[gcc r16-3417] libstdc++: Use _M_reverse to reverse partial_ordering using operator<=>

[Tomasz Kaminski]

https://gcc.gnu.org/g:b3038e1ace2a4e521763ac872c8265b7c7142252

commit r16-3417-gb3038e1ace2a4e521763ac872c8265b7c7142252
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Wed Aug 27 16:43:16 2025 +0200

    libstdc++: Use _M_reverse to reverse partial_ordering using operator<=>
    
    The patch r16-3414-gfcb3009a32dc33 changed the representation of unordered to
    optimize reversing of order, but it did not update implementation of reversing
    operator<=>(0, partial_order).
    
    libstdc++-v3/ChangeLog:
    
            * libsupc++/compare
            (operator<=>(__cmp_cat::__unspec, partial_ordering)):
            Implement using _M_reverse.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/libsupc++/compare | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index 2624fa9144fb..ef0f03769646 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -155,12 +155,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
     [[nodiscard]]
     friend constexpr partial_ordering
     operator<=>(__cmp_cat::__unspec, partial_ordering __v) noexcept
-    {
-      if (__v._M_value & 1)
-	return partial_ordering(__cmp_cat::_Ord(-__v._M_value));
-      else
-	return __v;
-    }
+    { return partial_ordering(__cmp_cat::_Ncmp(__v._M_reverse())); }
   };
 
   // valid values' definitions