A always gets deinit before B

deinit order.swift

class A {
    var b: B
    
    init(b: B) {
        self.b = b
    }
    
    deinit {
        print("  Destroying A")
    }
}

class B {
    weak var a: A?
    
    deinit {
        print("  Destroying B")
    }
}

func setup(_ a: inout A?, _ b: inout B?) {
    b = B()
    a = A(b: b!)
    b?.a = a
}

func teardown(_ a: inout A?, _ b: inout B?) {
    a = nil
    b = nil
}

var bravo: B?
var alpha: A?

print("Scenario 1: nil bravo only")
setup(&alpha, &bravo)
bravo = nil
print("Scenario 1: end")

print("Teardown 1")
teardown(&alpha, &bravo)
print("Teardown 1: end")

print("Scenario 2: nil alpha only")
setup(&alpha, &bravo)
alpha = nil
print("Scenario 2: end")

print("Teardown 2")
teardown(&alpha, &bravo)
print("Teardown 2: end")

print("Scenario 3: nil alpha then bravo")
setup(&alpha, &bravo)
teardown(&alpha, &bravo)
print("Scenario 3: end")

print("Scenario 4: nil bravo then alpha")
setup(&alpha, &bravo)
bravo = nil
alpha = nil
print("Scenario 4: end")