Why is `index_type` in `std::extents` allowed to be signed and how can I know the lower and upper bound for such signed integer types?

Question

If I understand https://en.cppreference.com/w/cpp/container/mdspan/extents right, index_type can be signed. Else it would not make sense to use std::make_unsigned_t<index_type> for size_type.

This means I cannot expect that 0 is the lower bound of an index value.

This implies that https://en.cppreference.com/w/cpp/container/mdspan/extent gives me the size of the extent, but does not allow me to deduce the highest index value.

So how can I find the lower and upper bound for an extent to be used in a generic function?

I have read https://en.cppreference.com/w/cpp/container/mdspan and looked into the reference implementation at https://github.com/kokkos/mdspan .

However, the documentation seems not to be ready, which is not that surprising for something still in development.

Using std::numeric_limits does not make sense, because for unsigned integers, it would give me a number far beyond usual sizes of extents.