If you want to support Visual Studio's OpenMP which is compiler flag"/openmp" , it has been at version OpenMP 2.0 for around a decade, and complains about size_t for the index and the error says it needs to be a signed int.

So the above size_t index works when it changed to a signed int for VS compilers.

I didn't want to make a pull since I'm not quite sure what the best way to add "/openmp" to how you have your CMakeLists.txt arranged for Visual Studio.