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 iterators such as back_insert_iterator and
front_insert_iterator whose value_type is defined to be void do not have an
iter_value_t according to [readable.traits] p4.  The entire __use_memmove
condition can probably be refactored, 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.
---
 libstdc++-v3/include/bits/ranges_algobase.h   |  4 +-
 libstdc++-v3/include/bits/stl_iterator.h      | 22 ++++++++++
 .../back_insert_iterator/pr93884.C            | 44 +++++++++++++++++++
 .../front_insert_iterator/pr93884.C           | 44 +++++++++++++++++++
 4 files changed, 112 insertions(+), 2 deletions(-)
 create mode 100644 
libstdc++-v3/testsuite/24_iterators/back_insert_iterator/pr93884.C
 create mode 100644 
libstdc++-v3/testsuite/24_iterators/front_insert_iterator/pr93884.C

diff --git a/libstdc++-v3/include/bits/ranges_algobase.h 
b/libstdc++-v3/include/bits/ranges_algobase.h
index 807822e99c8..739424e1a2d 100644
--- a/libstdc++-v3/include/bits/ranges_algobase.h
+++ b/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 = 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 = iterator_traits<_Out>::value_type;
          constexpr bool __use_memmove
            = (is_trivially_copyable_v<_ValueTypeI>
               && is_same_v<_ValueTypeI, _ValueTypeO>
diff --git a/libstdc++-v3/include/bits/stl_iterator.h 
b/libstdc++-v3/include/bits/stl_iterator.h
index 372df223113..0f2742ae4cf 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/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 = nullptr;
+#else
       _Container* container;
+#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)
diff --git a/libstdc++-v3/testsuite/24_iterators/back_insert_iterator/pr93884.C 
b/libstdc++-v3/testsuite/24_iterators/back_insert_iterator/pr93884.C
new file mode 100644
index 00000000000..5a6287ddd40
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/back_insert_iterator/pr93884.C
@@ -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();
+}
diff --git 
a/libstdc++-v3/testsuite/24_iterators/front_insert_iterator/pr93884.C 
b/libstdc++-v3/testsuite/24_iterators/front_insert_iterator/pr93884.C
new file mode 100644
index 00000000000..ee3d9ac4ed9
--- /dev/null
+++ b/libstdc++-v3/testsuite/24_iterators/front_insert_iterator/pr93884.C
@@ -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, std::front_inserter(v));
+  VERIFY( ranges::equal(v | views::reverse, i) );
+}
+
+int
+main()
+{
+  test01();
+}
-- 
2.25.1.291.ge68e29171c

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