[libcxx-commits] [libcxx] [libc++] Resolve LWG4366: Heterogeneous comparison of `expected` may be ill-formed (PR #185342)

[William Tran-Viet via libcxx-commits]

https://github.com/smallp-o-p created https://github.com/llvm/llvm-project/pull/185342

- Implement proposed resolution
- Refactor `operator==` code to be more in line with the standard as the current way was making an explicit `bool()` conversion in the `x.meow() == y.meow()` cases
- Add test cases
- Update issues paper

>From 748e9c28dc0e0f2f377fd4c4985cf397570dbf3b Mon Sep 17 00:00:00 2001
From: William Tran-Viet <wtranviet at proton.me>
Date: Sun, 8 Mar 2026 22:54:08 -0400
Subject: [PATCH] Resolve LWG4366

---
 libcxx/docs/Status/Cxx2cIssues.csv            |  2 +-
 libcxx/include/__expected/expected.h          | 16 +++++++++++----
 .../equality/equality.T2.pass.cpp             | 13 ++++++++++++
 .../equality/equality.unexpected.pass.cpp     | 20 +++++++++++++++++++
 libcxx/test/support/test_comparisons.h        | 17 ++++++++++++++++
 5 files changed, 63 insertions(+), 5 deletions(-)

diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 60b1bd6ff70da..008c594d8dd1d 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -201,7 +201,7 @@
 "`LWG4351 <https://wg21.link/LWG4351>`__","``integral-constant-like`` needs more ``remove_cvref_t``","2025-11 (Kona)","","","`#171359 <https://github.com/llvm/llvm-project/issues/171359>`__",""
 "`LWG4358 <https://wg21.link/LWG4358>`__","§[exec.as.awaitable] is using ""Preconditions:"" when it should probably be described in the constraint","2025-11 (Kona)","","","`#171360 <https://github.com/llvm/llvm-project/issues/171360>`__",""
 "`LWG4360 <https://wg21.link/LWG4360>`__","``awaitable-sender`` concept should qualify use of ``awaitable-receiver`` type","2025-11 (Kona)","","","`#171361 <https://github.com/llvm/llvm-project/issues/171361>`__",""
-"`LWG4366 <https://wg21.link/LWG4366>`__","Heterogeneous comparison of ``expected`` may be ill-formed","2025-11 (Kona)","","","`#171362 <https://github.com/llvm/llvm-project/issues/171362>`__",""
+"`LWG4366 <https://wg21.link/LWG4366>`__","Heterogeneous comparison of ``expected`` may be ill-formed","2025-11 (Kona)","|Complete|","22","`#171362 <https://github.com/llvm/llvm-project/issues/171362>`__",""
 "`LWG4369 <https://wg21.link/LWG4369>`__","``check-types`` function for ``upon_error`` and ``upon_stopped`` is wrong","2025-11 (Kona)","","","`#171363 <https://github.com/llvm/llvm-project/issues/171363>`__",""
 "`LWG4370 <https://wg21.link/LWG4370>`__","Comparison of ``optional<T>`` to ``T`` may be ill-formed","2025-11 (Kona)","|Complete|","22","`#171364 <https://github.com/llvm/llvm-project/issues/171364>`__",""
 "`LWG4372 <https://wg21.link/LWG4372>`__","Weaken *Mandates:* for dynamic padding values in padded layouts","2025-11 (Kona)","","","`#171365 <https://github.com/llvm/llvm-project/issues/171365>`__",""
diff --git a/libcxx/include/__expected/expected.h b/libcxx/include/__expected/expected.h
index 24ae33d4e3af8..fc451b6511200 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -1169,7 +1169,9 @@ class expected : private __expected_base<_Tp, _Err> {
     }
 #  endif
   {
-    return __x.__has_val() && static_cast<bool>(__x.__val() == __v);
+    if (__x.__has_val())
+      return __x.__val() == __v;
+    return false;
   }
 
   template <class _E2>
@@ -1180,7 +1182,9 @@ class expected : private __expected_base<_Tp, _Err> {
     }
 #  endif
   {
-    return !__x.__has_val() && static_cast<bool>(__x.__unex() == __e.error());
+    if (!__x.__has_val())
+      return __x.__unex() == __e.error();
+    return false;
   }
 };
 
