[PATCH] Fix libstdc++/6811

[Gabriel Dos Reis]

Paolo Carlini <pcarlini@unitus.it> writes:

| Gabriel Dos Reis wrote:
| 
| >| which mixes a const_iterator and an iterator. The issue is becoming 
| >| clearer to me (read DR179 and DR280) but I'm not sure which is the best fix.
| >
| >Well, since DR179 has a Reday status, I think we should implement the
| >resolution.  However, I propose we be cautious about DR280.
| >
| Thanks for your feedback.
| 
| Therefore, if I understand well, #ifdef _GLIBCPP_RESOLVE_LIB_DEFECTS we 
| should make operator- also non-member (as already happens for ==, !=, 
| <=, >=, <, >), right?

Yes, for X::iterator and X::const_iterator only.  

Thanks,

-- Gaby