Ensuring template argument is descendant of class

[Ross Hays]

> Yeah, is-expression syntax is one of the dark, ugly corners of 
> D that
> unfortunately we're stuck with, because changing it now will 
> totally
> break a LOT of code for merely cosmetic reasons. I honestly 
> wish that
> one day this mess could be cleared up, though I'm not holding 
> my breath
> for it.
>
>
> T

I know this is probably obvious, but if D were allowed to just 
make one big breaking change for D3 or something, and fix every 
dark corner it has. Would that really be so bad? I know it would 
break things but... I kind of long for it.