[gcc r11-8541] libstdc++: Fix miscellaneous issues with elements_view::_Sentinel [PR100631]

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

https://gcc.gnu.org/g:fd4db3ddc3639e03f557010b8e0e947e4b51633e

commit r11-8541-gfd4db3ddc3639e03f557010b8e0e947e4b51633e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue May 18 00:26:07 2021 -0400

    libstdc++: Fix miscellaneous issues with elements_view::_Sentinel [PR100631]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100631
            * include/std/ranges (elements_view::_Iterator): Also befriend
            _Sentinel<!_Const>.
            (elements_view::_Sentinel::_M_equal): Templatize.
            (elements_view::_Sentinel::_M_distance_from): Split out from ...
            (elements_view::_Sentinel::operator-): ... here.  Depend on
            _Base2 instead of _Base in the return type.
            * testsuite/std/ranges/adaptors/elements.cc (test06, test07):
            New tests.
    
    (cherry picked from commit 2e2eef80ac0c21f9533e6791ccf5e29458cbb77c)

Diff:
---
 libstdc++-v3/include/std/ranges                    | 18 ++++++++-----
 .../testsuite/std/ranges/adaptors/elements.cc      | 31 ++++++++++++++++++++++
 2 files changed, 43 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 0dd4fb6a9a5..c2ccba942ae 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -3657,16 +3657,22 @@ namespace views::__adaptor
 	    requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
 	  { return __x._M_current - __y._M_current; }
 
-	  friend _Sentinel<_Const>;
+	  template <bool> friend struct _Sentinel;
 	};
 
       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>();
@@ -3707,9 +3713,9 @@ namespace views::__adaptor
 	  template<bool _Const2,
 		   typename _Base2 = __detail::__maybe_const_t<_Const2, _Vp>>
 	    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
-	    friend constexpr range_difference_t<_Base>
+	    friend constexpr range_difference_t<_Base2>
 	    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 a0feb10accf..ac3e14b38af 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/elements.cc
@@ -118,6 +118,35 @@ test05()
   VERIFY( r2[0] == 1 && r2[1] == 3 );
 }
 
+void
+test06()
+{
+  // PR libstdc++/100631
+  auto r = views::iota(0)
+    | views::filter([](int){ return true; })
+    | views::take(42)
+    | views::reverse
+    | views::transform([](int) { return std::make_pair(42, "hello"); })
+    | views::take(42)
+    | views::keys;
+  auto b = r.begin();
+  auto e = r.end();
+  e - b;
+}
+
+void
+test07()
+{
+  // PR libstdc++/100631 comment #2
+  auto r = views::iota(0)
+    | views::transform([](int) { return std::make_pair(42, "hello"); })
+    | views::keys;
+  auto b = ranges::cbegin(r);
+  auto e = ranges::end(r);
+  b.base() == e.base();
+  b == e;
+}
+
 int
 main()
 {
@@ -126,4 +155,6 @@ main()
   test03();
   test04();
   test05();
+  test06();
+  test07();
 }