UDA and trait for non-GC managed pointers

[Nordlöw via Digitalmars-d]

On Saturday, 15 October 2016 at 17:48:13 UTC, Basile B. wrote:
> I agree. We are several people to do it already. I've started 
> to during last spring (see 
> https://forum.dlang.org/post/ficbsdfokvbvslatmomr@forum.dlang.org).
> So far I use "@NoGc" and "@TellGcRange" added.

Why does the template argument to `MustAddGcRange` default to 
`void`?

And why can't `MustAddGcRange` just be called any type instead of 
just `struct`, `union` or `class`?