Hello,

I have a question regarding the design of the free functions:subspan, first and last:
As they do not explicitly accept span<T> are they intended to be used with any type that can be accepted by make_span function? If it is the case then the implementation seems to have an issue: as ADL is not finding them with non-span-specialization parameters the call to this functions should be qualified (currently they are not available in the nonstd namespace) and make_span in the trailing return type expression should be visible for non-ADL. Unfortunately the unit-tests cover only the cases where the span specialization is passed to the functions (works because of ADL), so it is not clear if the use-case was considered, although I personally find it to be useful (would be glad to contribute here).

If the functions are designed to be used with span-s only as wrappers for methods to avoid .template then maybe they could accept span<T> ?

Could you please clarify the use cases and rational behind the design?

Thank you.