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4304. std::optional<NonReturnable&> is ill-formed due to value_or

Section: 22.5.4.6 [optional.ref.observe] Status: New Submitter: Jiang An Opened: 2025-07-25 Last modified: 2025-08-22

Priority: Not Prioritized

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Discussion:

Currently, if T is an array type or a function type, instantiation of std::optional<T&> is still ill-formed, because the return type of its value_or member function is specified as remove_cv_t<T>, which is invalid as a return type.

However, we don't exclude such T& from valid contained types. Given only value_or is problematic here, perhaps we can avoid providing it if T is not returnable.

Proposed resolution:

This wording is relative to N5014.

1. Modify 22.5.4.1 [optional.optional.ref.general], header <iterator> synopsis, as indicated:

   namespace std {
     template<class T>
     class optional<T&> {
       […]
       constexpr T& value() const; // freestanding-deleted
       template<class U = remove_cv_t<T>>
         constexpr remove_cv_t<T> value_or(U&& u) const; // not always present
       […]
     };
   }
   

2. Modify 22.5.4.6 [optional.ref.observe] as indicated:

   template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& u) const;
   

   -8- Let X be remove_cv_t<T>.

   -9- Mandates: is_constructible_v<X, T&> && is_convertible_v<U, X> is true.

   -10- Effects: Equivalent to:

   return has_value() ? *val : static_cast<X>(std::forward<U>(u));
   

   -?- Remarks: This function template is present if and only if T is a non-array object type.