I am doing a little work on my constexpr bitpacking library and wanted to upgrade nonstd::span from 0.7.0 to 0.10.0. In doing so I noticed that operator[] is no longer constexpr if contracts are on. If I simply mark impl::is_positive as constexpr then all my tests pass. Unlike #70 I don't want to disable bounds checking, I like to leave it on in debug builds to enforce the contracts.

Seems like a trivial change, but i can open a MR if you would like. I have the change in my fork and all your tests passed.