final class Node { var next: Node? }
func breakCycle(_ node: Node) { node.next = nil }

weak var observedA: Node?
weak var observedB: Node?
do {
    let a = Node()
    let b = Node()
    a.next = b
    b.next = a
    observedA = a
    observedB = b
}
precondition(observedA != nil && observedB != nil)
breakCycle(observedA!)
precondition(observedA == nil && observedB == nil)

final class Owner { var child: Child? }
final class Child { weak var owner: Owner? }
weak var observedOwner: Owner?
weak var observedChild: Child?
do {
    let owner = Owner()
    let child = Child()
    owner.child = child
    child.owner = owner
    observedOwner = owner
    observedChild = child
}
precondition(observedOwner == nil && observedChild == nil)

final class Borrower {
    unowned var owner: Owner?
    init(_ owner: Owner) { self.owner = owner }
}
do {
    let owner = Owner()
    let borrower = Borrower(owner)
    withExtendedLifetime(owner) {
        precondition(borrower.owner === owner)
        borrower.owner = nil
        precondition(borrower.owner == nil)
    }
}
print("cycle=retained broken=released weak=nil unowned-optional=manually-cleared")