Relax std::move_if_noexcept for std::pair

[Ville Voutilainen]

On Thu, 20 Dec 2018 at 08:29, François Dumont <frs.dumont@gmail.com> wrote:
>
> Hi
>
>      I eventually find out what was the problem with the
> std::move_if_noexcept within associative containers.
>
>      The std::pair move default constructor might not move both first
> and second member. If any is not moveable it will just copy it. And then

..as it should..

> the noexcept qualification of the copy constructor will participate in
> the noexcept qualification of the std::pair move constructor. So
> std::move_if_noexcept can eventually decide to not use move because a
> _copy_ constructor not noexcept qualified.

..and again, as it should.

>      This is why I am partially specializing __move_if_noexcept_cond. As
> there doesn't seem to exist any Standard meta function to find out if
> move will take place I resort using std::is_const as in this case for
> sure the compiler won't call the move constructor.

That seems wrong; just because a type is or is not const has nothing
to do whether
it's nothrow_move_constructible.

I don't understand what problem this is solving, and how it's not
introducing new problems.