Simplify _Rb_tree instantiation

[Ville Voutilainen]

On 25 May 2018 at 19:50, François Dumont <frs.dumont@gmail.com> wrote:
> Hi
>
> As we are at working on associative containers I'd like to propose this last
> patch to remove the copy constructible constraint on the _Compare functor
> when it is supposed to be default constructed.
>
> This way the _Compare is built directly at its final place.

Why is this patch removing _Compare() calls? That changes the initialization
of _Compare from value-initialization to default-initialization, which
is a breaking change.