Swift playground demonstrating crash from UIView subview cycle

crashingsubviews.playground

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    override func loadView() {
        let crash = true
        let viewA = UIView.singleLabelNamedView("a")
        let viewB = UIView.singleLabelNamedView("b")
        let viewC = UIView.singleLabelNamedView("c")
        let viewD = UIView.singleLabelNamedView("d")
        
        viewA.addSubview(viewB)
        viewB.addSubview(viewC)
        viewC.addSubview(viewD)
        
        if crash {
            print("Should crash")
            viewD.addSubview(viewA)
        }
        print("Did not crash...")
        self.view = viewA
        
        self.view.printViewAndSubviews()
    }
}

extension UIView {
    static func singleLabelNamedView(_ name : String) -> UIView {
        let view = UIView()
        view.backgroundColor = .white
        
        let label = UILabel()
        label.frame = CGRect(x: 150 / Int.random(in: 2...10), y: 150 / Int.random(in: 2...10), width: 200 / Int.random(in: 2...10), height: 20)
        label.text = name
        label.textColor = .black
        
        view.addSubview(label)
        
        return view
    }
    
    func printViewAndSubviews() {
        print(self)
        for view in subviews {
            view.printViewAndSubviews()
        }
    }
}

PlaygroundPage.current.liveView = MyViewController()