DeallocationCrash.swift

import Foundation

protocol Delegate: class {
}

@objc class Thing: NSObject {
    weak var delegate: Delegate?

    init(delegate: Delegate) {
        self.delegate = delegate
    }

    func nothing() {
    }
}

@objc

class MyClass: NSObject, Delegate {
    lazy var thing: Thing = {
        return Thing(delegate: self)
    }()

    deinit {
        thing.nothing() // This crashes due to a cyclical reference to self.
    }
}

func doThing() {
    let thing = MyClass()
}

doThing()