g++ -c -std=c++20 -fmodules-ts my_math.cxx
creates \gcm.cache\my_math.gcm and my_math.o
g++ -std=c++20 -fmodules-ts client.cpp my_math.o -o App
./App
cmake . -B build
cmake --build build/
./build/Application
cl.exe (Microsoft) uses the required extension ixx. The ixx stands for a module interface source.
In GCC, these files would need to be build using the line:
g++ -c -std=c++20 -fmodules-ts -x c++ my_math.ixx
-x c++ is required if it is a non-standard file extension (e.g. .ixx)