Issue 3886: Monad mo' problems (original) (raw)

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of WP status.

3886. Monad mo' problems

Section: 22.5.3.1 [optional.optional.general], 22.8.6.1 [expected.object.general] Status: WP Submitter: Casey Carter Opened: 2023-02-13 Last modified: 2024-11-28

Priority: 3

View all other issues in [optional.optional.general].

View all issues with WP status.

Discussion:

While implementing P2505R5 "Monadic Functions for std::expected" we found it odd that the template type parameter for the assignment operator that accepts an argument by forwarding reference is defaulted, but the template type parameter for value_or is not. For consistency, it would seem that_meow_.value_or(_woof_) should accept the same arguments _woof_ as does_meow_ = _woof_, even when those arguments are braced-initializers.

That said, it would be peculiar to default the template type parameter of value_or to Tinstead of remove_cv_t<T>. For expected<const vector<int>, int> _meow_{unexpect, 42};, for example, _meow_.value_or({1, 2, 3}) would create a temporary const vector<int>for the argument and return a copy of that argument. Were the default template argument insteadremove_cv_t<T>, _meow_.value_or({1, 2, 3}) could move construct its return value from the argument vector<int>. For the same reason, the constructor that accepts a forwarding reference with a default template argument of T should default that argument to remove_cv_t<T>.

For consistency, it would be best to default the template argument of the perfect-forwarding construct, perfect-forwarding assignment operator, and value_or to remove_cv_t<T>. Since all of the arguments presented apply equally to optional, we believe optional should be changed consistently with expected. MSVCSTL has prototyped these changes successfully.

[2023-03-22; Reflector poll]

Set priority to 3 after reflector poll.

[2024-09-18; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Wrocław 2024-11-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

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

   namespace std {
   template
   class optional {
   public:
   […]
   template<class U = remove_cv_t>
   constexpr explicit(see below) optional(U&&);

[…]
template<class U = remove_cv_t> constexpr optional& operator=(U&&);
[…]
template<class U = remove_cv_t> constexpr T value_or(U&&) const &;
template<class U = remove_cv_t> constexpr T value_or(U&&) &&;
[…]
};
[…]
} 2. Modify 22.5.3.2 [optional.ctor] as indicated:
template<class U = remove_cv_t> constexpr explicit(see below) optional(U&& v);

-23- Constraints: […] 3. Modify 22.5.3.4 [optional.assign] as indicated:
template<class U = remove_cv_t> constexpr optional& operator=(U&& v);

-12- Constraints: […] 4. Modify 22.5.3.7 [optional.observe] as indicated:
template<class U = remove_cv_t> constexpr T value_or(U&& v) const &;

-15- Mandates: […]

[…]

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

-17- Mandates: […] 5. Modify 22.8.6.1 [expected.object.general] as indicated:
namespace std {
template<class T, class E>
class expected {
public:
[…]
template<class U = remove_cv_t>
constexpr explicit(see below) expected(U&& v);
[…]
template<class U = remove_cv_t> constexpr expected& operator=(U&&);
[…]
template<class U = remove_cv_t> constexpr T value_or(U&&) const &;
template<class U = remove_cv_t> constexpr T value_or(U&&) &&;
[…]
};
[…]
} 6. Modify 22.8.6.2 [expected.object.cons] as indicated:
template<class U = remove_cv_t>
constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);

-23- Constraints: […] 7. Modify 22.8.6.4 [expected.object.assign] as indicated:
template<class U = remove_cv_t>
constexpr expected& operator=(U&& v);

-9- Constraints: […] 8. Modify 22.8.6.6 [expected.object.obs] as indicated:
template<class U = remove_cv_t> constexpr T value_or(U&& v) const &;

-16- Mandates: […]

[…]

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

-18- Mandates: […]