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

Title: optional::value_or should never return a cv-qualified type
Status: new
Section: [optional.observe]
Submitter: Casey Carter

Created on 2020-04-02.00:00:00 last changed 21 months ago

Messages

Date: 2023-02-10.09:41:36

Proposed resolution:

This wording is relative to N4928.

Modify [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&&) &&;
[…]

Modify [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(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(v));
```

Modify [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&&) &&;
[…]

Modify [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(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(v)).

msg11223 (view)

Date: 2023-02-09.00:00:00

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

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

msg11198 (view)

Date: 2020-04-18.00:00:00

[ 2020-04-18 Issue Prioritization ]

Priority to 3 after reflector discussion.

Previous resolution [SUPERSEDED]:

This wording is relative to N4861.
Modify [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&&) &&;
[…]
Modify [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(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(v));

msg11197 (view)

Date: 2020-04-02.00:00:00

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.

History
Date	User	Action	Args
2023-02-10 09:41:36	admin	set	messages: + msg13330
2020-04-18 12:19:25	admin	set	messages: + msg11223
2020-04-05 16:33:53	admin	set	messages: + msg11198
2020-04-02 00:00:00	admin	create