[PATCH 3/3] Fix condition for std::variant to be copy constructible

[Jonathan Wakely]

On 17/04/19 17:12 +0100, Jonathan Wakely wrote:
>The standard says the std::variant copy constructor is defined as
>deleted unless all alternative types are copy constructible, but we were
>making it also depend on move constructible.

It turns out that was changed by https://wg21.link/lwg2904 and we're
missing the rest of that resolution too. Patch coming soon ...