diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -48,6 +48,7 @@
   __ranges/semiregular_box.h
   __ranges/size.h
   __ranges/subrange.h
+  __ranges/transform_view.h
   __ranges/ref_view.h
   __ranges/view_interface.h
   __split_buffer
diff --git a/libcxx/include/__ranges/concepts.h b/libcxx/include/__ranges/concepts.h
--- a/libcxx/include/__ranges/concepts.h
+++ b/libcxx/include/__ranges/concepts.h
@@ -72,6 +72,9 @@
       range<_Range> &&
       (is_lvalue_reference_v<_Range> || enable_borrowed_range<remove_cvref_t<_Range>>);
 
+  template<bool _Const, class _Tp>
+  using __maybe_const = conditional_t<_Const, const _Tp, _Tp>;
+
 } // namespace ranges
 
 #endif // !defined(_LIBCPP_HAS_NO_RANGES)
diff --git a/libcxx/include/__ranges/subrange.h b/libcxx/include/__ranges/subrange.h
--- a/libcxx/include/__ranges/subrange.h
+++ b/libcxx/include/__ranges/subrange.h
@@ -65,6 +65,7 @@
     _Iter __begin = _Iter();
     _Sent __end = _Sent();
 
+    __subrange_base() = default;
     constexpr __subrange_base(_Iter __iter, _Sent __sent)
       : __begin(_VSTD::move(__iter)), __end(__sent) { }
   };
@@ -76,6 +77,7 @@
     _Sent __end = _Sent();
     make_unsigned_t<iter_difference_t<_Iter>> __size = 0;
 
+    __subrange_base() = default;
     constexpr __subrange_base(_Iter __iter, _Sent __sent, decltype(__size) __size)
       : __begin(_VSTD::move(__iter)), __end(__sent), __size(__size) { }
   };
diff --git a/libcxx/include/__ranges/transform_view.h b/libcxx/include/__ranges/transform_view.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__ranges/transform_view.h
@@ -0,0 +1,353 @@
+// -*- 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___RANGES_TRANSFORM_VIEW_H
+#define _LIBCPP___RANGES_TRANSFORM_VIEW_H
+
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/concepts.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/empty.h>
+#include <__ranges/semiregular_box.h>
+#include <__ranges/view.h>
+#include <__ranges/view_interface.h>
+#include <type_traits>
+
+#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)
+
+// REVIEW-NOTE: This is *not* part of the transform_view patch and will be added by a *separate* PR.
+template<class _I1, class _I2 = _I1>
+concept indirectly_swappable =
+    indirectly_readable<_I1> &&
+    indirectly_readable<_I2> &&
+    requires(const _I1 i1, const _I2 i2) {
+        ranges::iter_swap(i1, i1);
+        ranges::iter_swap(i1, i2);
+        ranges::iter_swap(i2, i1);
+        ranges::iter_swap(i2, i2);
+    };
+// END-REVIEW-NOTE.
+
+namespace ranges {
+
+template<input_range _View, copy_constructible _Fn>
+  requires view<_View> && is_object_v<_Fn> &&
+           regular_invocable<_Fn&, range_reference_t<_View>> &&
+           __referenceable<invoke_result_t<_Fn&, range_reference_t<_View>>>
+class transform_view : public view_interface<transform_view<_View, _Fn>> {
+  template<bool> class __iterator;
+  template<bool> class __sentinel;
+
+  _View __base = _View();
+  __semiregular_box<_Fn> __func;
+
+public:
+  transform_view() = default;
+  constexpr transform_view(_View __base, _Fn __func)
+    : __base(_VSTD::move(__base)), __func(_VSTD::move(__func)) {}
+
+  constexpr _View base() const& requires copy_constructible<_View> { return __base; }
+  constexpr _View base() && { return _VSTD::move(__base); }
+
+  constexpr __iterator<false> begin() { return __iterator<false>{*this, ranges::begin(__base)}; }
+  constexpr __iterator<true>  begin() const
+    requires range<const _View> &&
+             regular_invocable<const _Fn&, range_reference_t<const _View>>
+  {
+    return __iterator<true>{*this, ranges::begin(__base)};
+  }
+
+  constexpr __sentinel<false> end() { return __sentinel<false>{ranges::end(__base)}; }
+  constexpr __iterator<false> end()
+    requires common_range<_View>
+  {
