With the release of macOS 10.15 (Catalina), Apple has dropped support for running 32-bit executables and removed the 32-bit versions of system frameworks and libraries. Most Windows applications our users run with CrossOver are 32-bit and CrossOver uses a 32-bit Mac executable, system frameworks, and libraries to run them. This will break with Catalina.

And then comes the fun part:

We have built a modified version of the standard C language compiler for macOS, Clang, to automate many of the changes we need to make to Wine’s behavior without pervasive changes to Wine’s source code.

First, our version of Clang understands both 32- and 64-bit pointers. We are able to control from a broad level down to a detailed level which pointers in Wine’s source code need to be 32-bit and which 64-bit. Any code which substitutes for Windows at the interface with the Windows app has to use 32-bit pointers. On the other hand, the interfaces to the system libraries are always 64-bit.

source: grugq