[v3 PATCH] Don't revisit a variant we are already visiting.

[Ville Voutilainen]

On Thu, 28 Mar 2019 at 15:07, Ville Voutilainen
<ville.voutilainen@gmail.com> wrote:
>
> This shaves off some unnecessary codegen. In the assignment
> operators and swap, we are already visiting the rhs, so we shouldn't
> visit it again to get to its guts, we already have them in-hand.
>
> 2019-03-28  Ville Voutilainen  <ville.voutilainen@gmail.com>
>
>     Don't revisit a variant we are already visiting.
>     * include/std/variant (__variant_construct_single): New.
>     (__variant_construct): Use it.
>     (_M_destructive_move): Likewise, turn into a template.
>     (_M_destructive_copy): Likewise.
>     (_Copy_assign_base::operator=): Adjust.
>     (_Move_assign_base::operator=): Likewise.
>     (swap): Likewise.

Minor adjustment, use direct-init in all cases:

-+                  auto __tmp = std::move(__rhs_mem);
++                  auto __tmp(std::move(__rhs_mem));
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