[PATCH] Fix libstdc++/6811

[Paolo Carlini]

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?

This both for __normal_iterator and the special _Deque_iterator.

(actual commits only modulo ABI issues, of course)

Ciao, Paolo.