Is this source file generated as part of the build process.
If a source file is generated by the build process CMake will handle it differently in terms of dependency checking etc. Otherwise having a non-existent source file could create problems.
© 2000–2017 Kitware, Inc. and Contributors
Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.9/prop_sf/GENERATED.html