diff --git a/libcxx/docs/OneRangesProposalStatus.csv b/libcxx/docs/OneRangesProposalStatus.csv
--- a/libcxx/docs/OneRangesProposalStatus.csv
+++ b/libcxx/docs/OneRangesProposalStatus.csv
@@ -77,7 +77,7 @@
 [move.sentinel],,[predef.iterators],Unassigned,Not started
 [common.iterator],,"| [iterator.concepts]
 | [iterator.cust.swap]
-| [iterator.cust.move]",Zoe Carver,Not started
+| [iterator.cust.move]",Zoe Carver,`D103335 <https://llvm.org/D103335>`_,✅
 [default.sentinels],std::default_sentinel_t.,No dependencies,Unassigned,Not started
 [counted.iterator],,"| [iterator.concepts]
 | [iterator.cust.swap]
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -132,6 +132,7 @@
   __hash_table
   __iterator/advance.h
   __iterator/back_insert_iterator.h
+  __iterator/common_iterator.h
   __iterator/concepts.h
   __iterator/default_sentinel.h
   __iterator/front_insert_iterator.h
diff --git a/libcxx/include/__iterator/common_iterator.h b/libcxx/include/__iterator/common_iterator.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__iterator/common_iterator.h
@@ -0,0 +1,217 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ITERATOR_COMMON_ITERATOR_H
+#define _LIBCPP___ITERATOR_COMMON_ITERATOR_H
+
+#include <__config>
+#include <__iterator/concepts.h>
+#include <__iterator/iter_swap.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/readable_traits.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+
+template<input_or_output_iterator _Iter, sentinel_for<_Iter> _Sent>
+  requires (!same_as<_Iter, _Sent> && copyable<_Iter>)
+class common_iterator {
+  union _Hold {
+    _Iter __iter; // __index == 0;
+    _Sent __sent; // __index == 1;
+
+    _Hold() = default;
+    constexpr _Hold(_Iter __iter) : __iter(_VSTD::move(__iter)) {}
+    constexpr _Hold(_Sent __sent) : __sent(_VSTD::move(__sent)) {}
+  } __u;
+  bool __index;
+
+  class __proxy {
+    friend common_iterator;
+
+    iter_value_t<_Iter> __value;
+    constexpr __proxy(iter_reference_t<_Iter>&& __x)
+      : __value(__x) {}
+
+  public:
+    constexpr const iter_value_t<_Iter>* operator->() const {
+      return _VSTD::addressof(__value);
+    }
+  };
+
+  class __postfix_proxy {
+    friend common_iterator;
+
+    iter_value_t<_Iter> __value;
+    constexpr __postfix_proxy(iter_reference_t<_Iter>&& __x)
+      : __value(__x) {}
+
+  public:
+    constexpr const iter_value_t<_Iter>& operator*() const {
+      return __value;
+    }
+  };
+
+public:
+  common_iterator() requires default_initializable<_Iter> = default;
+
+  constexpr common_iterator(_Iter __i) : __u(_VSTD::move(__i)), __index(false) {}
+  constexpr common_iterator(_Sent __s) : __u(_VSTD::move(__s)), __index(true ) {}
+
+  template<class _I2, class _S2>
+    requires convertible_to<const _I2, _Iter> && convertible_to<const _S2&, _Sent>
+  constexpr common_iterator(const common_iterator<_I2, _S2>& __other)
+    : __u(__other.__u), __index(__other.__index) {}
+
+  template<class _I2, class _S2>
+    requires convertible_to<const _I2, _Iter> && convertible_to<const _S2&, _Sent> &&
+             assignable_from<const _I2, _Iter> && assignable_from<const _S2&, _Sent>
+  constexpr common_iterator& operator=(const common_iterator<_I2, _S2>& __other) {
+    __u = __other.__u;
+    __index = __other.__index;
+  }
+
+  constexpr decltype(auto) operator*() { return *__u.__iter; }
+  constexpr decltype(auto) operator*() const
+    requires __dereferenceable<const _Iter>
+  { return *__u.__iter; }
+
+  constexpr decltype(auto) operator->() const
+    requires indirectly_readable<const _Iter> &&
+    (requires(const _Iter& __i) { __i.operator->(); } ||
+     is_reference_v<iter_reference_t<_Iter>> ||
+     constructible_from<iter_value_t<_Iter>, iter_reference_t<_Iter>>) {
+    if constexpr (is_pointer_v<_Iter> || requires(const _Iter& __i) { __i.operator->(); })    {
+      return __u.__iter;
+    } else if constexpr (is_reference_v<iter_reference_t<_Iter>>) {
+      auto&& __tmp = *__u.__iter;
+      return _VSTD::addressof(__tmp);
+    } else {
+      return __proxy(*__u.__iter);
+    }
+  }
+
