> A more interesting question would be how well the C++ interoperability works

They’re using it in their work on FoundationDB. Looks good, but has limitations. Swift can call C++ and vice versa, Swift classes can inherit from C++ and vice versa, but not for all code, and may need work adding annotations on the C++ side. See https://github.com/apple/foundationdb/blob/main/SWIFT_GUIDE.....

There’s a good video on that work from a few years ago that was discussed on HN in https://news.ycombinator.com/item?id=38444876.