Timur Doumler via Std-Proposals:
> std::span is not a container.
>
> Cheers,
> Timur

But not having size_type break templates that don't take ownership the
the container and would otherwise be happy with std::span. I think
having size_type vs index_type duality does nothing good.

If we want people to take containers then add Container concept to the
standard library.