Swfit weak reference undefined behaviour case. (Swift 2.0)

1. Source Code

class AAA {
	deinit {
		print("AAA deinit")
		callbackOnDeinit!(self)
	}

	var callbackOnDeinit: (AAA->())?
}

class BBB {
	weak var aaa: AAA? {
		didSet {
			aaa!.callbackOnDeinit	=	{ [weak self] in
				print("self.aaa when it deinit: \(self!.aaa)")
				print("$0 when it deinit: \($0)")
				self!.strongRefAfterDeinit	=	$0
			}
		}
	}

	var strongRefAfterDeinit: AAA?
}

var	aaa	=	AAA() as AAA?
let	bbb	=	BBB()

bbb.aaa		=	aaa
aaa		=	nil
print("now aaa = \(aaa)")
print("bbb.aaa = \(bbb.aaa)")

/// Result of this line is undefined. It crashes, or even *works*.
print("bbb.strongRefAfterDeinit = \(bbb.strongRefAfterDeinit)")

2. Result A

AAA deinit
self.aaa when it deinit: nil
$0 when it deinit: CommonModelArchTest.AAA
now aaa = nil
bbb.aaa = nil
(program crashes here)

2. Result B

AAA deinit
self.aaa when it deinit: nil
$0 when it deinit: CommonModelArchTest.AAA
now aaa = nil
bbb.aaa = nil
bbb.strongRefAfterDeinit = Optional(CommonModelArchTest.AAA)
Program ended with exit code: 0