+  constexpr common_iterator& operator++() { ++__u.__iter; return *this; }
+  constexpr decltype(auto) operator++(int) {
+    if constexpr (forward_iterator<_Iter>) {
+      auto __tmp = *this;
+      ++*this;
+      return __tmp;
+    } else if constexpr (requires (_Iter& __i) { { *__i++ } -> __referenceable; } ||
+                         !constructible_from<iter_value_t<_Iter>, iter_reference_t<_Iter>>) {
+      return __u.__iter++;
+    } else {
+      __postfix_proxy __p(*this);
+      ++*this;
+      return __p;
+    }
+  }
+
+  template<class _I2, sentinel_for<_Iter> _S2>
+    requires sentinel_for<_Sent, _I2>
+  friend constexpr bool operator==(const common_iterator& __x, const common_iterator<_I2, _S2>& __y) {
+    if (__x.__index == __y.__index)
+      return true;
+
+    if (__x.__index == 0)
+      return __x.__u.__iter == __y.__u.__sent;
+
+    return __x.__u.__sent == __y.__u.__iter;
+  }
+
+  template<class _I2, sentinel_for<_Iter> _S2>
+    requires sentinel_for<_Sent, _I2> && equality_comparable_with<_Iter, _I2>
+  friend constexpr bool operator==(const common_iterator& __x, const common_iterator<_I2, _S2>& __y) {
+    if (__x.__index && __y.__index)
+      return true;
+
+    if (__x.__index == 0 && __y.__index == 0)
+      return __x.__u.__iter == __y.__u.__iter;
+
+    if (__x.__index == 0)
+      return __x.__u.__iter == __y.__u.__sent;
+
+    return __x.__u.__sent == __y.__u.__iter;
+  }
+
+  template<sized_sentinel_for<_Iter> _I2, sized_sentinel_for<_Iter> _S2>
+    requires sized_sentinel_for<_Sent, _I2>
+  friend constexpr iter_difference_t<_I2> operator-(const common_iterator& __x, const common_iterator<_I2, _S2>& __y) {
+    if (__x.__index && __y.__index)
+      return 0;
+
+    if (__x.__index == 0 && __y.__index == 0)
+      return __x.__u.__iter - __y.__u.__iter;
+
+    if (__x.__index == 0)
+      return __x.__u.__iter - __y.__u.__sent;
+
+    return __x.__u.__sent - __y.__u.__iter;
+  }
+
+  friend constexpr iter_rvalue_reference_t<_Iter> iter_move(const common_iterator& __i)
+    noexcept(noexcept(ranges::iter_move(declval<const _Iter&>())))
+      requires input_iterator<_Iter>
+  { return ranges::iter_move(__i.__u.__iter); }
+
+  template<indirectly_swappable<_Iter> _I2, class _S2>
+  friend constexpr void iter_swap(const common_iterator& __x, const common_iterator<_I2, _S2>& __y)
+      noexcept(noexcept(ranges::iter_swap(declval<const _Iter&>(), declval<const _I2&>())))
+  { return ranges::iter_swap(__x.__u.__iter, __y.__u.__iter); }
+};
+
+template<class _Iter, class _Sent>
+struct incrementable_traits<common_iterator<_Iter, _Sent>> {
+  using difference_type = iter_difference_t<_Iter>;
+};
+
+template<class _Iter>
+concept __denotes_forward_iter =
+  requires {
+    typename iterator_traits<_Iter>::iterator_category;
+  } &&
+  derived_from<typename iterator_traits<_Iter>::iterator_category, forward_iterator_tag>;
+
+template<class _Iter, class _Sent>
+concept __common_iter_has_ptr_op = requires(const common_iterator<_Iter, _Sent> __a) {
+  __a.operator->();
+};
+
+template<class _Iter, class _Sent>
+struct iterator_traits<common_iterator<_Iter, _Sent>> {
+  using iterator_concept = conditional_t<forward_iterator<_Iter>,
+                                         forward_iterator_tag,
+                                         input_iterator_tag>;
+  using iterator_category = conditional_t<__denotes_forward_iter<_Iter>,
+                                          forward_iterator_tag,
+                                          input_iterator_tag>;
+  using pointer = conditional_t<__common_iter_has_ptr_op<_Iter, _Sent>,
+                                decltype(declval<const common_iterator<_Iter, _Sent>>().operator->()),
+                                void>;
+  using value_type = iter_value_t<_Iter>;
+  using difference_type = iter_difference_t<_Iter>;
+  using reference = iter_reference_t<_Iter>;
+};
+
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___ITERATOR_COMMON_ITERATOR_H
diff --git a/libcxx/include/iterator b/libcxx/include/iterator
--- a/libcxx/include/iterator
+++ b/libcxx/include/iterator
