Trying to make a func visible to ObjC via protocol extension

CrashyViewController.swift

import UIKit

protocol Tapper: class {
    func install()
    func foo()
}

extension Tapper where Self: UIViewController {
    func install() {
        view.addGestureRecognizer(UITapGestureRecognizer(target: self, action: "foo"))
        
    }
    
    // CRASH
    // This doesn't get found by the tap gesture.  Though it satisfies the protocol.
    func foo() {
        print("hello!")
    }
}

class ViewController: UIViewController, Tapper {
    
    // Defining foo here does get found.
    
    override func viewDidLoad() {
        super.viewDidLoad()
        install()
    }
}