[PATCH] libstdc++: Fix access issue in iota_view::_Sentinel [PR100690]

[Patrick Palka via Gcc-patches]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk/11/10?

libstdc++-v3/ChangeLog:

        PR libstdc++/100690
        * include/std/ranges (iota_view::_Sentinel::_M_distance_from):
        Split out into this member function from ...
        (iota_view::_Sentinel::operator-): ... here, for sake of access
        checking.
        * testsuite/std/ranges/iota/iota_view.cc:
---
 libstdc++-v3/include/std/ranges                     |  8 ++++++--
 libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc | 11 +++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 704f924557c..76add252ca6 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -499,6 +499,10 @@ namespace ranges
        _M_equal(const _Iterator& __x) const
        { return __x._M_value == _M_bound; }
 
+       constexpr auto
+       _M_distance_from(const _Iterator& __x) const
+       { return _M_bound - __x._M_value; }
+
        _Bound _M_bound = _Bound();
 
       public:
@@ -515,12 +519,12 @@ namespace ranges
        friend constexpr iter_difference_t<_Winc>
        operator-(const _Iterator& __x, const _Sentinel& __y)
          requires sized_sentinel_for<_Bound, _Winc>
-       { return __x._M_value - __y._M_bound; }
+       { return -__y._M_distance_from(__x); }
 
        friend constexpr iter_difference_t<_Winc>
        operator-(const _Sentinel& __x, const _Iterator& __y)
          requires sized_sentinel_for<_Bound, _Winc>
-       { return -(__y - __x); }
+       { return __x._M_distance_from(__y); }
       };
 
       _Winc _M_value = _Winc();
diff --git a/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc 
b/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
index be8695120ad..362ef1f7f78 100644
--- a/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
+++ b/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
@@ -80,6 +80,16 @@ test04()
 // Verify we optimize away the 'bound' data member of an unbounded iota_view.
 static_assert(sizeof(std::ranges::iota_view<char>) == 1);
 
+void
+test05()
+{
+  // PR libstdc++/100690
+  int x[] = {42, 42, 42};
+  auto r = std::views::iota(std::ranges::begin(x), std::ranges::cbegin(x) + 3);
+  VERIFY( r.end() - r.begin() == 3 );
+  VERIFY( r.begin() - r.end() == -3 );
+}
+
 int
 main()
 {
@@ -87,4 +97,5 @@ main()
   test02();
   test03();
   test04();
+  test05();
 }
-- 
2.32.0.rc0