Currently, the `submdspan` function can call a `submdspan_mapping` customization with any valid slice type. This means that `submdspan_mapping` customizations may be ill-formed, possibly even without a diagnosic, if future C++ versions expand the set of valid slice types. This will happen if [P2769R3](https://wg21.link/p2769r3) is merged into the Working Draft for C++29, as that would generalize *`tuple-like`* from a concrete list of types to a concept. It may also happen in the future for other categories of slice types.