libstdc++: Add missing bits of P0896R4 pertaining to [back|front]_insert_iterator

This adds some missing pieces of the Ranges TS that make back_insert_iterator and
front_insert_iterator conform to the new output_iterator requirements.

It also fixes a bug in ranges::__copy_or_move and
ranges::__copy_or_move_backward in which we were inspecting the iter_value_t of
the output iterator, but output iterators such as back_insert_iterator and
front_insert_iterator whose value_type = void do not have an iter_value_t
according to [readable.traits] p4.  The entire __use_memmove condition should
probably be rewritten, but the simplest fix for now is to inspect the
iterator_traits of the output iterator instead.

libstdc++-v3/ChangeLog:

	PR libstdc++/93884
	* include/bits/ranges_algobase.h (__copy_or_move,
	__copy_or_move_backward): Don't inspect the iter_value_t of the output
	iterator, instead inspect its iterator_traits directly.
	* include/bits/stl_iterator.h (back_insert_iterator::container):
	Conditionally initialize.
	(back_insert_iterator::difference_type): Conditionally define.
	(back_insert_iterator::back_insert_iterator): Conditionally define this
	default constructor.
	(front_insert_iterator::container): Conditionally initialize.
	(front_insert_iterator::difference_type): Conditionally define.
	(front_insert_iterator::front_insert_iterator): Conditionally define
	this default constructor.
	* 24_iterators/back_insert_iterator/pr93884.cc: New test.
	* 24_iterators/front_insert_iterator/pr93884.cc: New test.

libstdc++-v3/ChangeLog

@@ -1,5 +1,21 @@
 2020-02-24  Patrick Palka  <ppalka@redhat.com>
 
+	PR libstdc++/93884
+	* include/bits/ranges_algobase.h (__copy_or_move,
+	__copy_or_move_backward): Don't inspect the iter_value_t of the output
+	iterator, instead inspect its iterator_traits directly.
+	* include/bits/stl_iterator.h (back_insert_iterator::container):
+	Conditionally initialize.
+	(back_insert_iterator::difference_type): Conditionally define.
+	(back_insert_iterator::back_insert_iterator): Conditionally define this
+	default constructor.
+	(front_insert_iterator::container): Conditionally initialize.
+	(front_insert_iterator::difference_type): Conditionally define.
+	(front_insert_iterator::front_insert_iterator): Conditionally define
+	this default constructor.
+	* 24_iterators/back_insert_iterator/pr93884.cc: New test.
+	* 24_iterators/front_insert_iterator/pr93884.cc: New test.
+
 	P0769R2 Add shift to <algorithm>
 	* include/bits/ranges_algo.h (shift_left, shift_right): New.
 	* testsuite/25_algorithms/shift_left/1.cc: New test.

libstdc++-v3/include/bits/ranges_algobase.h

@@ -249,7 +249,7 @@ namespace ranges
       else if constexpr (sized_sentinel_for<_Sent, _Iter>)
 	{
 	  using _ValueTypeI = iter_value_t<_Iter>;
-	  using _ValueTypeO = iter_value_t<_Out>;
+	  using _ValueTypeO = typename iterator_traits<_Out>::value_type;
 	  constexpr bool __use_memmove
 	    = (is_trivially_copyable_v<_ValueTypeI>
 	       && is_same_v<_ValueTypeI, _ValueTypeO>
@@ -386,7 +386,7 @@ namespace ranges
       else if constexpr (sized_sentinel_for<_Sent, _Iter>)
 	{
 	  using _ValueTypeI = iter_value_t<_Iter>;
-	  using _ValueTypeO = iter_value_t<_Out>;
+	  using _ValueTypeO = typename iterator_traits<_Out>::value_type;
 	  constexpr bool __use_memmove
 	    = (is_trivially_copyable_v<_ValueTypeI>
 	       && is_same_v<_ValueTypeI, _ValueTypeO>

libstdc++-v3/include/bits/stl_iterator.h

@@ -496,11 +496,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public iterator<output_iterator_tag, void, void, void, void>
     {
     protected:
+#if __cplusplus <= 201703L
       _Container* container;
+#else
+      _Container* container = nullptr;
+#endif
 
     public:
       /// A nested typedef for the type of whatever container you used.
       typedef _Container          container_type;
+#if __cplusplus > 201703L
+      using difference_type = ptrdiff_t;
+#endif
+
+#if __cplusplus > 201703L
+      constexpr back_insert_iterator() noexcept = default;
+#endif
 
       /// The only way to create this %iterator is with a container.
       explicit
@@ -588,11 +599,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : public iterator<output_iterator_tag, void, void, void, void>
     {
     protected:
+#if __cplusplus <= 201703L
       _Container* container;
+#else
+      _Container* container = nullptr;
+#endif
 
     public:
       /// A nested typedef for the type of whatever container you used.
       typedef _Container          container_type;
+#if __cplusplus > 201703L
+      using difference_type = ptrdiff_t;
+#endif
+
+#if __cplusplus > 201703L
+      constexpr front_insert_iterator() noexcept = default;
+#endif
 
       /// The only way to create this %iterator is with a container.
       explicit front_insert_iterator(_Container& __x)

libstdc++-v3/testsuite/24_iterators/back_insert_iterator/pr93884.cc

@@ -0,0 +1,44 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <iterator>
+#include <algorithm>
+#include <vector>
+#include <testsuite_hooks.h>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+void
+test01()
+{
+  auto v = std::vector<int>{};
+  auto i = views::iota(0, 10);
+  auto o = std::back_inserter(v);
+  static_assert(std::output_iterator<decltype(o), int>);
+  ranges::copy(i, o);
+  VERIFY( ranges::equal(v, i) );
+}
+
+int
+main()
+{
+  test01();
+}

libstdc++-v3/testsuite/24_iterators/front_insert_iterator/pr93884.cc

@@ -0,0 +1,44 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <iterator>
+#include <algorithm>
+#include <deque>
+#include <testsuite_hooks.h>
+
+namespace ranges = std::ranges;
+namespace views = std::views;
+
+void
+test01()
+{
+  auto v = std::deque<int>{};
+  auto i = views::iota(0, 10);
+  auto o = std::front_inserter(v);
+  static_assert(std::output_iterator<decltype(o), int>);
+  ranges::copy(i, o);
+  VERIFY( ranges::equal(v | views::reverse, i) );
+}
+
+int
+main()
+{
+  test01();
+}