std::is_copy_constructible::value is always true for variant<Ts...>

Nobody has claimed this yet.

The copy constructor only actually compiles when all member types are copyable, as it should. However, when checking whether the variant type is copy-constructible via SFINAE, e.g. with `std::is_copy_constructible`, you will always get `true`, because the copy constructor always exists, whether or not it is valid. The same applies to move constructors, and copy / move assignment operators.

I first thought I could fix this with a simple `enable_if` on each of the methods, but as it turns out I can't because if you make templates out of any of these special methods, they are not those special methods anymore. I did find a [solution] on StackOverflow, though I'm not entirely certain if it might introduce unwanted side effects. Could you have a look? In case it seems unproblematic to you, I would be happy to try implementing it here.

[solution]: https://stackoverflow.com/a/27073263/1592377

Start by reviewing the variant implementation's special member constructors and the linked Stack Overflow solution; the issue names no source file or test. Determine whether copy and move type traits correctly reflect non-copyable alternatives without unwanted side effects, then verify the resulting behavior with focused coverage.

Written by the indexing model from the issue text.