cast problem about alias this

[Sobaya via Digitalmars-d-learn]

class Parent {
    int x;
    alias x this;
}

class Child : Parent {
}

void main() {
    Parent p = new Child;
    Child c = cast(Child)p; // cannot cast `int` to `Child`
}

In this code, I got a compile error.
How can I cast p to Child?