file generated by add_custom_command cannot be found within add_subdirectory

Question asked at: https://stackoverflow.com/questions/75546798/file-generated-by-add-custom-command-cannot-be-found-within-add-subdirectory

I'm in the process of generating an api using flatbuffers within cmake. Now when on top level of the cmake directory tree the api is generated, the include files that should be generated are not found when they are needed in the branches.

./CMakeLists.txt

set(INTERFACE_FBS ${CMAKE_CURRENT_SOURCE_DIR}/fbs/testapi.fbs)

flatbuffers_generate_headers(TARGET incl
                             SCHEMAS ${INTERFACE_FBS}
                             FLAGS --scoped-enums --cpp-std c++17)
target_link_libraries(incl INTERFACE flatbuffers::flatbuffers)
set_directory_properties(PROPERTIES ADDITIONAL_CLEAN_FILES incl)

#add_executable(testapp app/src/test.cpp)
#target_link_libraries(testapp incl)

add_subdirectory(app)

./app/CMakeLists.txt

add_executable(app src/test.cpp)
target_link_libraries(app incl)

Running the system like this results in the following error:

CMake Error at app/CMakeLists.txt:2 (add_executable):
  Cannot find source file:

    /home/xxx/build.test/incl/testapi_generated.h

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx

The testapi_generated.h can't be found as it's not generated. But it should be generated as the function flatbuffers_generate_headers sets an add_custom_command for the wanted .h file.

When I move the building of the executable to the main cmake file, so replacing the bottom of: ./CMakeLists.txt

add_executable(testapp app/src/test.cpp)
target_link_libraries(testapp incl)

#add_subdirectory(app)

the .h file is generated and the simple application builds without problems. Also enabling the subdirectory at that point will result in a working executable. Performing a make clean and the building will fail again.

What am I doing wrong? Is it not possible to first use files created by an add_custom_command in a subdirectory?