JSONValueConverter requires vector<std::unique<T>> whenever you're setting up a repeated converter. The possible reason for this is to avoid an extra copy/move operation when checking if the conversion should go ahead.

It would be good to consider if the possibility of having an extra move. This would be really beneficial for cases in which it is required std::vector<std::unique<int>>.

We can also consider a) checking the value or b) having a is convertible that doesn't involve really converting the value, but rather wrapping the possible access in a adapter.