Small annoyance (I haven't read the entire thing, but I have strong feelings about this): gsl::index is disgustingly verbose. If you use stl, just use size_t. If you're using something else, follow the conventions they use (be it size_t, int, or whatever). Their examples [1] conveniently omit size_t to push their alternative.

[1] https://github.com/isocpp/CppCoreGuidelines/blob/master/CppC...