Let's not make invariants const

[bearophile]

Alex Rønne Petersen:

> So what if, for whatever reason, the invariant needs to track 
> and maintain some state in order to catch some kind of ugly bug?

This is legitimate practical need. I know two or more ways to 
solve that problem, one of them is to implement the prestate 
(old). That's probably the largest hole in the D implementation 
of contract programming, and I have felt its need several times.

Another solution (that doesn't replace the need for the 
prestate!) is ghost fields, that are class instance/struct fields 
usable only inside contracts and that vanish in release mode:
http://d.puremagic.com/issues/show_bug.cgi?id=5027

Bye,
bearophile