How do you return a subclass instance from a base class method?

[Steven Schveighoffer]

On 11/16/22 11:25 PM, Daniel Donnelly wrote:
> I have SubclassOf derived from PosetRelation.  For any poset relation, 
> the transitivity law applies, however, I'd like to return the correct type:
> 
> ```
>     PosetRelation transitivity(PosetRelation R, PosetRelation S)
>     {
>        if (R.op == S.op)
>        {
>           if (R.right is S.left)
>              return new SubclassOf(R.left, S.right);
>        }
> 
>        return null;
>     }
> 
> ```
> 
> How does one accomplish this in D?  Because PosetRelation doesn't know 
> about SubclassOf, in general.

I'd use a template:

```d
T transitivity(T : PosetRelation)(T R, T S)
{
       if (R.op == S.op)
       {
          if (R.right is S.left)
             return new T(R.left, S.right);
       }

       return null;
}
```

-Steve