@@ -136,6 +136,10 @@
 template<class I1, class I2 = I1>
   concept indirectly_swappable = see below;                // since C++20
 
+template<input_or_output_iterator I, sentinel_for<I> S>
+  requires (!same_as<I, S> && copyable<I>)
+class common_iterator;                                     // since C++20
+
 template<class Category, class T, class Distance = ptrdiff_t,
          class Pointer = T*, class Reference = T&>
 struct iterator                                            // deprecated in C++17
@@ -563,6 +567,7 @@
 #include <__functional_base>
 #include <__iterator/advance.h>
 #include <__iterator/back_insert_iterator.h>
+#include <__iterator/common_iterator.h>
 #include <__iterator/concepts.h>
 #include <__iterator/default_sentinel.h>
 #include <__iterator/front_insert_iterator.h>
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/arrow.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/arrow.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/arrow.pass.cpp
@@ -0,0 +1,74 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// decltype(auto) operator->() const
+//   requires see below;
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = simple_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(commonIter1.operator->() == buffer);
+    assert(commonIter2.operator->() == buffer);
+  }
+  {
+    auto iter1 = value_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(*commonIter1.operator->().operator->() == 1);
+    assert(*commonIter2.operator->().operator->() == 1);
+  }
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(commonIter1.operator->().base() == buffer);
+    assert(commonIter2.operator->().base() == buffer);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(commonIter1.operator->().base() == buffer);
+    assert(commonIter2.operator->().base() == buffer);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(commonIter1.operator->().base() == buffer);
+    assert(commonIter2.operator->().base() == buffer);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/assign.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/assign.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/assign.pass.cpp
@@ -0,0 +1,93 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// template<class I2, class S2>
+//   requires convertible_to<const I2&, I> && convertible_to<const S2&, S> &&
+//            assignable_from<I&, const I2&> && assignable_from<S&, const S2&>
+//     common_iterator& operator=(const common_iterator<I2, S2>& x);
+
+#include <iterator>
+#include <ranges>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(cpp17_input_iterator<int*>(buffer + 1));
+
+    assert(*commonIter1 == 1);
+    assert(*commonIter2 == 2);
+    assert(commonIter1 != commonIter2);
+
+    commonIter1 = commonIter2;
+
+    assert(*commonIter1 == 2);
+    assert(*commonIter2 == 2);
+    assert(commonIter1 == commonIter2);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(forward_iterator<int*>(buffer + 1));
+
+    assert(*commonIter1 == 1);
+    assert(*commonIter2 == 2);
+    assert(commonIter1 != commonIter2);
+
+    commonIter1 = commonIter2;
+
+    assert(*commonIter1 == 2);
+    assert(*commonIter2 == 2);
+    assert(commonIter1 == commonIter2);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1 + 1);
+    auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 7});
+
+    assert(*commonIter1 == 1);
+    assert(*commonIter2 == 2);
+    assert(commonIter1 != commonIter2);
+
+    commonIter1 = commonIter2;
+
+    assert(*commonIter1 == 2);
+    assert(*commonIter2 == 2);
+    assert(commonIter1 == commonIter2);
+
+    assert(std::ranges::next(commonIter1, 6) != commonSent1);
+    assert(std::ranges::next(commonIter1, 6) == commonSent2);
+
+    commonSent1 = commonSent2;
+
+    assert(std::ranges::next(commonIter1, 6) == commonSent1);
+    assert(std::ranges::next(commonIter1, 6) == commonSent2);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/ctor.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/ctor.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/ctor.pass.cpp
