[optional.monadic]/8 currently states:

Mandates: U is a non-array object type other than in_place_t or nullopt_t. The declaration
U u(invoke(std::forward<F>(f), value()));
is well-formed for some invented variable u.

In this declartion, invoke(std::forward<F>(f), value()) is a prvalue of cv U since U is remove_cv_t<invoke_result_t<F>>, so the declaration would be well-formed as long as U is well-formed.

Should we just say U is well-formed instead, or don't explicitly say any additional requirement given it is implicitly required that U is well-formed?

expected::transform and expected::transform_error have the same issue.