[libcxx-commits] [libcxx] 2b07bbf - [libc++] Resolve LWG4366: Heterogeneous comparison of `expected` may be ill-formed (#185342)
via libcxx-commits
libcxx-commits at lists.llvm.org
Tue Jun 30 23:49:35 PDT 2026
Author: William Tran-Viet
Date: 2026-07-01T14:49:31+08:00
New Revision: 2b07bbf52cd734b81dd1d8529dc42c76359d5283
URL: https://github.com/llvm/llvm-project/commit/2b07bbf52cd734b81dd1d8529dc42c76359d5283
DIFF: https://github.com/llvm/llvm-project/commit/2b07bbf52cd734b81dd1d8529dc42c76359d5283.diff
LOG: [libc++] Resolve LWG4366: Heterogeneous comparison of `expected` may be ill-formed (#185342)
Resolves #171362
- 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
---------
Co-authored-by: A. Jiang <de34 at live.cn>
Added:
Modified:
libcxx/docs/Status/Cxx26Issues.csv
libcxx/include/__expected/expected.h
libcxx/test/std/utilities/expected/expected.expected/equality/equality.T2.pass.cpp
libcxx/test/std/utilities/expected/expected.expected/equality/equality.unexpected.pass.cpp
libcxx/test/support/test_comparisons.h
Removed:
################################################################################
diff --git a/libcxx/docs/Status/Cxx26Issues.csv b/libcxx/docs/Status/Cxx26Issues.csv
index 1ea6f3ce5bec1..10cad5d52e2f1 100644
--- a/libcxx/docs/Status/Cxx26Issues.csv
+++ b/libcxx/docs/Status/Cxx26Issues.csv
@@ -201,7 +201,7 @@
"`LWG4351 <https://wg21.link/LWG4351>`__","``integral-constant-like`` needs more ``remove_cvref_t``","2025-11 (Kona)","|Complete|","23","`#171359 <https://github.com/llvm/llvm-project/issues/171359>`__",""
"`LWG4358 <https://wg21.link/LWG4358>`__","ยง[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|","23","`#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 32ed81a392702..9b0e0322106ba 100644
--- a/libcxx/include/__expected/expected.h
+++ b/libcxx/include/__expected/expected.h
@@ -446,6 +446,10 @@ class __expected_base {
_LIBCPP_NO_UNIQUE_ADDRESS __conditional_no_unique_address<__allow_reusing_expected_tail_padding, __repr> __repr_;
};
+// Helper to handle comparisons that produce a value whose type is not bool,
+// but allows only implicit (and not explicit) conversions to bool.
+constexpr bool __into_bool(bool __b) noexcept { return __b; }
+
template <class _Tp, class _Err>
class expected : private __expected_base<_Tp, _Err> {
static_assert(!is_reference_v<_Tp> && !is_function_v<_Tp> && !is_same_v<remove_cv_t<_Tp>, in_place_t> &&
@@ -1171,7 +1175,7 @@ class expected : private __expected_base<_Tp, _Err> {
}
# endif
{
- return __x.__has_val() && static_cast<bool>(__x.__val() == __v);
+ return __x.__has_val() && std::__into_bool(__x.__val() == __v);
}
template <class _E2>
@@ -1182,7 +1186,7 @@ class expected : private __expected_base<_Tp, _Err> {
}
# endif
{
- return !__x.__has_val() && static_cast<bool>(__x.__unex() == __e.error());
+ return !__x.__has_val() && std::__into_bool(__x.__unex() == __e.error());
}
};
@@ -1884,7 +1888,7 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
if (__x.__has_val() != __y.__has_val()) {
return false;
} else {
- return __x.__has_val() || static_cast<bool>(__x.__unex() == __y.__unex());
+ return __x.__has_val() || std::__into_bool(__x.__unex() == __y.__unex());
}
}
@@ -1896,7 +1900,7 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
}
# endif
{
- return !__x.__has_val() && static_cast<bool>(__x.__unex() == __y.error());
+ return !__x.__has_val() && std::__into_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 6a57dc4643354..e0d7457808b8d 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
@@ -54,6 +54,18 @@ constexpr bool test() {
assert(p1 == p2);
}
#endif
+ // 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
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