no size yet for forward reference for nested structures

[qqiang via Digitalmars-d]

The following code:
```D
template PowerHeap(T) {
    import std.container : SList;
    private alias PowerForest = SList!PowerNode;

    private final class PowerNode {
        ...
    }

    final class PowerHead {
        ...
    }
}

unittest {
    PowerHeap!int h;
}
```

failed to compile and get the error: PowerHeap!int.PowerNode: no 
size yet for forward reference.

I've googled and found no straightforward solution to this issue. 
The how can I modify my code to eliminate this error?