'CMake generator expression evaluated to "" instead of nothing

I'm using generator expressions in a custom command to compile hlsl shaders:

add_custom_command(TARGET Shaders
                           COMMAND vendor/shader-compiling/fxc.exe /nologo /Emain /Tvs_5_0 $<$<CONFIG:Debug>:/Od> /Zi /Fo ${TARGET_SHADER_PATH}/hlsl/${FILE_WE}_vs.cso /Fd ${CMAKE_BINARY_DIR}/${FILE_WE}_vs.pdb ${TARGET_SHADER_PATH}/hlsl/${FILE_WE}_vs.hlsl
                           COMMAND vendor/shader-compiling/fxc.exe /nologo /Emain /Tps_5_0 $<$<CONFIG:Debug>:/Od> /Zi /Fo ${TARGET_SHADER_PATH}/hlsl/${FILE_WE}_fs.cso /Fd ${CMAKE_BINARY_DIR}/${FILE_WE}_fs.pdb ${TARGET_SHADER_PATH}/hlsl/${FILE_WE}_fs.hlsl
                           MAIN_DEPENDENCY ${FILE}
                           WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                           COMMENT "Compiling HLSL shader ${TARGET_SHADER_PATH}/hlsl/${FILE_WE}.hlsl"
                           VERBATIM)

and in debug mode everything works fine. However, in release mode, my expected result is the /Od flag does not get included in the command at all. However, what ended up happening is $<$<CONFIG:Debug>:/Od> gets evaluated to two double quotes and the command doesn't work.

Any ideas on why this is happening?

cmake


Solution 1:[1]

Just in case anybody finds this useful, I solved the problem by adding COMMAND_EXPAND_LISTS to my add_custom_command command. Not sure why that works but it does get rid of the empty pair of quotes in the middle of the command therefore the command works.

Sources

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Source: Stack Overflow

Solution Source
Solution 1 SunnyMonster

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