[Patch] Fix variant::operator= on references

[Jonathan Wakely]

On 22/09/16 03:40 -0700, Tim Shen wrote:
>On Thu, Sep 22, 2016 at 3:36 AM, Tim Shen <timshen@google.com> wrote:
>> Then my question is, what about type traits uses like
>> is_copy_constructible? I have seen non-qualified uses in std::any and
>> std::optional and other places. Should all of them be qualified?
>
>Ah never mind, I realized that *usually* a type trait use is not part
>of a function call, so ADL is not triggered.

ADL is only used to do name lookup for unqualified functions, so it is
never necessary to qualify those types inside namespace std. Name
lookup will always find the right type.