BadExtensions.swift

import AVFoundation
import UIKit

class ViewController: UIViewController {

    var discoverSession:MyDiscoverySession = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInDualCamera, .builtInWideAngleCamera, .builtInTelephotoCamera], mediaType: .video, position: .back)

    @IBAction func didTapCrashButton(_ sender: Any) {
        let devices = discoverSession.myDevices
        print("Got devices: \(devices)")
    }
}

protocol MyDevice:class {
    
}

protocol MyDiscoverySession:class {
    var myDevices:[MyDevice] { get }
}

extension AVCaptureDevice:MyDevice {
    
}

extension AVCaptureDevice.DiscoverySession: MyDiscoverySession {
    var myDevices:[MyDevice] {
        return self.devices
    }
}