For 3.19.0 If projects mark .mm with the LANGUAGE of CXX they will fail if you actually meant objective-c++. The previous behavior will be restored in 3.19.1. Looking at the OBS project I see no files being marked explicitly as CXX.
Again CMake 3.20.0 will introduce a policy where the CMake 3.18 and older behavior will become deprecated and you will need to mark objective-c files as OBJC and not C.