cmake 3.18: generator expressions with add_subdirectory fail
*Generator expressions in custom command outputs are not implemented*
Win10, Cmake3.18 a cmakeLists.txt contains generator expressions, and is configured and generated successfully, if tried alone.
If I try to add_subdirectory(source_path/../parallel_direcory build_dir) where the build_dir contains generator expressions, I get the above error message.
What is the problem exactly and how can be solved?