invariant constancy: revisited

[Jakob Ovrum]

Here we go again.

Back in 2012, invariants were misguidedly made const by default 
without going through any review whatsoever, breaking a ton of 
code in the process[1]. That means bitwise constancy is enforced 
for the this-reference in the body of the invariant. There is no 
way to opt out of this.

The well-intentioned argument is that invariants should not have 
side effects, which I wholeheartedly agree with. The problem is 
measuring side effects in terms of bitwise constancy and not 
logical constancy. I was using logically constant invariants at 
the time, but had to abandon them because of this change (again, 
no way to opt out). Also, debug code inside invariants is also 
hampered by bitwise const.

Pull requests were since made to revert the change, while 
allowing invariants to be type-qualified explicitly to enforce 
bitwise-constancy[1][2]. They eventually rot away, and now two 
years later, invariants are still artificially limited by bitwise 
const; as I had stopped using invariants, I never noticed.

I think we're running out of time to fix this, if we haven't 
already. Specifically, the reversion was never commented on by 
Walter, though Andrei did seem supportive. Please approve the 
change. Hopefully this is the time to reclaim invariants ;)

[1] 
https://github.com/D-Programming-Language/dmd/commit/3c138718365541cde67abbdcfc031d755c3a7969
[2] https://github.com/D-Programming-Language/dmd/pull/1073
[3] https://github.com/D-Programming-Language/dmd/pull/1560