[Patch] Fix variant::operator= on references

[Tim Shen]

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.

-- 
Regards,
Tim Shen