3424. optional::value_or should never return a cv-qualified type

Section: 22.5.3.7 [optional.observe] Status: New Submitter: Casey Carter Opened: 2020-04-02 Last modified: 2023-02-10

Priority: 3

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Discussion:

The optional<T>::value_or overloads are specified to return T. This seems silly when T is const or volatile qualified — return types should never be cv-qualified. (In the volatile case, it is even deprecated since merging P1152R4 "Deprecating volatile" into the working draft.) We should strip cv-qualifiers from these return types.

[2020-04-18 Issue Prioritization]

Priority to 3 after reflector discussion.

Previous resolution [SUPERSEDED]:

This wording is relative to N4861.

1. Modify 22.5.3 [optional.optional] as indicated:

   […]
   template<class U> constexpr remove_cv_t<T> value_or(U&&) const&;
   template<class U> constexpr remove_cv_t<T> value_or(U&&) &&;
   […]
   

2. Modify 22.5.3.7 [optional.observe] as indicated:

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

   -?- Let R be remove_cv_t<T>.

   -17- Mandates: is_copy_constructible_v<T>is_convertible_v<const T&, R> && is_convertible_v<U&&, T> is true.

   -18- Effects: Equivalent to:

   return bool(*this) ? **this : static_cast<TR>(std::forward<U>(v));
   

   template<class U> constexpr remove_cv_t<T> value_or(U&& v) &&;
   

   -?- Let R be remove_cv_t<T>.

   -19- Mandates: is_move_constructible_v<T>is_convertible_v<T, R> && is_convertible_v<U&&, T> is true.

   -20- Effects: Equivalent to:

   return bool(*this) ? std::move(**this) : static_cast<TR>(std::forward<U>(v));
   

[2023-02-09 Casey improves wording and expands to cover expected::value_or]

Since expected was modeled on optional, it has the same issue.

Proposed resolution:

This wording is relative to N4928.

1. Modify 22.5.3.1 [optional.optional.general] as indicated:

   […]
   template<class U> constexpr remove_cv_t<T> value_or(U&&) const&;
   template<class U> constexpr remove_cv_t<T> value_or(U&&) &&;
   […]
   

2. Modify 22.5.3.7 [optional.observe] as indicated:

   [Drafting note: The two removals of the && in is_convertible_v<U&&, T> below is a simplification to restore consistency with the wording for expected::value_or.]

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

   -?- Let R be remove_cv_t<T>.

   -15- Mandates: is_copy_constructible_v<T> is_convertible_v<const T&, R> && is_convertible_v<U&&, TR> is true.

   -16- Effects: Equivalent to:

   return bool(*this) ? **this : static_cast<TR>(std::forward<U>(v));
   

   template<class U> constexpr remove_cv_t<T> value_or(U&& v) &&;
   

   -?- Let R be remove_cv_t<T>.

   -17- Mandates: is_move_constructible_v<T> is_convertible_v<T, R> && is_convertible_v<U&&, TR> is true.

   -18- Effects: Equivalent to:

   return bool(*this) ? std::move(**this) : static_cast<TR>(std::forward<U>(v));
   

3. Modify 22.8.6.1 [expected.object.general] as indicated:

   […]
   template<class U> constexpr remove_cv_t<T> value_or(U&&) const &;
   template<class U> constexpr remove_cv_t<T> value_or(U&&) &&;
   […]
   

4. Modify 22.8.6.6 [expected.object.obs] as indicated:

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

   -?- Let R be remove_cv_t<T>.

   -16- Mandates: is_copy_constructible_v<T> is_convertible_v<const T&, R> && is_convertible_v<U, TR> is true.

   -17- Returns: has_value() ? **this : static_cast<TR>(std::forward<U>(v)).

   template<class U> constexpr remove_cv_t<T> value_or(U&& v) &&;
   

   -?- Let R be remove_cv_t<T>.

   -18- Mandates: is_move_constructible_v<T> is_convertible_v<T, R> && is_convertible_v<U, TR> is true.

   -19- Returns: has_value() ? std::move(**this) : static_cast<TR>(std::forward<U>(v)).