No way to check whether a scheduler is a stdexec scheduler

[t3nsor]

P2300R10 has derived_from<typename remove_cvref_t<Sch>::scheduler_concept, scheduler_t> as part of the definition of the concept scheduler, but stdexec doesn't have this yet.

This is inconvenient because if I want to check whether some type S is a scheduler that's compatible with stdexec, by evaluating the concept-id stdexec::scheduler<S>, I will always get a hard error if S happens to be a scheduler from some other execution framework: stdexec checks whether it has a schedule member function and then gives a hard error if the result of calling schedule is not a stdexec sender.

[ericniebler]

this is a very good point. i should loosen the requirement.