superclass methods to return sublass class in swift

I have this code:

class Dark: NSObject {
  class var className: String {
    return NSStringFromClass(self)!.componentsSeparatedByString(".").last!
  }

  var success = 0

  class func devour<T: Dark>(params: Int) -> T {
    var s = T()
    return assign(s, params: params)
  }

  class func assign<T: Dark>(item: T, params: Int) -> T{
    if item.dynamicType === self {
      item.success = params
    }
    return item
  }
}

class Black: Dark {
}

class Violet: Dark {
}

Black.className // returns "Black"
Violet.className // returns "Violet"

The problem occurs when I do this:

var ret = Black.devour(5)
ret.success //returns 0 which is suppose to be 5

ret.self // returns {Dark success 0} which is suppose to be {Black success 0}

I want to be able to return the subclass type and not the superclass type when calling devour. I hope someone can help me on this. :) Im really out of ideas.