[PATCH] Fix libstdc++/6811

[Paolo Carlini]

Gabriel Dos Reis wrote:

>I gave it more thought: maybe one could try instantiating it and
>pass a flag to GCC to warn about missing return statement.  I don't
>know how far we can get with that.
>
Anyway, I find this very interesting! It seems that many stupid but 
*very* serious bugs don't get caught 'til you have a testing framework 
which thorughly instantiates the various classes...

>| 
>| Ok for both? (tested x86-linux, as usual)
>
>Yes.
>
Thanks. Done.

Could you possibly help a bit with libstdc++/6642?
Currently we cannot compile the trivial:

#include <string>

class MyClass {
private:
  std::string s;
  std::string::iterator it;
public:
  unsigned pos() const {
    return it - s.begin();
  }
};

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.

Ciao, Paolo.