+    return __iterator<false>{*this, ranges::end(__base)};
+  }
+  constexpr __sentinel<true> end() const
+    requires range<const _View> &&
+             regular_invocable<const _Fn&, range_reference_t<const _View>>
+  {
+    return __sentinel<true>{ranges::end(__base)};
+  }
+  constexpr __iterator<true> end() const
+    requires common_range<const _View> &&
+             regular_invocable<const _Fn&, range_reference_t<const _View>>
+  {
+    return __iterator<true>{*this, ranges::end(__base)};
+  }
+
+  constexpr auto size() requires sized_range<_View> { return ranges::size(__base); }
+  constexpr auto size() const requires sized_range<const _View> { return ranges::size(__base); }
+};
+
+template<class _Range, class _Fn>
+transform_view(_Range&&, _Fn)
+  -> transform_view<decltype(views::all(std::declval<_Range>())), _Fn>;
+
+template<class _View>
+struct __iterator_concept {
+  using type = conditional_t<
+    random_access_range<_View>,
+    random_access_iterator_tag,
+    conditional_t<
+      bidirectional_range<_View>,
+      bidirectional_iterator_tag,
+      conditional_t<
+        forward_range<_View>,
+        forward_iterator_tag,
+        input_iterator_tag
+      >
+    >
+  >;
+};
+
+template<class, class>
+struct __iterator_category_base {};
+
+template<forward_range _View, class _Fn>
+struct __iterator_category_base<_View, _Fn> {
+  using _Cat = typename iterator_traits<iterator_t<_View>>::iterator_category;
+
+  using iterator_category = conditional_t<
+    is_lvalue_reference_v<invoke_result_t<_Fn&, range_reference_t<_View>>>,
+    conditional_t<
+      derived_from<_Cat, contiguous_iterator_tag>,
+      random_access_iterator_tag,
+      _Cat
+    >,
+    input_iterator_tag
+  >;
+};
+
+template<input_range _View, copy_constructible _Fn>
+  requires view<_View> && is_object_v<_Fn> &&
+           regular_invocable<_Fn&, range_reference_t<_View>> &&
+           __referenceable<invoke_result_t<_Fn&, range_reference_t<_View>>>
+template<bool _Const>
+class transform_view<_View, _Fn>::__iterator
+  : public __iterator_category_base<_View, _Fn> {
+
+  using _Parent = __maybe_const<_Const, transform_view>;
+  using _Base = __maybe_const<_Const, _View>;
+
+  iterator_t<_Base> __current = iterator_t<_Base>();
+  _Parent *__parent = nullptr;
+
+  template<bool>
+  friend class transform_view<_View, _Fn>::__sentinel;
+
+public:
+  using iterator_concept = typename __iterator_concept<_View>::type;
+  using value_type = remove_cvref_t<invoke_result_t<_Fn&, range_reference_t<_Base>>>;
+  using difference_type = range_difference_t<_Base>;
+
+  __iterator() = default;
+
+  constexpr __iterator(_Parent& __parent, iterator_t<_Base> __current)
+    : __current(_VSTD::move(__current)), __parent(_VSTD::addressof(__parent)) {}
+  constexpr __iterator(__iterator<false> __i)
+    requires _Const && convertible_to<iterator_t<_View>, iterator_t<_Base>>
+    : __current(_VSTD::move(__i.__current)), __parent(__i.__parent) {}
+
+  constexpr iterator_t<_Base> base() const&
+    requires copyable<iterator_t<_Base>>
+  {
+    return __current;
+  }
+  constexpr iterator_t<_Base> base() && {
+    return _VSTD::move(__current);
+  }
+
+  constexpr decltype(auto) operator*() const { return _VSTD::invoke(*__parent->__func, *__current); }
+
+  constexpr __iterator& operator++() {
+    ++__current;
+    return *this;
+  }
+  constexpr void        operator++(int) { ++__current; }
+  constexpr __iterator  operator++(int)
+    requires forward_range<_Base>
+  {
+    auto __tmp = *this;
+    ++*this;
+    return __tmp;
+  }
+
+  constexpr __iterator& operator--()
+    requires bidirectional_range<_Base>
+  {
+    --__current;
+    return *this;
+  }
+  constexpr __iterator  operator--(int)
+    requires bidirectional_range<_Base>
+  {
+    auto __tmp = *this;
+    --*this;
+    return __tmp;
