[PATCH] libstdc++: Fix access issues in elements_view::_Sentinel [PR100631]

[Patrick Palka]

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

libstdc++-v3/ChangeLog:

	PR libstdc++/100631
	* include/std/ranges (elements_view::_Iterator): Befriend
	_Sentinel<!_Const>.
	(elements_view::_Sentinel::_M_equal): Templatize.
	(elements_view::_Sentinel::_M_distance_from): Split out from ...
	(elements_view::_Sentinel::operator-): Here.
	* testsuite/std/ranges/adaptors/elements.cc (test06, test07):
	New tests.
---
 libstdc++-v3/include/std/ranges               | 15 ++++++---
 .../testsuite/std/ranges/adaptors/elements.cc | 31 +++++++++++++++++++
 2 files changed, 42 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index fe6379fb858..bf52074ca05 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3749,15 +3749,22 @@ namespace views::__adaptor
 	  { return __x._M_current - __y._M_current; }
 
 	  friend _Sentinel<_Const>;
+	  friend _Sentinel<!_Const>;
 	};
 
       template<bool _Const>
 	struct _Sentinel
 	{
 	private:
-	  constexpr bool
-	  _M_equal(const _Iterator<_Const>& __x) const
-	  { return __x._M_current == _M_end; }
+	  template<bool _Const2>
+	    constexpr bool
+	    _M_equal(const _Iterator<_Const2>& __x) const
+	    { return __x._M_current == _M_end; }
+
+	  template<bool _Const2>
+	    constexpr auto
+	    _M_distance_from(const _Iterator<_Const2>& __i) const
+	    { return _M_end - __i._M_current; }
 
 	  using _Base = elements_view::_Base<_Const>;
 	  sentinel_t<_Base> _M_end = sentinel_t<_Base>();
@@ -3800,7 +3807,7 @@ namespace views::__adaptor
 	    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
 	    friend constexpr range_difference_t<_Base>
 	    operator-(const _Sentinel& __x, const _Iterator<_Const2>& __y)
-	    { return __x._M_end - __y._M_current; }
+	    { return __x._M_distance_from(__y); }
 
 	  friend _Sentinel<!_Const>;
 	};
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
index 134afd6a873..27aba2c0ff0 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
@@ -115,6 +115,35 @@ test05()
   VERIFY( r2[0] == 1 && r2[1] == 3 );
 }
 
+void
+test06()
+{
+  // PR libstdc++/100631
+  auto r = std::views::iota(0)
+    | std::views::filter([](int){ return true; })
+    | std::views::take(42)
+    | std::views::reverse
+    | std::views::transform([](int) { return std::make_pair(42, "hello"); })
+    | std::views::take(42)
+    | std::views::keys;
+  auto b = r.begin();
+  auto e = r.end();
+  e - b;
+}
+
+void
+test07()
+{
+  // PR libstdc++/100631 comment #2
+  auto r = std::views::iota(0)
+    | std::views::transform([](int) { return std::make_pair(42, "hello"); })
+    | std::views::keys;
+  auto b = std::ranges::cbegin(r);
+  auto e = std::end(r);
+  b.base() == e.base();
+  b == e;
+}
+
 int
 main()
 {
@@ -123,4 +152,6 @@ main()
   test03();
   test04();
   test05();
+  test06();
+  test07();
 }
-- 
2.31.1.621.g97eea85a0a