[comex]

That's nice, but the blog post doesn't say anything about migrating libraries across versions? Looking at the comment thread, I see tialaramex's sibling comment suggested the blog post was about migration, but it's not.

I suppose migration is another possible use case for the union trick, and for that case C++ inline namespaces can be used as part of an implementation that achieves a broadly similar goal, but in a completely different way. As tialaramex notes, with inline namespaces you still end up with two different types.