target_sources: CMake 3.26.0-rc4 refuses FILE_SET arguments

Hi,

I was trying to play with c++ modules with the new capabilities in CMake 3.26.0-rc4 but I got an error and I am not sure were the issue is. I was following this blog post.

The error:

CMake Error at ****/vcpkg/scripts/buildsystems/vcpkg.cmake:585 (_add_executable):
  Cannot find source file:

    FILE_SET

I opened an issue with vcpkg, but so far they are saying it is an issue with CMake so I am opening an issue here too.

That is what I do:

  add_executable(${targetNameTests})

  target_sources(${targetNameTests}
    PRIVATE
      ${ADD_TEST_TARGET_MAINFILE}
      ${ADD_TEST_TARGET_PRIVATEFILES}
      ${ADD_TEST_TARGET_TESTFILES}
    PUBLIC
      ${ADD_TEST_TARGET_PUBLICFILES}
      FILE_SET ${targetNameTests}_cxx_modules TYPE CXX_MODULES FILES ${ADD_TEST_TARGET_MODULEFILES}
      #FILE_SET ${targetNameTests}_cxx_header_unit TYPE CXX_MODULE_HEADER_UNITS FILES ${ADD_TEST_TARGET_HEADERUNITFILES}
    INTERFACE
      ${ADD_TEST_TARGET_INTERFACEFILES}
  )

Basically, the issue seems that executables refuse target_sources with FILESET of type CXX_MODULES ( I guess they would refuse CXX_MODULE_HEADER_UNITS also), I am pretty that is a bug with either CMake or vcpkg since if we are not allowed FILESET in executable that mean executable can't have c++ modules.

Thank you for your time!