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

Implements https://wg21.link/P2944R3 (partially):
- [pairs.spec](https://eel.is/c++draft/pairs.spec)

Related issues:
- Related to #105424
- Related to #118135
  - PR https://github.com/llvm/llvm-project/pull/135759
  - PR https://github.com/llvm/llvm-project/pull/117664

Closes: [#136763](https://github.com/llvm/llvm-project/issues/136763)

# References
- https://eel.is/c++draft/concept.booleantestable
- https://eel.is/c++draft/concept.equalitycomparable

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>
Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>

libcxx/docs/Status/Cxx2cPapers.csv

@@ -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",""
 "","","","","",""

libcxx/include/__utility/pair.h

@@ -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>
@@ -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;
+  }
+#endif
+{
   return __x.first == __y.first && __x.second == __y.second;
 }
 

libcxx/test/std/utilities/utility/pairs/pairs.spec/comparison.pass.cpp

@@ -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**)
 {
     {