C# type parameter constraint doesn't allow casting to constraint

So I generalized my code to the below example and I get the following compile error. My question is 'Why is assigning the reference back to 'container' not allowed?'. My assumption was that by restricting the type T to inherit from Element that 'this' should be easy to cast to Container<Element>. The point of this code is to keep a reference to the container in each element object in the case that the object (which gets passed around a lot) needs to have a reference to where it is contained.

Compilation error (line 16, col 23): Cannot implicitly convert type 'Container<T>' to 'Container<Element>'

If I add a type cast to the line containing 'this' I get the below instead.

Compilation error (line 16, col 23): Cannot convert type 'Container<T>' to 'Container<Element>'

using System;

public class Element
{
    public int datum;
    public Container<Element> container;
}

public class Container<T> where T : Element {

    T[] data;

    public Container() {
        data = new T[1];
        data[0].datum = 4;
        data[0].container = this;
    }

}