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

yronglin · 2025-04-15T07:55:59Z

Closes #118135

This PR implement P3379R0 Constrain std::expected equality operators.

llvmbot · 2025-04-15T07:56:34Z

@llvm/pr-subscribers-libcxx

Author: None (yronglin)

Changes

This PR implement P3379R0 Constrain std::expected equality operators.

Full diff: https://github.com/llvm/llvm-project/pull/135759.diff

8 Files Affected:

(modified) libcxx/docs/Status/Cxx2cPapers.csv (+1-1)
(modified) libcxx/include/__expected/expected.h (+27-6)
(modified) libcxx/test/std/utilities/expected/expected.expected/equality/equality.T2.pass.cpp (+7-8)
(modified) libcxx/test/std/utilities/expected/expected.expected/equality/equality.other_expected.pass.cpp (+9-7)
(modified) libcxx/test/std/utilities/expected/expected.expected/equality/equality.unexpected.pass.cpp (+7-8)
(modified) libcxx/test/std/utilities/expected/expected.void/equality/equality.other_expected.pass.cpp (+8-6)
(modified) libcxx/test/std/utilities/expected/expected.void/equality/equality.unexpected.pass.cpp (+7-8)
(modified) libcxx/test/std/utilities/expected/types.h (+13)

diff --git a/libcxx/docs/Status/Cxx2cPapers.csv b/libcxx/docs/Status/Cxx2cPapers.csv
index b40d9eb58c24e..be591c9b6a587 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)","","",""
 "`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
index 03bbd1623ed5c..f8af6429c05ae 100644
--- 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
index bc8b9de97e4d2..2959834a85dd1 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 +17,17 @@
 #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
index 9325c6c61ad2d..e2da668728e0d 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
@@ -18,19 +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
index a8c469d01be28..cd2db0efc48b1 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 +17,17 @@
 #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
index 8b24875586852..224cbc610e78b 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 +26,13 @@ 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
index 4500971131b65..4d9afaf24e3a6 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 +17,17 @@
 #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
index df73ebdfe495e..11473ca3d97de 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

libcxx/docs/Status/Cxx2cPapers.csv

Zingam · 2025-04-15T20:36:15Z

@yronglin A few comments:

We now use GitHub issues to track implementation: P3379R0: Constrain std::expected equality operators #118135 You can use GitHub syntax to associte PR to the issue https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue
If you are interested in implementing the rest of P2944R3: Comparisons for reference_wrapper #105424 please feel to do so.
I haven't thought about it much: would it make sense to generalize and reuse the test data between the types?

yronglin · 2025-04-15T23:45:57Z

Thanks for your review!

We now use GitHub issues to track implementation: P3379R0: Constrain std::expected equality operators #118135 You can use GitHub syntax to associte PR to the issue https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue

Thanks for reminding me, I didn't aware the change in the process before.

If you are interested in implementing the rest of P2944R3: Comparisons for reference_wrapper #105424 please feel to do so.

Yes, I'd like to implement the rest of P2944R3.

I haven't thought about it much: would it make sense to generalize and reuse the test data between the types?

I reuse the test data between the types because it's very similar, but I have no preference. Should we revert the reuses things?

libcxx/include/__expected/expected.h

Signed-off-by: yronglin <yronglin777@gmail.com>

…26 feature Signed-off-by: yronglin <yronglin777@gmail.com>

libcxx/include/__cxx03/module.modulemap

Signed-off-by: yronglin <yronglin777@gmail.com>

H-G-Hristov · 2025-04-22T18:04:11Z

@yronglin Please add

Closes #118135

to the top comment in the PR to link this PR to the GitHub issue as @frederick-vs-ja suggested above.

libcxx/include/__concepts/core_convertible_to.h

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

…tible.h Signed-off-by: yronglin <yronglin777@gmail.com>

github-actions · 2025-04-24T14:17:47Z

✅ With the latest revision this PR passed the C/C++ code formatter.

Signed-off-by: yronglin <yronglin777@gmail.com>

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

Signed-off-by: yronglin <yronglin777@gmail.com>

libcxx/test/std/utilities/expected/expected.void/equality/equality.unexpected.pass.cpp

Signed-off-by: yronglin <yronglin777@gmail.com>

yronglin requested a review from a team as a code owner April 15, 2025 07:56

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Apr 15, 2025

yronglin self-assigned this Apr 15, 2025

yronglin changed the title ~~[libc++] Implement P3379R0 Constrain std::expected equality operators~~ [libc++] Implement P3379R0 Constrain std::expected equality operators Apr 15, 2025

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H-G-Hristov mentioned this pull request Apr 22, 2025

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

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yronglin added 2 commits April 22, 2025 22:37

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

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Signed-off-by: yronglin <yronglin777@gmail.com>

[libc++] Introduce __core_convertible_to and make this paper as a C++…

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…26 feature Signed-off-by: yronglin <yronglin777@gmail.com>

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Merge remote-tracking branch 'llorg/main' into P3379R0

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yronglin and others added 2 commits April 23, 2025 22:52

Update libcxx/include/__concepts/core_convertible_to.h

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[libc++] Move __core_convertible_to into __type_traits/is_core_conver…

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…tible.h Signed-off-by: yronglin <yronglin777@gmail.com>

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[NFC][libc++] Format

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[libc++] Fix build errors

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[libc++] Remove __concepts/core_convertible_to.h in BUILD.gn

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

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

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

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