CMake/Tests/RunCMake/CXXModules/examples/scan_properties/main.cxx

#if SCAN_AT_TARGET_LEVEL
#  ifndef CMAKE_SCANNED_THIS_SOURCE
#    error "This file should have been scanned"
#  endif
#else
#  ifdef CMAKE_SCANNED_THIS_SOURCE
#    error "This file should not have been scanned"
#  endif
#endif

int join();

int main(int argc, char** argv)
{
  return join();
}