Tested x86_64-linux. Pushed to trunk. Backports likely to follow.
-- >8 --
This is a workaround for a possible compiler bug that causes constraint
recursion in the operator<=>(const optional<T>&, const U&) overload.
libstdc++-v3/ChangeLog:
PR libstdc++/104606
* include/std/optional (operator<=>(const optional<T>&, const U&)):
Reverse order of three_way_comparable_with template arguments.
* testsuite/20_util/optional/relops/104606.cc: New test.
---
libstdc++-v3/include/std/optional | 2 +-
.../20_util/optional/relops/104606.cc | 18 ++++++++++++++++++
2 files changed, 19 insertions(+), 1 deletion(-)
create mode 100644 libstdc++-v3/testsuite/20_util/optional/relops/104606.cc
diff --git a/libstdc++-v3/include/std/optional
b/libstdc++-v3/include/std/optional
index 1e88e8b8880..3507c36a4d8 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1430,7 +1430,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#ifdef __cpp_lib_three_way_comparison
template<typename _Tp, typename _Up>
requires (!__is_optional_v<_Up>)
- && three_way_comparable_with<_Tp, _Up>
+ && three_way_comparable_with<_Up, _Tp>
constexpr compare_three_way_result_t<_Tp, _Up>
operator<=>(const optional<_Tp>& __x, const _Up& __v)
{ return bool(__x) ? *__x <=> __v : strong_ordering::less; }
diff --git a/libstdc++-v3/testsuite/20_util/optional/relops/104606.cc
b/libstdc++-v3/testsuite/20_util/optional/relops/104606.cc
new file mode 100644
index 00000000000..2b8df245219
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/optional/relops/104606.cc
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++17 } }
+
+// Bug 104606 comparison operator resolution with std::optional and -std=c++20
+
+#include <optional>
+#include <variant>
+#include <vector>
+
+struct Value : std::variant<std::vector<Value>> { };
+
+struct Comparator {
+ template <typename T> bool operator<=(const T &) { return true; }
+};
+
+std::optional<Value> o;
+Comparator c;
+
+auto x = c <= o;
--
2.44.0
