More templated type argument inference

[bearophile]

Philippe Sigaud:

> Doesn't that mean you're asking the compiler to have a full 
> unification
> engine? Your example is simple, but the general case is more 
> complicated:

I think the general case doesn't require a whole program type 
inference, just a local one. But maybe it's too much complex to 
do it in D :-) I don't know.

Bye,
bearophile