Corruption when assigning to stored properties in classes using generics

generic-ivar.swift

import Cocoa

class A<T> : NSViewController {
	var x:T? = nil
    override init?(nibName nibNameOrNil: String?, bundle nibBundleOrNil: NSBundle?) {
        super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    }
}
class B : A<NSString> {
    override init?(nibName nibNameOrNil: String?, bundle nibBundleOrNil: NSBundle?) {
        super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    }
}

let b = B(nibName:"bbb", bundle:NSBundle.mainBundle())!
print("b.nibName = \(b.nibName)")
b.x = "foo" as NSString
print("b.nibName = \(b.nibName)")

Updated to show more clearly that the write to the generic property overwrites an ivar on the superclass.