[gcc r10-9104] libstdc++: Apply proposed resolution for LWG 3449 [PR95322]

[Patrick Palka]

https://gcc.gnu.org/g:be5f22ebe964244223f178330a66b67d3f58334e

commit r10-9104-gbe5f22ebe964244223f178330a66b67d3f58334e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Oct 12 13:46:21 2020 -0400

    libstdc++: Apply proposed resolution for LWG 3449 [PR95322]
    
    Now that the frontend bug PR96805 is fixed, we can cleanly apply the
    proposed resolution for this issue.
    
    This slightly deviates from the proposed resolution by declaring _CI a
    member of take_view instead of take_view::_Sentinel, since it doesn't
    depend on anything within _Sentinel anymore.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/95322
            * include/std/ranges (take_view::_CI): Define this alias
            template as per LWG 3449 and remove ...
            (take_view::_Sentinel::_CI): ... this type alias.
            (take_view::_Sentinel::operator==): Adjust use of _CI
            accordingly.  Define a second overload that accepts an iterator
            of the opposite constness as per LWG 3449.
            (take_while_view::_Sentinel::operator==): Likewise.
            * testsuite/std/ranges/adaptors/95322.cc: Add tests for LWG 3449.
    
    (cherry picked from commit e066821b6f6b7332c7a67981f7b33c9ba0ccaee7)

Diff:
---
 libstdc++-v3/include/std/ranges                    | 23 +++++++++++++++---
 .../testsuite/std/ranges/adaptors/95322.cc         | 28 +++++++++++++++++++++-
 2 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index e2ad6a313c7..6edc4313533 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1994,13 +1994,15 @@ namespace views
     class take_view : public view_interface<take_view<_Vp>>
     {
     private:
+      template<bool _Const>
+	using _CI = counted_iterator<
+	  iterator_t<__detail::__maybe_const_t<_Const, _Vp>>>;
+
       template<bool _Const>
 	struct _Sentinel
 	{
 	private:
 	  using _Base = __detail::__maybe_const_t<_Const, _Vp>;
-	  using _CI = counted_iterator<iterator_t<_Base>>;
-
 	  sentinel_t<_Base> _M_end = sentinel_t<_Base>();
 
 	public:
@@ -2021,7 +2023,15 @@ namespace views
 	  base() const
 	  { return _M_end; }
 
-	  friend constexpr bool operator==(const _CI& __y, const _Sentinel& __x)
+	  friend constexpr bool
+	  operator==(const _CI<_Const>& __y, const _Sentinel& __x)
+	  { return __y.count() == 0 || __y.base() == __x._M_end; }
+
+	  template<bool _OtherConst = !_Const,
+		   typename _Base2 = __detail::__maybe_const_t<_OtherConst, _Vp>>
+	    requires sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
+	  friend constexpr bool
+	  operator==(const _CI<_OtherConst>& __y, const _Sentinel& __x)
 	  { return __y.count() == 0 || __y.base() == __x._M_end; }
 
 	  friend _Sentinel<!_Const>;
@@ -2174,6 +2184,13 @@ namespace views
 	  operator==(const iterator_t<_Base>& __x, const _Sentinel& __y)
 	  { return __y._M_end == __x || !std::__invoke(*__y._M_pred, *__x); }
 
+	  template<bool _OtherConst = !_Const,
+		   typename _Base2 = __detail::__maybe_const_t<_OtherConst, _Vp>>
+	    requires sentinel_for<sentinel_t<_Base>, iterator_t<_Base2>>
+	  friend constexpr bool
+	  operator==(const iterator_t<_Base2>& __x, const _Sentinel& __y)
+	  { return __y._M_end == __x || !std::__invoke(*__y._M_pred, *__x); }
+
 	  friend _Sentinel<!_Const>;
 	};
 
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc
index 67bc7d33917..41785a0a8fa 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/95322.cc
@@ -26,7 +26,7 @@ using __gnu_test::test_forward_range;
 void
 test01()
 {
-  // PR libstdc++/95322 and LWG 3488
+  // PR libstdc++/95322 and LWG 3448
   int a[2]{1, 2};
   test_forward_range<int> v{a};
   auto view1 = v | std::views::take(2);
@@ -51,8 +51,34 @@ test02()
   VERIFY( !eq );
 }
 
+void
+test03()
+{
+  // LWG 3449, for take_view
+  int a[2]{1, 2};
+  test_forward_range<int> v{a};
+  auto view1 = v | std::views::transform(std::identity{});
+  auto view2 = view1 | std::views::take(2);
+  const bool eq = std::ranges::cbegin(view2) == std::ranges::end(view2);
+  VERIFY( !eq );
+}
+
+void
+test04()
+{
+  // LWG 3449, for take_while_view
+  int a[2]{1, 2};
+  test_forward_range<int> v{a};
+  auto view1 = v | std::views::transform(std::identity{});
+  auto view2 = view1 | std::views::take_while([] (int i) { return true; });
+  const bool eq = std::ranges::cbegin(view2) == std::ranges::end(view2);
+  VERIFY( !eq );
+}
+
 int main()
 {
   test01();
   test02();
+  test03();
+  test04();
 }