[libc++] Implement P3379R0 Constrain `std::expected` equality operators (#135759)

Closes #118135

Co-authored-by: A. Jiang <de34@live.cn>

9 files changed, 110 insertions, 42 deletions

diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index e230002..3809446 100644
--- a/libcxx/docs/Status/Cxx2cPapers.csv
+++ b/libcxx/docs/Status/Cxx2cPapers.csv
@@ -78,7 +78,7 @@
 "","","","","",""
 "`P3136R1 <https://wg21.link/P3136R1>`__","Retiring niebloids","2024-11 (Wrocław)","|Complete|","14",""
 "`P3138R5 <https://wg21.link/P3138R5>`__","``views::cache_latest``","2024-11 (Wrocław)","","",""
-"`P3379R0 <https://wg21.link/P3379R0>`__","Constrain ``std::expected`` equality operators","2024-11 (Wrocław)","","",""
+"`P3379R0 <https://wg21.link/P3379R0>`__","Constrain ``std::expected`` equality operators","2024-11 (Wrocław)","|Complete|","21",""
 "`P2862R1 <https://wg21.link/P2862R1>`__","``text_encoding::name()`` should never return null values","2024-11 (Wrocław)","","",""
 "`P2897R7 <https://wg21.link/P2897R7>`__","``aligned_accessor``: An ``mdspan`` accessor expressing pointer over-alignment","2024-11 (Wrocław)","|Complete|","21",""
 "`P3355R1 <https://wg21.link/P3355R1>`__","Fix ``submdspan`` for C++26","2024-11 (Wrocław)","","",""
diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 03bbd16..6b3d335 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -25,6 +25,7 @@
 #include <__type_traits/is_assignable.h>
 #include <__type_traits/is_constructible.h>
 #include <__type_traits/is_convertible.h>
+#include <__type_traits/is_core_convertible.h>
 #include <__type_traits/is_function.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
@@ -1139,8 +1140,15 @@ public:
 
   // [expected.object.eq], equality operators
   template <class _T2, class _E2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y)
     requires(!is_void_v<_T2>)
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
+#  if _LIBCPP_STD_VER >= 26
+            && requires {
+                 { *__x == *__y } -> __core_convertible_to<bool>;
+                 { __x.error() == __y.error() } -> __core_convertible_to<bool>;
+               }
+#  endif
+  {
     if (__x.__has_val() != __y.__has_val()) {
       return false;
     } else {
@@ -1153,12 +1161,24 @@ public:
   }
 
   template <class _T2>
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v) {
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v)
+#  if _LIBCPP_STD_VER >= 26
+    requires(!__is_std_expected<_T2>::value) && requires {
+      { *__x == __v } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
     return __x.__has_val() && static_cast<bool>(__x.__val() == __v);
   }
 
   template <class _E2>
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e) {
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e)
+#  if _LIBCPP_STD_VER >= 26
+    requires requires {
+      { __x.error() == __e.error() } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
     return !__x.__has_val() && static_cast<bool>(__x.__unex() == __e.error());
   }
 };
@@ -1851,7 +1871,13 @@ public:
   // [expected.void.eq], equality operators
   template <class _T2, class _E2>
     requires is_void_v<_T2>
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y)
+#  if _LIBCPP_STD_VER >= 26
+    requires requires {
+      { __x.error() == __y.error() } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
     if (__x.__has_val() != __y.__has_val()) {
       return false;
     } else {
@@ -1860,7 +1886,13 @@ public:
   }
 
   template <class _E2>
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __y) {
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __y)
+#  if _LIBCPP_STD_VER >= 26
+    requires requires {
+      { __x.error() == __y.error() } -> __core_convertible_to<bool>;
+    }
+#  endif
+  {
     return !__x.__has_val() && static_cast<bool>(__x.__unex() == __y.error());
   }
 };
diff --git a/libcxx/include/__type_traits/is_core_convertible.h b/libcxx/include/__type_traits/is_core_convertible.h
index 93e23d2..ca3a346 100644
--- a/libcxx/include/__type_traits/is_core_convertible.h
+++ b/libcxx/include/__type_traits/is_core_convertible.h
@@ -30,6 +30,13 @@ template <class _Tp, class _Up>
 struct __is_core_convertible<_Tp, _Up, decltype(static_cast<void (*)(_Up)>(0)(static_cast<_Tp (*)()>(0)()))>
     : true_type {};
 
