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[libc++][pair] P2944R3: Constrain std::pair's equality operator #136672

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Merged
merged 8 commits into from
Apr 29, 2025
Merged

[libc++][pair] P2944R3: Constrain std::pair's equality operator #136672

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12fc430
[libc++][pair] P2944R3: Constrain `pair` equality operator
H-G-Hristov Apr 8, 2025
6d48213
Updated tests
H-G-Hristov Apr 18, 2025
92544cb
Cleanup test
H-G-Hristov Apr 22, 2025
672c7c3
Fix CI
H-G-Hristov Apr 22, 2025
3079b70
Fix docs gen
H-G-Hristov Apr 22, 2025
40b04ce
Update libcxx/docs/Status/Cxx2cPapers.csv
Zingam Apr 23, 2025
3ff0e71
Update libcxx/docs/Status/Cxx2cPapers.csv
Zingam Apr 29, 2025
292acd3
Merge branch 'main' into hgh/libcxx/P2944R3-constrained-equality-std_…
Zingam Apr 29, 2025
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2 changes: 1 addition & 1 deletion libcxx/docs/Status/Cxx2cPapers.csv
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Original file line number Diff line number Diff line change
Expand Up @@ -59,7 +59,7 @@
"`P2248R8 <https://wg21.link/P2248R8>`__","Enabling list-initialization for algorithms","2024-03 (Tokyo)","","",""
"`P2810R4 <https://wg21.link/P2810R4>`__","``is_debugger_present`` ``is_replaceable``","2024-03 (Tokyo)","","",""
"`P1068R11 <https://wg21.link/P1068R11>`__","Vector API for random number generation","2024-03 (Tokyo)","","",""
"`P2944R3 <https://wg21.link/P2944R3>`__","Comparisons for ``reference_wrapper``","2024-03 (Tokyo)","|Partial|","19","Implemented comparisons for ``reference_wrapper`` only"
"`P2944R3 <https://wg21.link/P2944R3>`__","Comparisons for ``reference_wrapper``","2024-03 (Tokyo)","|Partial|","","Implemented changes to ``reference_wrapper`` and ``pair``"
"`P2642R6 <https://wg21.link/P2642R6>`__","Padded ``mdspan`` layouts","2024-03 (Tokyo)","","",""
"`P3029R1 <https://wg21.link/P3029R1>`__","Better ``mdspan``'s CTAD","2024-03 (Tokyo)","|Complete|","19",""
"","","","","",""
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10 changes: 9 additions & 1 deletion libcxx/include/__utility/pair.h
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Expand Up @@ -11,6 +11,7 @@

#include <__compare/common_comparison_category.h>
#include <__compare/synth_three_way.h>
#include <__concepts/boolean_testable.h>
#include <__concepts/different_from.h>
#include <__config>
#include <__cstddef/size_t.h>
Expand Down Expand Up @@ -461,7 +462,14 @@ pair(_T1, _T2) -> pair<_T1, _T2>;

template <class _T1, class _T2, class _U1, class _U2>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
operator==(const pair<_T1, _T2>& __x, const pair<_U1, _U2>& __y) {
operator==(const pair<_T1, _T2>& __x, const pair<_U1, _U2>& __y)
#if _LIBCPP_STD_VER >= 26
requires requires {
{ __x.first == __y.first } -> __boolean_testable;
{ __x.second == __y.second } -> __boolean_testable;
}
Comment on lines +467 to +470
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Don't we have the same situation as in #135759? (that __boolean_testable might be too strict)

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The standard requirements are inconsistent among these new constraints. boolean-testable is used for pair and tuple's operator==s (perhaps due to boolean-testable in old Preconditions added by P2167R3), while plain implicit convertibility is used elsewhere.

I guess the difference was because of that && (and possibly ||?) is expected to be used in pair and tuple's operator==s, but not in other mentioned operators.

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As @frederick-vs-ja pointed out these are described as __boolean_testable in the standard so I prefer to keep them as such for consistency with the Standard but I'll update the reference_wrapper with the proposed concept in #135759 later.

#endif
{
return __x.first == __y.first && __x.second == __y.second;
}

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26 changes: 26 additions & 0 deletions libcxx/test/std/utilities/utility/pairs/pairs.spec/comparison.pass.cpp
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Expand Up @@ -19,9 +19,35 @@

#include <utility>
#include <cassert>
#include <concepts>

#include "test_macros.h"

#if TEST_STD_VER >= 26

// Test SFINAE.

struct EqualityComparable {
constexpr EqualityComparable(int value) : value_{value} {};

friend constexpr bool operator==(const EqualityComparable&, const EqualityComparable&) noexcept = default;

int value_;
};

static_assert(std::equality_comparable<EqualityComparable>);

static_assert(std::equality_comparable<std::pair<EqualityComparable, EqualityComparable>>);

struct NonComparable {};

static_assert(!std::equality_comparable<NonComparable>);

static_assert(!std::equality_comparable<std::pair<EqualityComparable, NonComparable>>);
static_assert(!std::equality_comparable<std::pair<NonComparable, EqualityComparable>>);

#endif // TEST_STD_VER >= 26

int main(int, char**)
{
{
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