set(oldname "${CMAKE_CURRENT_BINARY_DIR}/input")
set(newname "${CMAKE_CURRENT_BINARY_DIR}/output")
file(WRITE "${oldname}" "a")
file(WRITE "${newname}" "b")
file(COPY_FILE "${oldname}" "${newname}")
file(READ "${newname}" new)
if(NOT "${new}" STREQUAL "a")
  message(FATAL_ERROR "New name:\n  ${newname}\ndoes not contain expected content 'a'.")
endif()