Constrain std::expected equality operators

Revision History

• R1: Feature test macro should be added to <expected>.

Introduction

P2944R3 (Comparisons for reference_wrapper) added constraints to the relational operators for pair, tuple, optional, and variant, but missed expected out. We should fix that.

Discussion

We needed LWG issue 4072 to fix the constraints for std::optional. We should do the same here.

This has been implemented and tested in libstdc++.

Proposed wording

The edits are shown relative to N4988.

Update the value of the __cpp_lib_constrained_equality feature test macro in [version.syn] and add it to <expected>.

#define __cpp_lib_constrained_equality  20YYMML // freestanding,
  // also in <utility>, <tuple>, <optional>, <variant>, <expected>

Change Mandates: to Constraints: in [expected.object.eq]:

22.8.6.8 Equality operators [expected.object.eq]

template<class T2, class E2> requires (!is_void_v<T2>)
  friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);

-1- Mandates Constraints: The expressions *x == *y and x.error() == y.error() are well-formed and their results are convertible to bool.

-2- Returns: If x.has_value() does not equal y.has_value(), false; otherwise if x.has_value() is true, *x == *y; otherwise x.error() == y.error().

template<class T2> friend constexpr bool operator==(const expected& x, const T2& v);

-3- Mandates Constraints: T2 is not a specialization of expected. The expression *x == v is well-formed and its result is convertible to bool.

[Note 1: T need not be Cpp17EqualityComparable. — end note]

-4- Returns: x.has_value() && static_cast<bool>(*x == v).

template<class E2> friend constexpr bool operator==(const expected& x, const unexpected<E2>& e);

-5- Mandates Constraints: The expression x.error() == e.error() is well-formed and its result is convertible to bool.

-6- Returns: !x.has_value() && static_cast<bool>(x.error() == e.error()).

Change Mandates: to Constraints: in [expected.void.eq]:

22.8.7.8 Equality operators [expected.void.eq]

template<class T2, class E2> requires is_void_v<T2>
  friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);

-1- Mandates Constraints: The expression x.error() == y.error() is well-formed and its result is convertible to bool.

-2- Returns: If x.has_value() does not equal y.has_value(), false; otherwise x.has_value() || static_cast<bool>(x.error() == y.error()).

template<class E2>
  friend constexpr bool operator==(const expected& x, const unexpected<E2>& e);

-3- Mandates Constraints: The expression x.error() == e.error() is well-formed and its result is convertible to bool.

-4- Returns: !x.has_value() && static_cast<bool>(x.error() == e.error()).

References

N4988, Working Draft - Programming Languages -- C++, Thomas Köppe, 2024.
P2944R3, Comparisons for reference_wrapper, Barry Revzin, 2024.