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

Some of the range adaptors have distinct constant and non-constant
iterator/sentinel types, along with converting constructors that can convert a
non-constant iterator/sentinel to a constant iterator/sentinel.  This patch adds
the missing appropriate friend declarations in order to make these converting
constructors well formed.

Strictly speaking it seems the friendship relations don't need to go both ways
-- we could get away with declaring e.g. friend _Iterator<false>; instead of
friend _Iterator<!_Const>; but both reference implementations seem to use the
latter symmetric form anyway.

libstdc++-v3/ChangeLog:

	* include/std/ranges (transform_view::_Iterator<_Const>): Befriend
	_Iterator<!_Const>.
	(transform_view::_Sentinel<_Const>): Befriend _Sentinel<!_Const>.
	(take_view::_Sentinel<_Const>): Likewise.
	(take_while_view::_Sentinel<_Const>): Likewise.
	(split_view::_OuterIter<_Const>): Befriend _OuterIter<!_Const>.
	* testsuite/std/ranges/adaptors/split.cc: Augment test.
	* testsuite/std/ranges/adaptors/take.cc: Augment test.
	* testsuite/std/ranges/adaptors/take_while.cc: Augment test.
	* testsuite/std/ranges/adaptors/transform.cc: Augment test.

libstdc++-v3/ChangeLog

 2020-02-27  Patrick Palka  <ppalka@redhat.com>
 
+	* include/std/ranges (transform_view::_Iterator<_Const>): Befriend
+	_Iterator<!_Const>.
+	(transform_view::_Sentinel<_Const>): Befriend _Sentinel<!_Const>.
+	(take_view::_Sentinel<_Const>): Likewise.
+	(take_while_view::_Sentinel<_Const>): Likewise.
+	(split_view::_OuterIter<_Const>): Befriend _OuterIter<!_Const>.
+	* testsuite/std/ranges/adaptors/split.cc: Augment test.
+	* testsuite/std/ranges/adaptors/take.cc: Augment test.
+	* testsuite/std/ranges/adaptors/take_while.cc: Augment test.
+	* testsuite/std/ranges/adaptors/transform.cc: Augment test.
+
 	* testsuite/25_algorithms/copy/constrained.cc: Don't assume that the
 	base() of a vector<>::iterator is a pointer.
 	* testsuite/25_algorithms/copy_backward/constrained.cc: Likewise.

libstdc++-v3/include/std/ranges

@@ -1759,6 +1759,7 @@ namespace views
 	    requires indirectly_swappable<_Base_iter>
 	  { return ranges::iter_swap(__x._M_current, __y._M_current); }
 
+	  friend _Iterator<!_Const>;
 	  friend _Sentinel<_Const>;
 	};
 
@@ -1811,6 +1812,8 @@ 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();
@@ -1916,6 +1919,8 @@ 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();
@@ -2055,6 +2060,8 @@ 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();
@@ -2730,6 +2737,7 @@ namespace views
 	  operator==(const _OuterIter& __x, default_sentinel_t)
 	  { return __x.__at_end(); };
 
+	  friend _OuterIter<!_Const>;
 	  friend _InnerIter<_Const>;
 	};
 

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();
 }

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();
 }

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();
 }

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();
 }