+#if _LIBCPP_STD_VER >= 20
+
+template <class _Tp, class _Up>
+concept __core_convertible_to = __is_core_convertible<_Tp, _Up>::value;
+
+#endif // _LIBCPP_STD_VER >= 20
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_IS_CORE_CONVERTIBLE_H
diff --git a/libcxx/test/std/utilities/expected/expected.expected/equality/equality.T2.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/equality/equality.T2.pass.cpp
index bc8b9de..25eb97a 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/equality/equality.T2.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/equality/equality.T2.pass.cpp
@@ -17,18 +17,19 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
-struct Data {
-  int i;
-  constexpr Data(int ii) : i(ii) {}
-
-  friend constexpr bool operator==(const Data& data, int ii) { return data.i == ii; }
-};
+#if TEST_STD_VER >= 26
+// https://wg21.link/P3379R0
+static_assert(CanCompare<std::expected<int, int>, int>);
+static_assert(CanCompare<std::expected<int, int>, EqualityComparable>);
+static_assert(!CanCompare<std::expected<int, int>, NonComparable>);
+#endif
 
 constexpr bool test() {
   // x.has_value()
   {
-    const std::expected<Data, int> e1(std::in_place, 5);
+    const std::expected<EqualityComparable, int> e1(std::in_place, 5);
     int i2 = 10;
     int i3 = 5;
     assert(e1 != i2);
@@ -37,7 +38,7 @@ constexpr bool test() {
 
   // !x.has_value()
   {
-    const std::expected<Data, int> e1(std::unexpect, 5);
+    const std::expected<EqualityComparable, int> e1(std::unexpect, 5);
     int i2 = 10;
     int i3 = 5;
     assert(e1 != i2);
diff --git a/libcxx/test/std/utilities/expected/expected.expected/equality/equality.other_expected.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/equality/equality.other_expected.pass.cpp
index 9325c6c..f0f549b 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/equality/equality.other_expected.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/equality/equality.other_expected.pass.cpp
@@ -18,20 +18,26 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
 // Test constraint
-template <class T1, class T2>
-concept CanCompare = requires(T1 t1, T2 t2) { t1 == t2; };
-
-struct Foo{};
-static_assert(!CanCompare<Foo, Foo>);
+static_assert(!CanCompare<NonComparable, NonComparable>);
 
 static_assert(CanCompare<std::expected<int, int>, std::expected<int, int>>);
 static_assert(CanCompare<std::expected<int, int>, std::expected<short, short>>);
 
-// Note this is true because other overloads are unconstrained
-static_assert(CanCompare<std::expected<int, int>, std::expected<void, int>>);
-
+#if TEST_STD_VER >= 26
+// https://wg21.link/P3379R0
+static_assert(!CanCompare<std::expected<int, int>, std::expected<void, int>>);
+static_assert(CanCompare<std::expected<int, int>, std::expected<int, int>>);
+static_assert(!CanCompare<std::expected<NonComparable, int>, std::expected<NonComparable, int>>);
+static_assert(!CanCompare<std::expected<int, NonComparable>, std::expected<int, NonComparable>>);
+static_assert(!CanCompare<std::expected<NonComparable, int>, std::expected<int, NonComparable>>);
+static_assert(!CanCompare<std::expected<int, NonComparable>, std::expected<NonComparable, int>>);
+#else
+// Note this is true because other overloads in expected<non-void> are unconstrained
+static_assert(CanCompare<std::expected<void, int>, std::expected<int, int>>);
+#endif
 constexpr bool test() {
   // x.has_value() && y.has_value()
   {
diff --git a/libcxx/test/std/utilities/expected/expected.expected/equality/equality.unexpected.pass.cpp b/libcxx/test/std/utilities/expected/expected.expected/equality/equality.unexpected.pass.cpp
index a8c469d..6c7d2f3 100644
--- a/libcxx/test/std/utilities/expected/expected.expected/equality/equality.unexpected.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.expected/equality/equality.unexpected.pass.cpp
@@ -17,18 +17,19 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
-struct Data {
-  int i;
-  constexpr Data(int ii) : i(ii) {}
-
-  friend constexpr bool operator==(const Data& data, int ii) { return data.i == ii; }
-};
+#if TEST_STD_VER >= 26
+// https://wg21.link/P3379R0
+static_assert(CanCompare<std::expected<EqualityComparable, EqualityComparable>, std::unexpected<int>>);
+static_assert(CanCompare<std::expected<EqualityComparable, int>, std::unexpected<EqualityComparable>>);
+static_assert(!CanCompare<std::expected<EqualityComparable, NonComparable>, std::unexpected<int>>);
+#endif
 
 constexpr bool test() {
   // x.has_value()
   {
-    const std::expected<Data, Data> e1(std::in_place, 5);
+    const std::expected<EqualityComparable, EqualityComparable> e1(std::in_place, 5);
     std::unexpected<int> un2(10);
     std::unexpected<int> un3(5);
     assert(e1 != un2);
@@ -37,7 +38,7 @@ constexpr bool test() {
 