@@ -0,0 +1,99 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// constexpr common_iterator() requires default_initializable<I> = default;
+// constexpr common_iterator(I i);
+// constexpr common_iterator(S s);
+// template<class I2, class S2>
+//   requires convertible_to<const I2&, I> && convertible_to<const S2&, S>
+//     constexpr common_iterator(const common_iterator<I2, S2>& x);
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+struct ConvertibleToForwardIter {
+  int *ptr;
+
+  constexpr operator forward_iterator<int*>() { return forward_iterator<int*>(ptr); }
+};
+
+template<class I, class S>
+concept ValidCommonIterator = requires {
+  typename std::common_iterator<I, S>;
+};
+
+template<class I, class S, class I2>
+concept ConvCtorEnabled = requires(I2 ci) {
+  std::common_iterator<I, S>(ci);
+};
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  static_assert( std::is_default_constructible_v<std::common_iterator<int*, sentienl_type<int*>>>);
+  static_assert(!std::is_default_constructible_v<std::common_iterator<no_default_constructor<int*>, sentienl_type<int*>>>);
+
+  // Not copyable:
+  static_assert(!ValidCommonIterator<cpp20_input_iterator<int*>, sentienl_type<int*>>);
+  // Same iter and sent:
+  static_assert(!ValidCommonIterator<cpp17_input_iterator<int*>, cpp17_input_iterator<int*>>);
+
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+  }
+
+  // Conversion constructor:
+  {
+    ConvertibleToForwardIter conv{buffer};
+    auto commonIter = std::common_iterator<forward_iterator<int*>, sentienl_type<int*>>(conv);
+    assert(*commonIter == 1);
+
+    static_assert(ConvCtorEnabled<forward_iterator<int*>, sentienl_type<int*>, ConvertibleToForwardIter>);
+    static_assert(!ConvCtorEnabled<forward_iterator<int*>, sentienl_type<int*>, random_access_iterator<int*>>);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/deref.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/deref.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/deref.pass.cpp
@@ -0,0 +1,150 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// decltype(auto) operator*();
+// decltype(auto) operator*() const
+//   requires dereferenceable<const I>;
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = simple_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = simple_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+  }
+  {
+    auto iter1 = value_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = value_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+  }
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = cpp17_input_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = forward_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = random_access_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/eq.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/eq.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/eq.pass.cpp
@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// template<class I2, sentinel_for<I> S2>
+//   requires sentinel_for<S, I2>
+// friend bool operator==(
+//   const common_iterator& x, const common_iterator<I2, S2>& y);
+// template<class I2, sentinel_for<I> S2>
+//   requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
+// friend bool operator==(
+//   const common_iterator& x, const common_iterator<I2, S2>& y);
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = simple_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(commonIter1 != commonSent1);
+    assert(commonIter2 != commonSent2);
+
+    for (auto i = 1; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = value_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(commonIter1 != commonSent1);
+    assert(commonIter2 != commonSent2);
+
+    for (auto i = 1; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(commonIter1 != commonSent1);
+    assert(commonIter2 != commonSent2);
+
+    for (auto i = 1; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(commonIter1 != commonSent1);
+    assert(commonIter2 != commonSent2);
+
+    for (auto i = 1; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(commonIter1 != commonSent1);
