Document number | P3379R0 |
Date | 2024-08-23 |
Project | Programming Language C++, Library Evolution Working Group |
Reply-to | Jonathan Wakely <cxx@kayari.org> |
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.
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++.
The edits are shown relative to N4988.
Update the value of the __cpp_lib_constrained_equality
feature test macro
in [version.syn].
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())
.
N4988, Working Draft - Programming Languages -- C++, Thomas Köppe, 2024.
P2944R3, Comparisons for reference_wrapper
, Barry Revzin, 2024.