[Patch] Fix variant::operator= on references

[Jonathan Wakely]

On 22/09/16 00:43 -0700, Tim Shen wrote:
>Hi, this patch fixes the following compilation failure:
>
>#include <variant>
>
>int main()
>{
>   float f1 = 1.0f, f2 = 2.0f;
>
>   std::variant<float&> v1(f1);
>
>   v1 = f2; // #1
>}
>
>The bug is caused by a misuse of __storage. I also examined other
>__storage usage, they all seem appropriate.

@@ -1147,8 +1147,7 @@ namespace __variant
        {
          constexpr auto __index = __accepted_index<_Tp&&>;
          if (index() == __index)
-           *static_cast<__storage<__to_type<__index>>*>(this->_M_storage())
-             = forward<_Tp>(__rhs);
+           std::get<__index>(*this) = forward<_Tp>(__rhs);

Please qualify std::forward here.

OK for trunk with that change, thanks for the quick fix.