[libc++] Implement P3379R0 Constrain std::expected equality operators

yronglin · yronglin · commit f32488c9f267 · 2025-04-15T15:54:07.000+08:00
Signed-off-by: yronglin &lt;yronglin777@gmail.com&gt;
diff --git 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)","","",""
 "`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)","","",""
 "`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
@@ -10,6 +10,7 @@
 #define _LIBCPP___EXPECTED_EXPECTED_H
 
 #include <__assert>
+#include <__concepts/boolean_testable.h>
 #include <__config>
 #include <__expected/bad_expected_access.h>
 #include <__expected/unexpect.h>
@@ -1139,8 +1140,12 @@ class expected : private __expected_base<_Tp, _Err> {
 
   // [expected.object.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)
+    requires(!is_void_v<_T2>) && requires {
+      { *__x == *__y } -> __boolean_testable;
+      { __x.error() == __y.error() } -> __boolean_testable;
+    }
+  {
     if (__x.__has_val() != __y.__has_val()) {
       return false;
     } else {
@@ -1153,12 +1158,20 @@ class expected : private __expected_base<_Tp, _Err> {
   }
 
   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)
+    requires(!__is_std_expected<_T2>::value) && requires {
+      { *__x == __v } -> __boolean_testable;
+    }
+  {
     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)
+    requires requires {
+      { __x.error() == __e.error() } -> __boolean_testable;
+    }
+  {
     return !__x.__has_val() && static_cast<bool>(__x.__unex() == __e.error());
   }
 };
@@ -1851,7 +1864,11 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
   // [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)
+    requires requires {
+      { __x.error() == __y.error() } -> __boolean_testable;
+    }
+  {
     if (__x.__has_val() != __y.__has_val()) {
       return false;
     } else {
@@ -1860,7 +1877,11 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
   }
 
   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)
+    requires requires {
+      { __x.error() == __y.error() } -> __boolean_testable;
+    }
+  {
     return !__x.__has_val() && static_cast<bool>(__x.__unex() == __y.error());
   }
 };
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
@@ -17,18 +17,17 @@
 #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; }
-};
+// 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>);
 
 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 +36,7 @@ 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
@@ -18,19 +18,21 @@
 #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>>);
+// 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>>);
 
 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
@@ -17,18 +17,17 @@
 #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; }
-};
+// 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>>);
 
 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 +36,7 @@ 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
@@ -18,19 +18,21 @@
 #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>);
 
 static_assert(CanCompare<std::expected<void, int>, std::expected<void, int>>);
 static_assert(CanCompare<std::expected<void, int>, std::expected<void, short>>);
 
-// Note this is true because other overloads in expected<non-void> are unconstrained
-static_assert(CanCompare<std::expected<void, int>, std::expected<int, int>>);
+// 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>>);
 
 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
@@ -17,18 +17,17 @@
 #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; }
-};
+// 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>>);
 
 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 +36,7 @@ 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
@@ -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
