[gcc r14-9771] libstdc++: Reverse arguments in constraint for std::optional's <=> [PR104606]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:7f65d8267fbfd19cf21a3dc71d27e989e75044a3

commit r14-9771-g7f65d8267fbfd19cf21a3dc71d27e989e75044a3
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 27 21:51:13 2024 +0000

    libstdc++: Reverse arguments in constraint for std::optional's <=> [PR104606]
    
    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.

Diff:
---
 libstdc++-v3/include/std/optional                      |  2 +-
 .../testsuite/20_util/optional/relops/104606.cc        | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

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;