+    assert(commonIter2 != commonSent2);
+
+    for (auto i = 1; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/iter_move.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/iter_move.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/iter_move.pass.cpp
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// friend iter_rvalue_reference_t<I> iter_move(const common_iterator& i)
+//   noexcept(noexcept(ranges::iter_move(declval<const I&>())))
+//     requires input_iterator<I>;
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    assert(std::ranges::iter_move(commonIter1) == 1);
+    ASSERT_SAME_TYPE(decltype(std::ranges::iter_move(commonIter1)), int&&);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    assert(std::ranges::iter_move(commonIter1) == 1);
+    ASSERT_SAME_TYPE(decltype(std::ranges::iter_move(commonIter1)), int&&);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    assert(std::ranges::iter_move(commonIter1) == 1);
+    ASSERT_SAME_TYPE(decltype(std::ranges::iter_move(commonIter1)), int&&);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/iter_swap.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/iter_swap.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/iter_swap.pass.cpp
@@ -0,0 +1,68 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// template<indirectly_swappable<I> I2, class S2>
+//   friend void iter_swap(const common_iterator& x, const common_iterator<I2, S2>& y)
+//     noexcept(noexcept(ranges::iter_swap(declval<const I&>(), declval<const I2&>())));
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    for (auto i = 0; i < 4; ++i) ++commonIter2;
+    assert(*commonIter2 == 5);
+    std::ranges::iter_swap(commonIter1, commonIter2);
+    assert(*commonIter1 == 5);
+    assert(*commonIter2 == 1);
+    std::ranges::iter_swap(commonIter2, commonIter1);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    for (auto i = 0; i < 4; ++i) ++commonIter2;
+    assert(*commonIter2 == 5);
+    std::ranges::iter_swap(commonIter1, commonIter2);
+    assert(*commonIter1 == 5);
+    assert(*commonIter2 == 1);
+    std::ranges::iter_swap(commonIter2, commonIter1);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    for (auto i = 0; i < 4; ++i) ++commonIter2;
+    assert(*commonIter2 == 5);
+    std::ranges::iter_swap(commonIter1, commonIter2);
+    assert(*commonIter1 == 5);
+    assert(*commonIter2 == 1);
+    std::ranges::iter_swap(commonIter2, commonIter1);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/iterator_traits.compile.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/iterator_traits.compile.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/iterator_traits.compile.pass.cpp
@@ -0,0 +1,83 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// template<input_iterator I, class S>
+//   struct iterator_traits<common_iterator<I, S>>;
+
+#include <iterator>
+
+#include "test_macros.h"
+#include "types.h"
+
+void test() {
+  {
+    using Iter = simple_iterator<int*>;
+    using CommonIter = std::common_iterator<Iter, sentienl_type<int*>>;
+    using IterTraits = std::iterator_traits<CommonIter>;
+
+    static_assert(std::same_as<IterTraits::iterator_concept, std::input_iterator_tag>);
+    static_assert(std::same_as<IterTraits::iterator_category, std::input_iterator_tag>);
+    static_assert(std::same_as<IterTraits::value_type, int>);
+    static_assert(std::same_as<IterTraits::difference_type, std::ptrdiff_t>);
+    static_assert(std::same_as<IterTraits::pointer, int*>);
+    static_assert(std::same_as<IterTraits::reference, int&>);
+  }
+  {
+    using Iter = value_iterator<int*>;
+    using CommonIter = std::common_iterator<Iter, sentienl_type<int*>>;
+    using IterTraits = std::iterator_traits<CommonIter>;
+
+    static_assert(std::same_as<IterTraits::iterator_concept, std::input_iterator_tag>);
+    static_assert(std::same_as<IterTraits::iterator_category, std::input_iterator_tag>);
+    static_assert(std::same_as<IterTraits::value_type, int>);
+    static_assert(std::same_as<IterTraits::difference_type, std::ptrdiff_t>);
+    // Note: IterTraits::pointer == __proxy.
