Issue 4406: `optional::value_or` return statement is inconsistent with Mandates

Title: `optional::value_or` return statement is inconsistent with Mandates
Status: new
Section: [optional.observe] [optional.ref.observe] [expected.object.obs]
Submitter: Hewill Kang

Created on 2025-09-06.00:00:00 last changed 2 weeks ago

Messages

Date: 2025-11-19.18:55:35

Proposed resolution:

This wording is relative to the working draft after N5014.

Modify [optional.optional.general] as indicated:

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

Modify [optional.observe] as indicated:

template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& v) const &;
-?- Let X be remove_cv_t<T>.

-15- Mandates: is_copy_constructible_v<T> is_convertible_v<const T&, X> && is_convertible_v<U&&, T X> is `true`.
-16- Effects: Equivalent to:
return has_value() ? val : static_cast<T>(std::forward(v));
if (has_value())
 return val;
return std::forward(v);
template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& v) &&;
-?- Let X be remove_cv_t<T>.

-17- Mandates: is_move_constructible_v<T> is_convertible_v<T, X> && is_convertible_v<U&&, T X> is `true`.
-18- Effects: Equivalent to:
return has_value() ? std::move(val) : static_cast<T>(std::forward(v));
if (has_value())
 return std::move(val);
return std::forward(v);

Modify [optional.ref.observe] as indicated:

constexpr T& value() const;
-7- Effects: Equivalent to:
return has_value() ? *val : throw bad_optional_access();
if (has_value())
 return *val;
throw bad_optional_access();
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<T> is_convertible_v<T&, X> && is_convertible_v<U, X> is `true`.
-10- Effects: Equivalent to:
return has_value() ? *val : static_cast<X>(std::forward(u));
if (has_value())
 return *val;
return std::forward(u);

Modify [expected.object.general] as indicated:

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

Modify [expected.object.obs] as indicated:
```
template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& v) const &;
```
-?- Let X be remove_cv_t<T>.

-18- Mandates: ~~is_copy_constructible_v<T> is `true` and~~ is_convertible_v<const T&, X> && is_convertible_v<U, T> is `true`.
~~-19- Returns: has_value() ? **this : static_cast<T>(std::forward(v)).~~
-?- Effects: Equivalent to:
```
if (has_value())
 return val;
return std::forward(v);
```
```
template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& v) &&;
```
-?- Let X be remove_cv_t<T>.

-20- Mandates: ~~is_move_constructible_v<T> is `true` and~~ is_convertible_v<T, X> && is_convertible_v<U, T> is `true`.
~~-21- Returns: has_value() ? std::move(**this) : static_cast<T>(std::forward(v)).~~
-?- Effects: Equivalent to:
```
if (has_value())
 return std::move(val);
return std::forward(v);
```

msg15733 (view)

Date: 2025-11-15.00:00:00

[ 2025-11-19; Jonathan provides updated wording ]

Incorporate proposed resolution of 3424 as well, so that the return type is never cv-qualified.

msg15183 (view)

Date: 2025-11-15.00:00:00

[ 2025-11-11; Jonathan provides updated wording ]

Rebase after LWG 4015 which was approved in Kona. This would also resolve LWG 4281.

msg15134 (view)

Date: 2025-10-15.00:00:00

[ 2025-10-16; Reflector poll ]

Set priority to 3 after reflector poll.

This would need to use `val` instead of `**this` if LWG 4015 is accepted.

This wording is relative to N5014.

Modify [optional.observe] as indicated:

template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;
-15- Mandates: is_copy_constructible_v<T> && is_convertible_v<U&&, T> is `true`.
-16- Effects: Equivalent to:
return has_value() ? **this : static_cast<T>(std::forward(v));
if (has_value())
 return **this;
return std::forward(v);
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;
-17- Mandates: is_move_constructible_v<T> && is_convertible_v<U&&, T> is `true`.
-18- Effects: Equivalent to:
return has_value() ? std::move(**this) : static_cast<T>(std::forward(v));
if (has_value())
 return std::move(**this);
return std::forward(v);

Modify [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));
if (has_value())
 return *val;
return std::forward(u);

Modify [expected.object.obs] as indicated:
```
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) const &;
```
-18- Mandates: is_copy_constructible_v<T> is `true` and is_convertible_v<U, T> is `true`.
~~-19- Returns: has_value() ? **this : static_cast<T>(std::forward(v)).~~
-?- Effects: Equivalent to:
```
if (has_value())
 return **this;
return std::forward(v);
```
```
template<class U = remove_cv_t<T>> constexpr T value_or(U&& v) &&;
```
-20- Mandates: is_move_constructible_v<T> is `true` and is_convertible_v<U, T> is `true`.
~~-21- Returns: has_value() ? std::move(**this) : static_cast<T>(std::forward(v)).~~
-?- Effects: Equivalent to:
```
if (has_value())
 return std::move(**this);
return std::forward(v);
```

msg15133 (view)

Date: 2025-09-06.00:00:00

optional<T>::value_or(U&&) requires is_convertible_v<U&&, T> to ensure that `T` can be convert from `U` when `optional` has no value.

However, the return statement explicitly constructs `T` by `static_cast`, which is not checked by `is_convertible_v` since it only checks for implicit conversions.

This results in rare cases where Mandates may not be violated, but `value_or` is ill-formed (demo):

struct S {
  operator int() const;
  explicit operator int() = delete;
};

int main() {
   std::optional<int>{}.value_or(S{}); // fire
}

It is reasonable to create objects that stick to Mandates. The same goes for `expected::value_or`.

Daniel:

This issue has considerable overlap with LWG 4281.

History
Date	User	Action	Args
2025-11-19 18:55:35	admin	set	messages: + msg15755
2025-11-11 15:51:14	admin	set	messages: + msg15733
2025-10-16 11:35:46	admin	set	messages: + msg15183
2025-10-10 15:07:03	admin	set	messages: + msg15134
2025-09-06 00:00:00	admin	create