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4299. Missing Mandates: part in optional<T&>::transform

Section: 22.5.4.7 [optional.ref.monadic] Status: Tentatively Ready Submitter: Giuseppe D'Angelo Opened: 2025-07-15 Last modified: 2025-10-16

Priority: Not Prioritized

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Discussion:

In 22.5.4.7 [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 22.5.3.8 [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.

[2025-10-16; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

As optional<remove_cv_t<invoke_result_t<F, T&>>> is part of the signature (return type), we never enter the body to trigger the Mandates, so it's already implicitly ill-formed if the result of f is not a valid contained type. It's worth clarifying that though."

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.7 [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.