+    static_assert(!std::same_as<IterTraits::pointer, int*>);
+    static_assert(std::same_as<IterTraits::reference, int>);
+  }
+  {
+    using Iter = cpp17_input_iterator<int*>;
+    using CommonIter = std::common_iterator<Iter, sentienl_type<int*>>;
+    using IterTraits = std::iterator_traits<CommonIter>;
+
+    static_assert(std::same_as<IterTraits::iterator_concept, std::input_iterator_tag>);
+    static_assert(std::same_as<IterTraits::iterator_category, std::input_iterator_tag>);
+    static_assert(std::same_as<IterTraits::value_type, int>);
+    static_assert(std::same_as<IterTraits::difference_type, std::ptrdiff_t>);
+    static_assert(std::same_as<IterTraits::pointer, Iter>); // TODO: is this right?
+    static_assert(std::same_as<IterTraits::reference, int&>);
+  }
+  {
+    using Iter = forward_iterator<int*>;
+    using CommonIter = std::common_iterator<Iter, sentienl_type<int*>>;
+    using IterTraits = std::iterator_traits<CommonIter>;
+
+    static_assert(std::same_as<IterTraits::iterator_concept, std::forward_iterator_tag>);
+    static_assert(std::same_as<IterTraits::iterator_category, std::forward_iterator_tag>);
+    static_assert(std::same_as<IterTraits::value_type, int>);
+    static_assert(std::same_as<IterTraits::difference_type, std::ptrdiff_t>);
+    static_assert(std::same_as<IterTraits::pointer, Iter>);
+    static_assert(std::same_as<IterTraits::reference, int&>);
+  }
+  {
+    using Iter = random_access_iterator<int*>;
+    using CommonIter = std::common_iterator<Iter, sentienl_type<int*>>;
+    using IterTraits = std::iterator_traits<CommonIter>;
+
+    static_assert(std::same_as<IterTraits::iterator_concept, std::forward_iterator_tag>);
+    static_assert(std::same_as<IterTraits::iterator_category, std::forward_iterator_tag>);
+    static_assert(std::same_as<IterTraits::value_type, int>);
+    static_assert(std::same_as<IterTraits::difference_type, std::ptrdiff_t>);
+    static_assert(std::same_as<IterTraits::pointer, Iter>);
+    static_assert(std::same_as<IterTraits::reference, int&>);
+  }
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/members.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/members.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/members.pass.cpp
@@ -0,0 +1,208 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// decltype(auto) operator*();
+// decltype(auto) operator*() const
+//   requires dereferenceable<const I>;
+// decltype(auto) operator->() const
+//   requires see below;
+
+// common_iterator& operator++();
+// decltype(auto) operator++(int);
+
+// template<class I2, sentinel_for<I> S2>
+//   requires sentinel_for<S, I2>
+// friend bool operator==(
+//   const common_iterator& x, const common_iterator<I2, S2>& y);
+// template<class I2, sentinel_for<I> S2>
+//   requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
+// friend bool operator==(
+//   const common_iterator& x, const common_iterator<I2, S2>& y);
+
+// template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
+//   requires sized_sentinel_for<S, I2>
+// friend iter_difference_t<I2> operator-(
+//   const common_iterator& x, const common_iterator<I2, S2>& y);
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = simple_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = simple_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+    assert(commonIter1.operator->() == buffer);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+    assert(commonIter2 != commonSent2);
+    assert(commonIter2.operator->() == buffer);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = value_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = value_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+    assert(*commonIter1.operator->().operator->() == 1);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+    assert(commonIter2 != commonSent2);
+    assert(*commonIter2.operator->().operator->() == 1);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = cpp17_input_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+    assert(commonIter1.operator->().base() == buffer);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+    assert(commonIter2 != commonSent2);
+    assert(commonIter2.operator->().base() == buffer);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = forward_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+    assert(commonIter1.operator->().base() == buffer);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+    assert(commonIter2 != commonSent2);
