From a19d37044b35766d216d77c3fb5e5275e48438ea Mon Sep 17 00:00:00 2001
From: Hristo Hristov <hghristov.rmm@gmail.com>
Date: Fri, 27 Jun 2025 20:12:18 +0300
Subject: [PATCH] [libc++] P2944R3: Constrained comparisions - `tuple`
 (#145677)

Implements P2944R3 partially, which adds constrained comparisons
`std::tuple`.
The missing overloads introduced in [P2165R4](https://wg21.link/P2165R4)
are not implemented.

Uses
[`__all`](https://github.com/llvm/llvm-project/blob/f7af33a9eb5b3876f219075023dc9c565d75849b/libcxx/include/__type_traits/conjunction.h#L45)
instead of a fold expression, see comment:
https://github.com/llvm/llvm-project/pull/141396#discussion_r2161166077

Relates to #136765

# References

[tuple.rel](https://wg21.link//tuple.rel)
---
 libcxx/docs/Status/Cxx2cPapers.csv            |  2 +-
 libcxx/include/tuple                          |  6 +++++
 .../tuple/tuple.tuple/tuple.rel/eq.pass.cpp   | 25 +++++++++++++++++--
 .../size_incompatible_comparison.verify.cpp   |  3 +++
 4 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index 00fad3ff802a..dc18127525a3 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/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|","","The changes to ``optional`` and ``tuple`` are not yet implemented"
+"`P2944R3 <https://wg21.link/P2944R3>`__","Comparisons for ``reference_wrapper``","2024-03 (Tokyo)","|Partial|","","The changes to ``optional`` and ``tuple``'s equality overload from P2165R4 are not yet implemented"
 "`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",""
 "","","","","",""
diff --git a/libcxx/include/tuple b/libcxx/include/tuple
index 6e7a430d219e..75021f0ea51f 100644
--- a/libcxx/include/tuple
+++ b/libcxx/include/tuple
@@ -216,6 +216,7 @@ template <class... Types>
 #  include <__compare/common_comparison_category.h>
 #  include <__compare/ordering.h>
 #  include <__compare/synth_three_way.h>
+#  include <__concepts/boolean_testable.h>
 #  include <__config>
 #  include <__cstddef/size_t.h>
 #  include <__fwd/array.h>
@@ -1153,6 +1154,11 @@ struct __tuple_equal<0> {
 };
 
 template <class... _Tp, class... _Up>
+#    if _LIBCPP_STD_VER >= 26
+  requires(__all<requires(const _Tp& __t, const _Up& __u) {
+             { __t == __u } -> __boolean_testable;
+           }...>::value && (sizeof...(_Tp) == sizeof...(_Up)))
+#    endif
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
 operator==(const tuple<_Tp...>& __x, const tuple<_Up...>& __y) {
   static_assert(sizeof...(_Tp) == sizeof...(_Up), "Can't compare tuples of different sizes");
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/eq.pass.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/eq.pass.cpp
index a8de656313d4..779a89b163f0 100644
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/eq.pass.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/eq.pass.cpp
@@ -16,12 +16,33 @@
 
 // UNSUPPORTED: c++03
 
-#include <tuple>
-#include <string>
+#include <array>
 #include <cassert>
+#include <tuple>
 
+#include "test_comparisons.h"
 #include "test_macros.h"
 
+#if TEST_STD_VER >= 26
+
+// Test SFINAE.
+
+static_assert(std::equality_comparable<std::tuple<EqualityComparable>>);
+static_assert(std::equality_comparable<std::tuple<EqualityComparable, EqualityComparable>>);
+
+static_assert(!std::equality_comparable<std::tuple<NonComparable>>);
+static_assert(!std::equality_comparable<std::tuple<EqualityComparable, NonComparable>>);
+static_assert(!std::equality_comparable<std::tuple<NonComparable, EqualityComparable>>);
+static_assert(!std::equality_comparable_with<std::tuple<EqualityComparable>, std::tuple<NonComparable>>);
+static_assert(!std::equality_comparable_with<std::tuple<NonComparable>, std::tuple<EqualityComparable>>);
+// Size mismatch.
+static_assert(
+    !std::equality_comparable_with<std::tuple<EqualityComparable>, std::tuple<EqualityComparable, EqualityComparable>>);
+static_assert(
+    !std::equality_comparable_with<std::tuple<EqualityComparable, EqualityComparable>, std::tuple<EqualityComparable>>);
+
+#endif
+
 int main(int, char**)
 {
     {
diff --git a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/size_incompatible_comparison.verify.cpp b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/size_incompatible_comparison.verify.cpp
index 851f6fcd1fba..c05438f838f1 100644
--- a/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/size_incompatible_comparison.verify.cpp
+++ b/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.rel/size_incompatible_comparison.verify.cpp
@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+// Disabled in C++26 and later because tuple comparison between different sizes is constrained since P2944R3.
+// UNSUPPORTED: std-at-least-cxx26
+
 // <tuple>
 
 // template <class... Types> class tuple;