[gcc r10-9904] libstdc++: Fix access issue in elements_view::_Sentinel [PR100631]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:b1f54b2b7d876c72ab05afb0df214aba205f94ad

commit r10-9904-gb1f54b2b7d876c72ab05afb0df214aba205f94ad
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue May 18 10:21:27 2021 -0400

    libstdc++: Fix access issue in elements_view::_Sentinel [PR100631]
    
    In the earlier commit r12-854 I forgot to also rewrite the other operator-
    overload in terms of the split-out member function _M_distance_from.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100631
            * include/std/ranges (elements_view::_Sentinel::operator-): Use
            _M_distance_from in the other operator- overload too.
            * testsuite/std/ranges/adaptors/elements.cc (test06): Augment test.
    
    (cherry picked from commit 38751c4d5e15bd1c682ac4c868a2c4ce48ca5503)

Diff:
---
 libstdc++-v3/include/std/ranges                        | 2 +-
 libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc | 3 ++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 27bbbc8cec4..5f67f830174 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3768,7 +3768,7 @@ namespace views
 	    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
 	    friend constexpr range_difference_t<_Base2>
 	    operator-(const _Iterator<_Const2>& __x, const _Sentinel& __y)
-	    { return __x._M_current - __y._M_end; }
+	    { return -__y._M_distance_from(__x); }
 
 	  template<bool _Const2,
 		   typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
index c14f5b9fca3..1ad05361d00 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
@@ -120,7 +120,8 @@ test06()
     | views::keys;
   auto b = r.begin();
   auto e = r.end();
-  e - b;
+  VERIFY( e - b == 42 );
+  VERIFY( b - e == -42 );
 }
 
 void