@@ -1881,8 +1885,10 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
   {
     if (__x.__has_val() != __y.__has_val()) {
       return false;
+    } else if (__x.__has_val()) {
+      return true;
     } else {
-      return __x.__has_val() || static_cast<bool>(__x.__unex() == __y.__unex());
+      return __x.__unex() == __y.__unex();
     }
   }
 
@@ -1894,7 +1900,9 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
     }
 #  endif
   {
-    return !__x.__has_val() && static_cast<bool>(__x.__unex() == __y.error());
+    if (!__x.__has_val())
+      return __x.__unex() == __y.error();
+    return false;
   }
 };
 
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 16c6986ae670e..ebd8ebabbd2c8 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
@@ -45,6 +45,19 @@ constexpr bool test() {
     assert(e1 != i3);
   }
 
+  // LWG4366
+  { // x.has_value()
+    const std::expected<ImplicitBool::E1, int> e1(ImplicitBool::E1{1});
+    assert(e1 == ImplicitBool::E2{1});
+    assert(e1 != ImplicitBool::E2{3});
+  }
+
+  { // !x.has_value()
+    const std::expected<ImplicitBool::E1, int> e1(std::unexpect, 1);
+    assert(e1 != ImplicitBool::E2{1});
+    assert(e1 != ImplicitBool::E2{2});
+  }
+
   return true;
 }
 
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 153cbbddf3062..24a6f3d0e906e 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
@@ -45,6 +45,26 @@ constexpr bool test() {
     assert(e1 == un3);
   }
 
+  // LWG4366
+  { // x.has_value()
+    const std::expected<int, ImplicitBool::E1> e1(std::in_place, 1);
+    const std::unexpected<ImplicitBool::E2> u2{ImplicitBool::E2{1}};
+    const std::unexpected<ImplicitBool::E2> u3{ImplicitBool::E2{2}};
+
+    assert(e1 != u2);
+    assert(e1 != u3);
+  }
+
+  { // !x.has_value()
+    const std::unexpected<ImplicitBool::E1> u1{ImplicitBool::E1{1}};
+    const std::unexpected<ImplicitBool::E2> u2{ImplicitBool::E2{1}};
+    const std::unexpected<ImplicitBool::E2> u3{ImplicitBool::E2{2}};
+
+    const std::expected<int, ImplicitBool::E1> e1(u1);
+    assert(e1 == u2);
+    assert(e1 != u3);
+  }
+
   return true;
 }
 
diff --git a/libcxx/test/support/test_comparisons.h b/libcxx/test/support/test_comparisons.h
index e37ab44828c70..9d8bf910a2ebc 100644
--- a/libcxx/test/support/test_comparisons.h
+++ b/libcxx/test/support/test_comparisons.h
@@ -332,6 +332,23 @@ static_assert(HasOperatorLessThanEqual<ThreeWayComparable>);
 static_assert(HasOperatorNotEqual<ThreeWayComparable>);
 static_assert(HasOperatorSpaceship<ThreeWayComparable>);
 
+// LWG4366: Heterogeneous comparison of expected may be ill-formed
+struct ImplicitBool {
+  bool val;
+  constexpr operator bool() const { return val; };
+  constexpr explicit operator bool() = delete;
+
+  struct E1 {
+    int x;
+  };
+
+  struct E2 {
+    int y;
+  };
+};
+
+constexpr ImplicitBool operator==(ImplicitBool::E1 lhs, ImplicitBool::E2 rhs) { return {lhs.x == rhs.y}; }
+
 #endif // TEST_STD_VER >= 20
 
 #endif // TEST_COMPARISONS_H