confused with <T>

Question

watched tutorial from sebastian lague for pathfiending.and code example

public class Heap<T> where T : HeapIndex<T>
{

}
public interface HeapIndex<T> : IComparable<T>
{
    int IndexHeap
    {
        get;
        set;
    }
}
public class Node : HeapIndex<Node> 
{
    public int IndexHeap { get; set; }

    public int CompareTo([AllowNull] Node other)
    {
        return IndexHeap.CompareTo(other.IndexHeap);
    }
}

and Heap<Node> heapNode = new Heap<Node>(); all its work. i start read for and i confused. why i cant do with this way

public class NodeInt : HeapIndex<int>
{
    public int IndexHeap { get; set; }

    public int CompareTo([AllowNull] int other)
    {
        return IndexHeap.CompareTo(other);
    }
}
        NodeInt nd = new NodeInt();
        HeapIndex<NodeInt> hpD = nd as HeapIndex<NodeInt>;
        Heap<NodeInt> heapInt = hpD as Heap<NodeInt>;

bacause its not work Heap<NodeInt> heapNodeInt = new Heap<NodeInt>(); I probably just do not fully understand, and terribly blurred my eyes. but I thought that his code works, any instance that meets the requirement where T is an interface implementation, then you can use it to create a heap why i cant just Heap<NodeInt> heapNodeInt = new Heap<NodeInt>();

Answer 1

Heap<T> where T: HeapIndex<T> means that if you want Heap<NodeInt> , then the NodeInt should inherit HeapIndex<NodeInt> just because T = NodeInt in the Heap declaration. But your NodeInt is HeapIndex<int> , so it does not meet requirement and this code won't compile.