safe unordered local iterators

[Paolo Carlini]

Hi,
> Yes, if it's possible for users to invalidate and misuse those 
> iterators then it's a good idea to have debug mode checks. 
Indeed. But my rough feeling would be that such misuses are much less 
frequent and relevant than the other, with normal iterators. Francois, 
can you provide a breakdown about the cost of tracking those local 
iterators too? In terms of memory use, maybe performance too? We could 
even imagine tracking those only in pedantic-mode or when an additional 
macro is set, in case.

By the way, talking about fixing the ABI, etc, we have definitely at 
least a serious issue to resolve in the normal-mode unordered 
containers, that is LWG 579 (aka libstdc++/41975), it's in my to do list 
for 4.7.0, I'm not sure whether something will change in the details of 
the local iterators which would impact the debug-mode version too..

Paolo.