+  }
+
+  constexpr __iterator& operator+=(difference_type __n)
+    requires random_access_range<_Base>
+  {
+    __current += __n;
+    return *this;
+  }
+  constexpr __iterator& operator-=(difference_type __n)
+    requires random_access_range<_Base>
+  {
+    __current -= __n;
+    return *this;
+  }
+  constexpr decltype(auto) operator[](difference_type __n) const
+    requires random_access_range<_Base>
+  {
+    return _VSTD::invoke(*__parent->__func, __current[__n]);
+  }
+
+  friend constexpr bool operator==(const __iterator& __x, const __iterator& __y)
+    requires equality_comparable<iterator_t<_Base>>
+  {
+    return __x.__current == __y.__current;
+  }
+
+  friend constexpr bool operator<(const __iterator& __x, const __iterator& __y)
+    requires random_access_range<_Base>
+  {
+    return __x.__current < __y.__current;
+  }
+  friend constexpr bool operator>(const __iterator& __x, const __iterator& __y)
+    requires random_access_range<_Base>
+  {
+    return __x.__current > __y.__current;
+  }
+  friend constexpr bool operator<=(const __iterator& __x, const __iterator& __y)
+    requires random_access_range<_Base>
+  {
+    return __x.__current <= __y.__current;
+  }
+  friend constexpr bool operator>=(const __iterator& __x, const __iterator& __y)
+    requires random_access_range<_Base>
+  {
+    return __x.__current >= __y.__current;
+  }
+//   friend constexpr auto operator<=>(const __iterator& __x, const __iterator& __y)
+//     requires random_access_range<_Base> && three_way_comparable<iterator_t<_Base>>
+//   {
+//     return __x.__current <=> __y.__current;
+//   }
+
+  friend constexpr __iterator operator+(__iterator __i, difference_type __n)
+    requires random_access_range<_Base>
+  {
+    return __iterator{*__i.__parent, __i.__current + __n};
+  }
+  friend constexpr __iterator operator+(difference_type __n, __iterator __i)
+    requires random_access_range<_Base>
+  {
+    return __iterator{*__i.__parent, __i.__current + __n};
+  }
+
+  friend constexpr __iterator operator-(__iterator __i, difference_type __n)
+    requires random_access_range<_Base>
+  {
+    return __iterator{*__i.__parent, __i.__current - __n};
+  }
+  friend constexpr difference_type operator-(const __iterator& __x, const __iterator& __y)
+    requires sized_sentinel_for<iterator_t<_Base>, iterator_t<_Base>>
+  {
+    return __x.__current - __y.__current;
+  }
+
+  friend constexpr decltype(auto) iter_move(const __iterator& __i)
+    // TODO: this noexcept looks wrong: why do we care about func's noexceptness?
+    noexcept(noexcept(_VSTD::invoke(*__i.__parent->__func, *__i.__current)))
+  {
+    // TODO: this looks wrong: shouldn't this be the oposite.
+    if constexpr (is_lvalue_reference_v<decltype(*__i)>)
+      return _VSTD::move(*__i);
+    else
+      return *__i;
+  }
+
+  friend constexpr void iter_swap(const __iterator& __x, const __iterator& __y)
+    noexcept(noexcept(ranges::iter_swap(__x.__current, __y.__current)))
+    requires indirectly_swappable<iterator_t<_Base>>
+  {
+    ranges::iter_swap(__x.__current, __y.__current);
+  }
+};
+
+template<input_range _View, copy_constructible _Fn>
+  requires view<_View> && is_object_v<_Fn> &&
+           regular_invocable<_Fn&, range_reference_t<_View>> &&
+           __referenceable<invoke_result_t<_Fn&, range_reference_t<_View>>>
+template<bool _Const>
+class transform_view<_View, _Fn>::__sentinel {
+  using _Parent = __maybe_const<_Const, transform_view>;
+  using _Base = __maybe_const<_Const, _View>;
+
+  sentinel_t<_Base> __end = sentinel_t<_Base>();
+
+  template<bool>
+  friend class transform_view<_View, _Fn>::__iterator;
+
+  template<bool>
+  friend class transform_view<_View, _Fn>::__sentinel;
+
+public:
+  __sentinel() = default;
+
+  constexpr explicit __sentinel(sentinel_t<_Base> __end) : __end(__end) {}
