Issue 3891: LWG 3870 breaks std::expected<cv T, E>

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3891. LWG 3870 breaks `std::expected<cv T, E>`

Section: 22.8.6.1 [expected.object.general] Status: New Submitter: Jiang An Opened: 2023-02-19 Last modified: 2024-10-02

Priority: 2

View all other issues in [expected.object.general].

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Discussion:

Currently the value_type of std::expected can be a cv-qualified type, which is possibly intended. However, LWG 3870⁽ⁱ⁾ disallows std::construct_at to construct objects via cv T*, which breaks std::expected<cv T, E> because some operations are specified with std::construct_at (22.8.6.4 [expected.object.assign], 22.8.6.5 [expected.object.swap]).

I think when T is cv-qualified, it would be better to store std::remove_cv_t<T> subobject while sometimes (other than construction/destruction) access it via a cv-qualified glvalue, which can also avoid UB associated with const/volatile objects.

[2023-03-22; Reflector poll]

Set priority to 2 after reflector poll. "Not clear if all these wording changes are needed or desired." "Unconvinced that the mixed-value-error swap should use value(), source is destroyed immediately anyway. The else branch should use remove_cv_t too."

Previous resolution [SUPERSEDED]:

This wording is relative to N4928.

[Drafting note: When assignment and swap need to backup the old value by move construction, the source should be considered cv-unqualified, as the backup mechanism is only used internally.]
Modify 22.8.6.1 [expected.object.general] as indicated:
[…]
bool has_val;      // exposition only
union {
  remove_cv_t<T> val;       // exposition only
  E unex;          // exposition only
};
[…]
Modify 22.8.6.4 [expected.object.assign] as indicated:
constexpr expected& operator=(const expected& rhs);
-2- Effects:

(2.1) — If this->has_value() && rhs.has_value() is true, equivalent to value()val = *rhs.

[…]

[…]
constexpr expected& operator=(expected&& rhs) noexcept(see below);
[…]
-6- Effects:

(6.1) — If this->has_value() && rhs.has_value() is true, equivalent to value()val = std::move(*rhs).

[…]

[…]
template<class U = T>
  constexpr expected& operator=(U&& v);
[…]
-10- Effects:

(10.1) — If has_value() is true, equivalent to value()val = std::forward<U>(v).

[…]
Modify Table 64: swap(expected&) effects [tab:expected.object.swap] as indicated:

Table 64 — swap(expected&) effects [tab:expected.object.swap]
this->has_value() !this->has_value()

rhs.has_value() equivalent to: using std::swap;
swap(value()val, rhs.value()val); calls rhs.swap(*this)

[…]
Modify 22.8.6.5 [expected.object.swap] as indicated:
constexpr void swap(expected& rhs) noexcept(see below);
-1- Constraints: […]
-2- Effects: See Table 64 [tab:expected.object.swap].
For the case where rhs.value() is false and this->has_value() is true, equivalent to:
if constexpr (is_nothrow_move_constructible_v<E>) {
  E tmp(std::move(rhs.unex));
  destroy_at(addressof(rhs.unex));
  try {
    construct_at(addressof(rhs.val), std::move(value()val));
    destroy_at(addressof(val));
    construct_at(addressof(unex), std::move(tmp));
  } catch(...) {
    construct_at(addressof(rhs.unex), std::move(tmp));
    throw;
  }
} else {
  T tmp(std::move(val));
  destroy_at(addressof(val));
  try {
    construct_at(addressof(unex), std::move(rhs.unex));
    destroy_at(addressof(rhs.unex));
    construct_at(addressof(rhs.val), std::move(tmp));
  } catch (...) {
    construct_at(addressof(val), std::move(tmp));
    throw;
  }
}
has_val = false;
rhs.has_val = true;

Table 64 — `swap(expected&)` effects [tab:expected.object.swap]
	`this->has_value()`	`!this->has_value()`
`rhs.has_value()`	equivalent to: `using std::swap;` `swap(value()val, rhs.value()val);`	calls `rhs.swap(*this)`
`[…]`

[2024-10-02; Jonathan provides improved wording]

Removed the use of value() in the [expected.object.swap] p2 Effects: and added remove_cv_t to the local T in the else-branch.

Proposed resolution:

This wording is relative to N4988.

[Drafting note: When assignment and swap need to backup the old value by move construction, the source should be considered cv-unqualified, as the backup mechanism is only used internally.]

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

[…]
bool has_val;      // exposition only
union {
  remove_cv_t<T> val;       // exposition only
  E unex;          // exposition only
};
[…]

Modify 22.8.6.4 [expected.object.assign] as indicated:
```
constexpr expected& operator=(const expected& rhs);
```
-2- Effects:
1. (2.1) — If this->has_value() && rhs.has_value() is true, equivalent to value()val = *rhs.
2. […]
[…]
```
constexpr expected& operator=(expected&& rhs) noexcept(see below);
```
[…]
-6- Effects:
1. (6.1) — If this->has_value() && rhs.has_value() is true, equivalent to value()val = std::move(*rhs).
2. […]
[…]
```
template<class U = T>
  constexpr expected& operator=(U&& v);
```
[…]
-10- Effects:
1. (10.1) — If has_value() is true, equivalent to value()val = std::forward<U>(v).
2. […]

Modify Table 64: swap(expected&) effects [tab:expected.object.swap] as indicated:

Table 64 — swap(expected&) effects [tab:expected.object.swap]
this->has_value() !this->has_value()

rhs.has_value() equivalent to: using std::swap;
swap(value()val, rhs.value()val); calls rhs.swap(*this)

[…]

Table 64 — `swap(expected&)` effects [tab:expected.object.swap]
	`this->has_value()`	`!this->has_value()`
`rhs.has_value()`	equivalent to: `using std::swap;` `swap(value()val, rhs.value()val);`	calls `rhs.swap(*this)`
`[…]`

Modify 22.8.6.5 [expected.object.swap] as indicated:

constexpr void swap(expected& rhs) noexcept(see below);

-1- Constraints: […]

-2- Effects: See Table 64 [tab:expected.object.swap].

For the case where rhs.value() is false and this->has_value() is true, equivalent to:

if constexpr (is_nothrow_move_constructible_v<E>) {
  E tmp(std::move(rhs.unex));
  destroy_at(addressof(rhs.unex));
  try {
    construct_at(addressof(rhs.val), std::move(val));
    destroy_at(addressof(val));
    construct_at(addressof(unex), std::move(tmp));
  } catch(...) {
    construct_at(addressof(rhs.unex), std::move(tmp));
    throw;
  }
} else {
  remove_cv_t<T> tmp(std::move(val));
  destroy_at(addressof(val));
  try {
    construct_at(addressof(unex), std::move(rhs.unex));
    destroy_at(addressof(rhs.unex));
    construct_at(addressof(rhs.val), std::move(tmp));
  } catch (...) {
    construct_at(addressof(val), std::move(tmp));
    throw;
  }
}
has_val = false;
rhs.has_val = true;

3891. LWG 3870 breaks std::expected<cv T, E>

3891. LWG 3870 breaks `std::expected<cv T, E>`