GitLab

cmake version 3.15.4 on Ubuntu 18.04 somepath/foo/CMakeLists.txt:

include(FetchContent)
FetchContent_Declare(foo
  URL https://foo.com/foo.tar.bz2
)
FetchContent_Populate(foo
  SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src
)

My goal is to still let FetchContent_Populate fetch the source but in a non-default location specified in SOURCE_DIR. FetchContent_Populate however assumes that I want to specify an already populated source directory and fails with the following error message:

Make Error at /usr/share/cmake-3.15/Modules/ExternalProject.cmake:2611 (message):
  No download info given for 'foo-populate' and its source directory:

   somepath/foo/src

  is not an existing non-empty directory.  Please specify one of:

   * SOURCE_DIR with an existing non-empty directory
   * DOWNLOAD_COMMAND
   * URL
   * GIT_REPOSITORY
   * SVN_REPOSITORY
   * HG_REPOSITORY
   * CVS_REPOSITORY and CVS_MODULE
Call Stack (most recent call first):
  /usr/share/cmake-3.15/Modules/ExternalProject.cmake:3204 (_ep_add_download_command)
  CMakeLists.txt:13 (ExternalProject_Add)

From the documentation, my understand is that a pre-populated source directory is specified by defining FETCHCONTENT_SOURCE_DIR_ucName; specifying SOURCE_DIR in FetchContent_Populate should only override the default source location.

If my understanding is incorrect, what is the recommended way to accomplish my goal?