I want to generate files in the per-configuration build directory:

add_executable(test_generator_gen test_generator.cpp)
target_compile_definitions(test_generator_gen PRIVATE GENERATE)
target_link_libraries(test_generator_gen YOMM2::yomm2)

add_custom_command(
    OUTPUT ${GENERATED_FILES}
    DEPENDS test_generator_gen
    COMMAND test_generator_gen
    WORKING_DIRECTORY $<TARGET_FILE_DIR:test_generator_gen>
)

The files are generated where I want (e.g. ./builds/headers-only/tests/Debug/test_generator_slots.hpp. My problem is, what do I put in ${GENERATED_FILES}? I tried what seemed natural to me:

add_custom_command(
    OUTPUT $<TARGET_FILE_DIR:test_generator_gen>/test_generator_slots.hpp
    DEPENDS test_generator_gen
    COMMAND test_generator_gen
    WORKING_DIRECTORY $<TARGET_FILE_DIR:test_generator_gen>
)
# ...

…but I get an error:

CMake Error at tests/CMakeLists.txt:95 (add_custom_command):
[cmake]   Error evaluating generator expression:
[cmake] 
[cmake]     $<TARGET_FILE_DIR:test_generator_gen>
[cmake] 
[cmake]   No target "test_generator_gen"

I am puzzled. Why is the same generator expression in the same command accepted for some arguments and not others? And how do I specify the OUTPUT correctly?

cmake 3.22.1