Why D functions paramter can not implicit infer type of Variant?

[Paul Backus]

On Wednesday, 13 January 2021 at 18:09:08 UTC, sighoya wrote:
> A more natural conclusion would be to infer c to the most 
> common supertype as other inferences would unnecessarily 
> exclude future assignments to c.  But the most common supertype 
> doesn't seem to exist, and I'm unsure if this type can be 
> modeled at all in D?

That's what Variant is--a struct that models the universal 
supertype (sometimes called "Top" or "Any").