Not implementable delegate in protocol extension.

playground.swift

import UIKit
import PlaygroundSupport

class MyViewController : UIViewController, Interface {
  lazy var tap = UITapGestureRecognizer(target: self, action: #selector(tapped))
  
  override func viewDidLoad() {
    super.viewDidLoad()
    tap.delegate = self
    view.addGestureRecognizer(tap)
  }
  
  @objc private func tapped(_ sender: UITapGestureRecognizer) {
    print("hoge")
  }
}

protocol Interface: UIGestureRecognizerDelegate {
  func setup()
}

extension Interface where Self: MyViewController {
  func setup() {
    tap.delegate = self
  }
  
  func gestureRecognizerShouldBegin(_ gestureRecognizer: UIGestureRecognizer) -> Bool {
    return false
  }
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()