+  constexpr __sentinel(__sentinel<false> __i)
+    requires _Const && convertible_to<sentinel_t<_View>, sentinel_t<_Base>>
+    : __end(_VSTD::move(__i.__end)) {}
+
+  constexpr sentinel_t<_Base> base() const { return __end; }
+
+  template<bool _OtherConst>
+    requires sentinel_for<sentinel_t<_Base>, iterator_t<__maybe_const<_OtherConst, _View>>>
+  friend constexpr bool operator==(const __iterator<_OtherConst>& __x, const __sentinel __y) {
+    return __x.__current == __y.__end;
+  }
+
+  template<bool _OtherConst>
+    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<__maybe_const<_OtherConst, _View>>>
+  friend constexpr range_difference_t<__maybe_const<_OtherConst, _View>>
+  operator-(const __iterator<_OtherConst>& __x, const __sentinel& __y) {
+    return __x.__current - __y.__end;
+  }
+
+  template<bool _OtherConst>
+    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<__maybe_const<_OtherConst, _View>>>
+  friend constexpr range_difference_t<__maybe_const<_OtherConst, _View>>
+  operator-(const __sentinel& __x, const __iterator<_OtherConst>& __y) {
+    return __x.__end - __y.__current;
+  }
+};
+
+} // namespace ranges
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___RANGES_TRANSFORM_VIEW_H
diff --git a/libcxx/include/ranges b/libcxx/include/ranges
--- a/libcxx/include/ranges
+++ b/libcxx/include/ranges
@@ -85,6 +85,7 @@
 #include <__ranges/view.h>
 #include <__ranges/size.h>
 #include <__ranges/subrange.h>
+#include <__ranges/transform_view.h>
 #include <__ranges/ref_view.h>
 #include <__ranges/view_interface.h>
 #include <compare>          // Required by the standard.
diff --git a/libcxx/test/std/ranges/range.adaptors/range.transform/range.transform.view.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.transform/range.transform.view.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/std/ranges/range.adaptors/range.transform/range.transform.view.pass.cpp
@@ -0,0 +1,607 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// class std::ranges::transform_view;
+
+#include <ranges>
+
+#include <vector>
+#include <list>
+#include <numeric>
+#include <string>
+#include <iostream>
+#include <cctype>
+
+#include <cassert>
+#include "test_macros.h"
+#include "test_iterators.h"
+
+int globalBuff[8] = {0,1,2,3,4,5,6,7};
+
+template<class T, class F>
+concept ValidDropView = requires { typename std::ranges::transform_view<T, F>; };
+
+struct View : std::ranges::view_base {
+  int start;
+  int *ptr;
+  constexpr View(int* ptr = globalBuff, int start = 0) : start(start), ptr(ptr) {}
+  constexpr View(View&&) = default;
+  constexpr View& operator=(View&&) = default;
+  constexpr friend int* begin(View& view) { return view.ptr + view.start; }
+  constexpr friend int* begin(View const& view) { return view.ptr + view.start; }
+  constexpr friend int* end(View& view) { return view.ptr + 8; }
+  constexpr friend int* end(View const& view) { return view.ptr + 8; }
+};
+
+struct CopyableView : std::ranges::view_base {
+  int start;
+  constexpr CopyableView(int start = 0) : start(start) {}
+  constexpr CopyableView(CopyableView const&) = default;
+  constexpr CopyableView& operator=(CopyableView const&) = default;
+  constexpr friend int* begin(CopyableView& view) { return globalBuff + view.start; }
+  constexpr friend int* begin(CopyableView const& view) { return globalBuff + view.start; }
+  constexpr friend int* end(CopyableView&) { return globalBuff + 8; }
+  constexpr friend int* end(CopyableView const&) { return globalBuff + 8; }
+};
+
+using ForwardIter = forward_iterator<int*>;
+struct ForwardView : std::ranges::view_base {
+  int *ptr;
+  constexpr ForwardView(int* ptr = globalBuff) : ptr(ptr) {}
+  constexpr ForwardView(ForwardView&&) = default;
+  constexpr ForwardView& operator=(ForwardView&&) = default;