   // !x.has_value()
   {
-    const std::expected<Data, Data> e1(std::unexpect, 5);
+    const std::expected<EqualityComparable, EqualityComparable> e1(std::unexpect, 5);
     std::unexpected<int> un2(10);
     std::unexpected<int> un3(5);
     assert(e1 != un2);
diff --git a/libcxx/test/std/utilities/expected/expected.void/equality/equality.other_expected.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/equality/equality.other_expected.pass.cpp
index 8b24875..b6c3d8d 100644
--- a/libcxx/test/std/utilities/expected/expected.void/equality/equality.other_expected.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/equality/equality.other_expected.pass.cpp
@@ -18,10 +18,7 @@
 #include <utility>
 
 #include "test_macros.h"
-
-// Test constraint
-template <class T1, class T2>
-concept CanCompare = requires(T1 t1, T2 t2) { t1 == t2; };
+#include "../../types.h"
 
 struct Foo{};
 static_assert(!CanCompare<Foo, Foo>);
@@ -29,8 +26,18 @@ static_assert(!CanCompare<Foo, Foo>);
 static_assert(CanCompare<std::expected<void, int>, std::expected<void, int>>);
 static_assert(CanCompare<std::expected<void, int>, std::expected<void, short>>);
 
+#if TEST_STD_VER >= 26
+// https://wg21.link/P3379R0
+static_assert(!CanCompare<std::expected<void, int>, std::expected<int, int>>);
+static_assert(CanCompare<std::expected<void, int>, std::expected<void, int>>);
+static_assert(CanCompare<std::expected<void, int>, std::expected<void, int>>);
+static_assert(!CanCompare<std::expected<void, NonComparable>, std::expected<void, NonComparable>>);
+static_assert(!CanCompare<std::expected<void, int>, std::expected<void, NonComparable>>);
+static_assert(!CanCompare<std::expected<void, NonComparable>, std::expected<void, int>>);
+#else
 // Note this is true because other overloads in expected<non-void> are unconstrained
 static_assert(CanCompare<std::expected<void, int>, std::expected<int, int>>);
+#endif
 
 constexpr bool test() {
   // x.has_value() && y.has_value()
diff --git a/libcxx/test/std/utilities/expected/expected.void/equality/equality.unexpected.pass.cpp b/libcxx/test/std/utilities/expected/expected.void/equality/equality.unexpected.pass.cpp
index 4500971..f37f38b 100644
--- a/libcxx/test/std/utilities/expected/expected.void/equality/equality.unexpected.pass.cpp
+++ b/libcxx/test/std/utilities/expected/expected.void/equality/equality.unexpected.pass.cpp
@@ -17,18 +17,19 @@
 #include <utility>
 
 #include "test_macros.h"
+#include "../../types.h"
 
-struct Data {
-  int i;
-  constexpr Data(int ii) : i(ii) {}
-
-  friend constexpr bool operator==(const Data& data, int ii) { return data.i == ii; }
-};
+#if TEST_STD_VER >= 26
+// https://wg21.link/P3379R0
+static_assert(CanCompare<std::expected<void, EqualityComparable>, std::unexpected<int>>);
+static_assert(CanCompare<std::expected<void, int>, std::unexpected<EqualityComparable>>);
+static_assert(!CanCompare<std::expected<void, NonComparable>, std::unexpected<int>>);
+#endif
 
 constexpr bool test() {
   // x.has_value()
   {
-    const std::expected<void, Data> e1;
+    const std::expected<void, EqualityComparable> e1;
     std::unexpected<int> un2(10);
     std::unexpected<int> un3(5);
     assert(e1 != un2);
@@ -37,7 +38,7 @@ constexpr bool test() {
 
   // !x.has_value()
   {
-    const std::expected<void, Data> e1(std::unexpect, 5);
+    const std::expected<void, EqualityComparable> e1(std::unexpect, 5);
     std::unexpected<int> un2(10);
     std::unexpected<int> un3(5);
     assert(e1 != un2);
diff --git a/libcxx/test/std/utilities/expected/types.h b/libcxx/test/std/utilities/expected/types.h
index df73ebd..11473ca 100644
--- a/libcxx/test/std/utilities/expected/types.h
+++ b/libcxx/test/std/utilities/expected/types.h
@@ -336,4 +336,17 @@ struct CheckForInvalidWrites : public CheckForInvalidWritesBase<WithPaddedExpect
   }
 };
 
+struct NonComparable {};
+
+struct EqualityComparable {
+  int i;
+  constexpr EqualityComparable(int ii) : i(ii) {}
+
+  friend constexpr bool operator==(const EqualityComparable& data, int ii) { return data.i == ii; }
+};
+
+// Test constraint
+template <class T1, class T2>
+concept CanCompare = requires(T1 t1, T2 t2) { t1 == t2; };
+
 #endif // TEST_STD_UTILITIES_EXPECTED_TYPES_H