do get called.

Here's what I'm talking about:

class FirstViewController: UIViewController, SlideDismissor {
    let transition: PercentDrivenInteractiveTransitionWithState? = PercentDrivenInteractiveTransitionWithState()
}

protocol SlideDismissor: UIViewControllerTransitioningDelegate {
    var transition: PercentDrivenInteractiveTransitionWithState? { get }
}

extension SlideDismissor {
    func animationControllerForDismissedController(dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return nil // I do return something here, but for this example it isn't necessary
    }

    func interactionControllerForDismissal(animator: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning? {
        return transition
    }
}

If I add the following code the functions do get called.

extension FirstViewController: UIViewControllerTransitioningDelegate {

    func animationControllerForDismissedController(dismissed: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        return nil // I do return something here, but for this example it isn't necessary
    }

    func interactionControllerForDismissal(animator: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning? {
        return transition
    }

}

These methods I'm trying to provide a default implementation for are optional methods.