class A {
    var b: B?
    deinit {
        print("A being deallocated")
    }
}

class B {
    var a: A?
    deinit {
        print("B being deallocated")
    }
}

@discardableResult
func makeCycle() -> A {
    let a = A()
    let b = B()
    b.a = a
    a.b = b
    return a
}

weak var a: A? = makeCycle()

print(a == nil ? "A is gone" : "A exists")

print("Setting a.b to nil")
a?.b = nil

print(a == nil ? "A is gone" : "A exists")