libstdc++: Add missing friend declarations in some range adaptors

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 2e3e298adcc..2f08cfd7f16 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -1844,6 +1844,7 @@  namespace views
 	    requires indirectly_swappable<_Base_iter>
 	  { return ranges::iter_swap(__x._M_current, __y._M_current); }
 
+	  friend _Iterator<!_Const>;
 	  friend _Sentinel<_Const>;
 	};
 
@@ -1896,6 +1897,8 @@  namespace views
 	  operator-(const _Sentinel& __y, const _Iterator<_Const>& __x)
 	    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base>>
 	  { return __y.__distance_from(__x); }
+
+	  friend _Sentinel<!_Const>;
 	};
 
       _Vp _M_base = _Vp();
@@ -2001,6 +2004,8 @@  namespace views
 
 	  friend constexpr bool operator==(const _CI& __y, const _Sentinel& __x)
 	  { return __y.count() == 0 || __y.base() == __x._M_end; }
+
+	  friend _Sentinel<!_Const>;
 	};
 
       _Vp _M_base = _Vp();
@@ -2140,6 +2145,8 @@  namespace views
 	  friend constexpr bool
 	  operator==(const iterator_t<_Base>& __x, const _Sentinel& __y)
 	  { return __y._M_end == __x || !std::__invoke(*__y._M_pred, *__x); }
+
+	  friend _Sentinel<!_Const>;
 	};
 
       _Vp _M_base = _Vp();
@@ -2831,6 +2838,7 @@  namespace views
 	  operator==(const _OuterIter& __x, default_sentinel_t)
 	  { return __x.__at_end(); };
 
+	  friend _OuterIter<!_Const>;
 	  friend _InnerIter<_Const>;
 	};
 
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/split.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/split.cc
index 52b015cf0c6..e7556725e4f 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/split.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/split.cc
@@ -108,6 +108,19 @@  test05()
 			str | views::filter(not_space_p)) );
 }
 
+void
+test06()
+{
+  std::string str = "hello world";
+  auto v = str | views::transform(std::identity{}) | views::split(' ');
+
+  // Verify that _Iterator<false> is implicitly convertible to _Iterator<true>.
+  static_assert(!std::same_as<decltype(ranges::begin(v)),
+			      decltype(ranges::cbegin(v))>);
+  auto b = ranges::cbegin(v);
+  b = ranges::begin(v);
+}
+
 int
 main()
 {
@@ -116,4 +129,5 @@  main()
   test03();
   test04();
   test05();
+  test06();
 }
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc
index e2d2edbe0a8..c42505b44cb 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/take.cc
@@ -19,6 +19,7 @@ 
 // { dg-do run { target c++2a } }
 
 #include <algorithm>
+#include <forward_list>
 #include <ranges>
 #include <testsuite_hooks.h>
 #include <testsuite_iterators.h>
@@ -85,6 +86,20 @@  test04()
   VERIFY( ranges::equal(v | views::take(5), (int[]){1,2,3}) );
 }
 
+void
+test05()
+{
+  std::forward_list<int> x = {1,2,3,4,5};
+  auto v = x | views::transform(std::negate{}) | views::take(4);
+
+  // Verify that _Sentinel<false> is implicitly convertible to _Sentinel<true>.
+  static_assert(!ranges::common_range<decltype(v)>);
+  static_assert(!std::same_as<decltype(ranges::end(v)),
+			      decltype(ranges::cend(v))>);
+  auto b = ranges::cend(v);
+  b = ranges::end(v);
+}
+
 int
 main()
 {
@@ -92,4 +107,5 @@  main()
   test02();
   test03();
   test04();
+  test05();
 }
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/take_while.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/take_while.cc
index b261ffd1aae..d587127b97e 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/take_while.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/take_while.cc
@@ -19,6 +19,7 @@ 
 // { dg-do run { target c++2a } }
 
 #include <algorithm>
+#include <forward_list>
 #include <ranges>
 #include <testsuite_hooks.h>
 #include <testsuite_iterators.h>
@@ -54,9 +55,25 @@  test02()
   static_assert(ranges::forward_range<R>);
 }
 
+void
+test03()
+{
+  std::forward_list<int> x = {1,2,3,4,5};
+  auto v
+    = x | views::transform(std::negate{}) | views::take_while(std::identity{});
+
+  // Verify that _Sentinel<false> is implicitly convertible to _Sentinel<true>.
+  static_assert(!ranges::common_range<decltype(v)>);
+  static_assert(!std::same_as<decltype(ranges::end(v)),
+			      decltype(ranges::cend(v))>);
+  auto b = ranges::cend(v);
+  b = ranges::end(v);
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
index 178544d6378..c14e36e0cef 100644
--- a/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/transform.cc
@@ -100,6 +100,28 @@  test04()
     }
 }
 
+void
+test05()
+{
+  int x[] = {1,2,3,4,5};
+  auto i = std::counted_iterator(x, 5);
+  auto r = ranges::subrange{i, std::default_sentinel};
+  auto v = r | views::transform(std::negate{});
+
+  // Verify that _Iterator<false> is implicitly convertible to _Iterator<true>.
+  static_assert(!std::same_as<decltype(ranges::begin(v)),
+			      decltype(ranges::cbegin(v))>);
+  auto a = ranges::cbegin(v);
+  a = ranges::begin(v);
+
+  // Verify that _Sentinel<false> is implicitly convertible to _Sentinel<true>.
+  static_assert(!ranges::common_range<decltype(v)>);
+  static_assert(!std::same_as<decltype(ranges::end(v)),
+			      decltype(ranges::cend(v))>);
+  auto b = ranges::cend(v);
+  b = ranges::end(v);
+}
+
 int
 main()
 {
@@ -107,4 +129,5 @@  main()
   test02();
   test03();
   test04();
+  test05();
 }