+    assert(commonIter2.operator->().base() == buffer);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    const auto iter2 = random_access_iterator<int*>(buffer);
+    const auto commonIter2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    const auto commonSent2 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*iter1 == 1);
+    assert(*commonIter1 == 1);
+    assert(commonIter1 != commonSent1);
+    assert(commonIter1.operator->().base() == buffer);
+
+    assert(*iter2 == 1);
+    assert(*commonIter2 == 1);
+    assert(commonIter2 != commonSent2);
+    assert(commonIter2.operator->().base() == buffer);
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sized_sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sized_sentienl_type<int*>>(sized_sentienl_type<int*>{buffer + 8});
+    assert(commonIter1 - commonSent1 == -8);
+    assert(commonSent1 - commonIter1 == 8);
+    assert(commonIter1 - commonIter1 == 0);
+    assert(commonSent1 - commonSent1 == 0);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/minus.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/minus.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/minus.pass.cpp
@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
+//   requires sized_sentinel_for<S, I2>
+// friend iter_difference_t<I2> operator-(
+//   const common_iterator& x, const common_iterator<I2, S2>& y);
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  auto iter1 = random_access_iterator<int*>(buffer);
+  auto commonIter1 = std::common_iterator<decltype(iter1), sized_sentienl_type<int*>>(iter1);
+  auto commonSent1 = std::common_iterator<decltype(iter1), sized_sentienl_type<int*>>(sized_sentienl_type<int*>{buffer + 8});
+  assert(commonIter1 - commonSent1 == -8);
+  assert(commonSent1 - commonIter1 == 8);
+  assert(commonIter1 - commonIter1 == 0);
+  assert(commonSent1 - commonSent1 == 0);
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/plus_plus.pass.cpp b/libcxx/test/std/iterators/predef.iterators/iterators.common/plus_plus.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/plus_plus.pass.cpp
@@ -0,0 +1,109 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+
+// common_iterator& operator++();
+// decltype(auto) operator++(int);
+
+#include <iterator>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  {
+    auto iter1 = simple_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = value_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = cpp17_input_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = forward_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+  {
+    auto iter1 = random_access_iterator<int*>(buffer);
+    auto commonIter1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(iter1);
+    auto commonSent1 = std::common_iterator<decltype(iter1), sentienl_type<int*>>(sentienl_type<int*>{buffer + 8});
+
+    assert(*(commonIter1++) == 1);
+    assert(*commonIter1 == 2);
+    assert(*(++commonIter1) == 3);
+    assert(*commonIter1 == 3);
+
+    for (auto i = 3; commonIter1 != commonSent1; ++i) {
+      assert(*(commonIter1++) == i);
+    }
+    assert(commonIter1 == commonSent1);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/iterators/predef.iterators/iterators.common/types.h b/libcxx/test/std/iterators/predef.iterators/iterators.common/types.h
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/iterators/predef.iterators/iterators.common/types.h
@@ -0,0 +1,88 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef TEST_STD_RANGES_ITERATORS_PREDEF_ITERATORS_ITERATORS_COMMON_TYPES_H
+#define TEST_STD_RANGES_ITERATORS_PREDEF_ITERATORS_ITERATORS_COMMON_TYPES_H
+
+#include "test_macros.h"
+#include "test_iterators.h"
+
+template <class It>
+class simple_iterator
+{
+    It it_;
+
+public:
+    typedef          std::input_iterator_tag                   iterator_category;
+    typedef typename std::iterator_traits<It>::value_type      value_type;
+    typedef typename std::iterator_traits<It>::difference_type difference_type;
+    typedef It                                                 pointer;
+    typedef typename std::iterator_traits<It>::reference       reference;
+
+    constexpr It base() const {return it_;}
+
+    simple_iterator() = default;
