Infinite loop from didSet in property

protocol AProtocol {
    var someString: String { get }
}


class A: AProtocol {
    var someString: String
    init() {
        someString = "A"
    }
}


class B {
    var myProperty: AProtocol! {
        didSet {
            self.doSomething()
        }
    }


    func doSomething() {
        NSLog(self.myProperty.someString)
    }
}


let myB = B()
myB.myProperty = A() // infinite recursion




protocol AProtocol: class {
    var someString: String { get }
}


class A: AProtocol {
    var someString: String
    init() {
        someString = "A"
    }
}


class B {
    var myProperty: AProtocol! {
        didSet {
            self.doSomething()
        }
    }


    func doSomething() {
        NSLog(self.myProperty.someString)
    }
}


let myB = B()
myB.myProperty = A() // now it works

So basically if I don't impose the restriction AProtocol: class I will get an infinite loop. Basically when in doSomething I try access the just set property it will trigger the didSet block again (B.(myProperty.materializeForSet).(closure #1))

Can someone explain me why is this or confirm this is a bug in Swift?

Cheers