How to check if object is an instance of generic class?

[ag0aep6g via Digitalmars-d-learn]

On 05/03/2017 08:04 PM, H. S. Teoh via Digitalmars-d-learn wrote:
> You only need a common interface if you wish to do something more with
> Box!X instantiations that's common across all Boxes.

The goal is to return `true` for two empty boxes with different payload 
types. From the OP: "Empty boxes are equal no matter the type."