Generator expression in ExternalProject_Add() / CMAKE_ARGS

Code
1

Hello,

When using the ExternalProject_Add(), I try to use a generator expression with the CMAKE_ARGS argument (the … contains actually some variables):

ExternalProject_Add(${APP_NAME}_${module}_${require_module_PLATFORM}
      SOURCE_DIR ${ROOT_DIR}/source/modules/${module}
      BINARY_DIR ${CMAKE_BINARY_DIR}/${module}_${require_module_PLATFORM}
      DOWNLOAD_COMMAND ""
      ${generator_params}
      CMAKE_ARGS
          -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
          -DROOT_DIR=${ROOT_DIR}
          -DPLATFORM=${PLATFORM}
          -DMODE=${MODE}
          $<$<BOOL:${OUTPUT_DIR}>:-DOUTPUT_DIR=${OUTPUT_DIR}>
          ${dependency_cmake_args}
          ${options_args}
      DEPENDS ${dependency_targets}
  )

The problem is with the generator expression. When OUTPUT_DIR doesn’t exist, I get the following warning :
Ignoring empty string ("") provided on the command line.

2

Does OUTPUT_DIR contain genexes? If not, you don’t need to use a genex either:

if(OUTPUT_DIR)
  set(OUTPUT_DIR_ARG -DOUTPUT_DIR=${OUTPUT_DIR})
else()
  set(OUTPUT_DIR_ARG "")
endif()
ExternalProject_Add(${APP_NAME}_${module}_${require_module_PLATFORM}
      SOURCE_DIR ${ROOT_DIR}/source/modules/${module}
      BINARY_DIR ${CMAKE_BINARY_DIR}/${module}_${require_module_PLATFORM}
      DOWNLOAD_COMMAND ""
      ${generator_params}
      CMAKE_ARGS
          -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
          -DROOT_DIR=${ROOT_DIR}
          -DPLATFORM=${PLATFORM}
          -DMODE=${MODE}
          ${OUTPUT_DIR_ARG} # <--- the only change from the original
          ${dependency_cmake_args}
          ${options_args}
      DEPENDS ${dependency_targets}
  )
3

No, OUTPUT_DIR is not a generator expression.
I have finally done a variant of what you suggest:

if(OUTPUT_DIR)
  set(options_args ${options_args} -DOUTPUT_DIR=${OUTPUT_DIR})
endif()

ExternalProject_Add(${APP_NAME}_${module}_${require_module_PLATFORM}
      SOURCE_DIR ${ROOT_DIR}/source/modules/${module}
      BINARY_DIR ${CMAKE_BINARY_DIR}/${module}_${require_module_PLATFORM}
      DOWNLOAD_COMMAND ""
      ${generator_params}
      CMAKE_ARGS
          -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
          -DROOT_DIR=${ROOT_DIR}
          -DPLATFORM=${PLATFORM}
          -DMODE=${MODE}
          ${dependency_cmake_args}
          ${options_args}
      DEPENDS ${dependency_targets}
  )