Proposed resolution:

This wording is relative to N5014.

1. Modify [optional.ref.monadic] as indicated:

   template<class F>
     constexpr optional<remove_cv_t<invoke_result_t<F, T&>>> transform(F&& f) const;
   

   -4- Let `U` be remove_cv_t<invoke_result_t<F, T&>>.

   -5- Mandates: `U` is a valid contained type for `optional`. The declaration

   U u(invoke(std::forward<F>(f), *val ));
   

   is well-formed for some invented variable `u`.

In [optional.ref.monadic] the specification of optional<T&>::transform is missing an additional part of the Mandates: element compared to the primary template's `transform` (in [optional.monadic] p8); that is, is missing to enforce that the `U` type is a valid contained type for `optional`.

The definition of "valid contained type" comes from P2988R12. The paper amended the Mandates: element of the primary template's `transform` to use this definition. The fact that the same wording has not been applied to optional<T&>::transform as well looks like an oversight. I would suggest to apply it.