I tried to follow the "Getting Started with C++ Interop" guide, but I can't get past "error: public headers ("include") directory path for 'CxxTest' is invalid or not contained in the target".
AFAIU an implicit include subdirectory is always required when building targets that contain module.modulemap with SwiftPM. We do need the "Getting Started" document for C++ to be updated to reflect that.
Unfortunately I could only compile it as a Swift Package from the command line. When I open that Swift Package in Xcode (via xed .), for some reason, Xcode seems to complain with
error: Driver threw unknown argument: '-cxx-interoperability-mode=swift-5.9' without emitting errors. (in target 'Swift-CANyonero' from project 'Swift-CANyonero')
although I downloaded and activated the latest Xcode toolchain from the 5.9/development branch.