Return object with an specific metatype in general metatype function

I have some issues with inheritance and generics classes. I want to return an object of a subclass (with an specific generic type) in a function that return an object of the parent class with a generic metatype.

More or less, I have this structure of classes and functions:

class Element {}
class Item:Element {}

class A<aType:Element> { }
class B:A<Item> { }

class SomeClass {
    func foo() -> A<Element> {
        return A<Element>()
    }
}
class OtheClass:SomeClass {
    override func foo() -> A<Element> {
        return B() // An error where!
    }
}

The thing that i can't do is return B() in OtherClass's foo function. The error is:

Cannot convert return expression of type 'B' to return type 'A< Element >'

Is there any way to do it?

The solution that I have found is:

class CoreElement {}
class CoreItem:CoreElement {}

class CoreA<aType:CoreElement> { }
class CoreB:CoreA<CoreItem> { }

class CoreSomeClass<T:CoreElement> {
    func foo() -> CoreA<T> {
        return CoreA<T>()
    }
}
class CoreOtherClass:CoreSomeClass<CoreItem> {
    override func foo() -> CoreA<CoreItem> {
        return CoreB()
    }
}