+    explicit constexpr simple_iterator(It it) : it_(it) {}
+
+    constexpr reference operator*() const {return *it_;}
+
+    constexpr simple_iterator& operator++() {++it_; return *this;}
+    constexpr simple_iterator operator++(int)
+        {simple_iterator tmp(*this); ++(*this); return tmp;}
+};
+
+template <class It>
+class value_iterator
+{
+    It it_;
+
+public:
+    typedef          std::input_iterator_tag                   iterator_category;
+    typedef typename std::iterator_traits<It>::value_type      value_type;
+    typedef typename std::iterator_traits<It>::difference_type difference_type;
+    typedef It                                                 pointer;
+    typedef typename std::iterator_traits<It>::reference       reference;
+
+    constexpr It base() const {return it_;}
+
+    value_iterator() = default;
+    explicit constexpr value_iterator(It it) : it_(it) {}
+
+    constexpr value_type operator*() const {return *it_;}
+
+    constexpr value_iterator& operator++() {++it_; return *this;}
+    constexpr value_iterator operator++(int)
+        {value_iterator tmp(*this); ++(*this); return tmp;}
+};
+
+
+template<class T>
+struct sentienl_type {
+  T base;
+
+  template<class U>
+  friend constexpr bool operator==(const sentienl_type& lhs, const U& rhs) { return lhs.base == rhs.base(); }
+  template<class U>
+  friend constexpr bool operator==(const U& lhs, const sentienl_type& rhs) { return lhs.base() == rhs.base; }
+};
+
+template<class T>
+struct sized_sentienl_type {
+  T base;
+
+  template<class U>
+  friend constexpr bool operator==(const sized_sentienl_type& lhs, const U& rhs) { return lhs.base - rhs.base(); }
+  template<class U>
+  friend constexpr bool operator==(const U& lhs, const sized_sentienl_type& rhs) { return lhs.base() - rhs.base; }
+  template<class U>
+  friend constexpr auto operator- (const sized_sentienl_type& lhs, const U& rhs) { return lhs.base - rhs.base(); }
+  template<class U>
+  friend constexpr auto operator- (const U& lhs, const sized_sentienl_type& rhs) { return lhs.base() - rhs.base; }
+};
+
+#endif // TEST_STD_RANGES_ITERATORS_PREDEF_ITERATORS_ITERATORS_COMMON_TYPES_H
diff --git a/libcxx/test/support/test_iterators.h b/libcxx/test/support/test_iterators.h
--- a/libcxx/test/support/test_iterators.h
+++ b/libcxx/test/support/test_iterators.h
@@ -160,6 +160,59 @@
     return !(x == y);
 }
 
+template <class It>
+class no_default_constructor
+{
+    It it_;
+
+    template <class U> friend class no_default_constructor;
+public:
+    typedef          std::forward_iterator_tag                 iterator_category;
+    typedef typename std::iterator_traits<It>::value_type      value_type;
+    typedef typename std::iterator_traits<It>::difference_type difference_type;
+    typedef It                                                 pointer;
+    typedef typename std::iterator_traits<It>::reference       reference;
+
+    TEST_CONSTEXPR_CXX14 It base() const {return it_;}
+
+    no_default_constructor() = delete;
+
+    explicit TEST_CONSTEXPR_CXX14 no_default_constructor(It it) : it_(it) {}
+    template <class U>
+        TEST_CONSTEXPR_CXX14 no_default_constructor(const no_default_constructor<U>& u) :it_(u.it_) {}
+
+    TEST_CONSTEXPR_CXX14 reference operator*() const {return *it_;}
+    TEST_CONSTEXPR_CXX14 pointer operator->() const {return it_;}
+
+    TEST_CONSTEXPR_CXX14 no_default_constructor& operator++() {++it_; return *this;}
+    TEST_CONSTEXPR_CXX14 no_default_constructor operator++(int)
+        {no_default_constructor tmp(*this); ++(*this); return tmp;}
+
+    friend TEST_CONSTEXPR_CXX14 bool operator==(const no_default_constructor& x, const no_default_constructor& y)
+        {return x.it_ == y.it_;}
+    friend TEST_CONSTEXPR_CXX14 bool operator!=(const no_default_constructor& x, const no_default_constructor& y)
+        {return !(x == y);}
+
+    template <class T>
+    void operator,(T const &) DELETE_FUNCTION;
+};
+
+template <class T, class U>
+inline
+bool TEST_CONSTEXPR_CXX14
+operator==(const no_default_constructor<T>& x, const no_default_constructor<U>& y)
+{
+    return x.base() == y.base();
+}
+
+template <class T, class U>
+inline
+bool TEST_CONSTEXPR_CXX14
+operator!=(const no_default_constructor<T>& x, const no_default_constructor<U>& y)
+{
+    return !(x == y);
+}
+
 template <class It>
 class bidirectional_iterator
 {