[Bug libstdc++/31247] std::vector::iterator::value_type is accessible

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31247

--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Marc Glisse from comment #10)
> that's exactly what you'll test, since iterator_traits is not guaranteed to
> be sfinae-friendly.

N.B. that's changed for C++14.

I'm still not motivated to implement this request though.