I does not work fine in C++ when N and M are not compile-time constants, which is basically always the case in any interesting numerical algorithm. Also not in Rust.
replyIt works fine in C though, or FORTRAN, or Ada, or ALGOL 60, ...
Which is why std::mdspan exists, and std::linalg.
replyNVidia has pivoted to design CUDA hardware with focus on C++ back in , and seems to be doing quite well for them.
CppCon 2017: "Designing (New) C++ Hardware”
https://www.youtube.com/watch?v=86seb-iZCnI
They were also the ones sponsoring the ISO work on mdspan, while HPC research labs are pushing for linalg on top.
I would rather be using Ada today, but that isn't how the world moves.