+  constexpr friend ForwardIter begin(ForwardView& view) { return ForwardIter(view.ptr); }
+  constexpr friend ForwardIter begin(ForwardView const& view) { return ForwardIter(view.ptr); }
+  constexpr friend ForwardIter end(ForwardView& view) { return ForwardIter(view.ptr + 8); }
+  constexpr friend ForwardIter end(ForwardView const& view) { return ForwardIter(view.ptr + 8); }
+};
+
+struct ForwardRange {
+  ForwardIter begin() const;
+  ForwardIter end() const;
+  ForwardIter begin();
+  ForwardIter end();
+};
+
+using RandomAccessIter = random_access_iterator<int*>;
+struct RandomAccessView : std::ranges::view_base {
+  RandomAccessIter begin() const;
+  RandomAccessIter end() const;
+  RandomAccessIter begin();
+  RandomAccessIter end();
+};
+
+using BidirectionalIter = bidirectional_iterator<int*>;
+struct BidirectionalView : std::ranges::view_base {
+  BidirectionalIter begin() const;
+  BidirectionalIter end() const;
+  BidirectionalIter begin();
+  BidirectionalIter end();
+};
+
+struct BorrowableRange {
+  friend int* begin(BorrowableRange const& range);
+  friend int* end(BorrowableRange const&);
+  friend int* begin(BorrowableRange& range);
+  friend int* end(BorrowableRange&);
+};
+
+template<>
+inline constexpr bool std::ranges::enable_borrowed_range<BorrowableRange> = true;
+
+struct InputView : std::ranges::view_base {
+  int *ptr;
+  constexpr InputView(int* ptr = globalBuff) : ptr(ptr) {}
+  constexpr cpp20_input_iterator<int*> begin() const { return cpp20_input_iterator<int*>(ptr); }
+  constexpr int* end() const { return ptr + 8; }
+  constexpr cpp20_input_iterator<int*> begin() { return cpp20_input_iterator<int*>(ptr); }
+  constexpr int* end() { return ptr + 8; }
+};
+
+constexpr bool operator==(const cpp20_input_iterator<int*> &lhs, int* rhs) { return lhs.base() == rhs; }
+constexpr bool operator==(int* lhs, const cpp20_input_iterator<int*> &rhs) { return rhs.base() == lhs; }
+
+struct SentinelView : std::ranges::view_base {
+  int count;
+  constexpr SentinelView(int count = 8) : count(count) {}
+  constexpr RandomAccessIter begin() const { return RandomAccessIter(globalBuff); }
+  constexpr int* end() const { return globalBuff + count; }
+  constexpr RandomAccessIter begin() { return RandomAccessIter(globalBuff); }
+  constexpr int* end() { return globalBuff + count; }
+};
+
+constexpr long operator- (const RandomAccessIter &lhs, int* rhs) { return lhs.base() - rhs; }
+constexpr long operator- (int* lhs, const RandomAccessIter &rhs) { return lhs - rhs.base(); }
+constexpr bool operator==(const RandomAccessIter &lhs, int* rhs) { return lhs.base() == rhs; }
+constexpr bool operator==(int* lhs, const RandomAccessIter &rhs) { return rhs.base() == lhs; }
+
+struct SizedSentinelNotConstView : std::ranges::view_base {
+  ForwardIter begin() const;
+  int *end() const;
+  ForwardIter begin();
+  int *end();
+  size_t size();
+};
+bool operator==(const ForwardIter &lhs, int* rhs);
+bool operator==(int* lhs, const ForwardIter &rhs);
+
+struct Range {
+  friend int* begin(Range const&);
+  friend int* end(Range const&);
+  friend int* begin(Range&);
+  friend int* end(Range&);
+};
+
+using CountedIter = stride_counting_iterator<forward_iterator<int*>>;
+struct CountedView : std::ranges::view_base {
+  constexpr CountedIter begin() { return CountedIter(ForwardIter(globalBuff)); }
+  constexpr CountedIter begin() const { return CountedIter(ForwardIter(globalBuff)); }
+  constexpr CountedIter end() { return CountedIter(ForwardIter(globalBuff + 8)); }
+  constexpr CountedIter end() const { return CountedIter(ForwardIter(globalBuff + 8)); }
+};
+
+struct PlusPlus {
+  constexpr int operator()(int x) { return ++x; }
+};
+
+struct PlusPlusConst {
+  constexpr int operator()(int x) const { return ++x; }
+};
+
+struct PlusPlusRef {
+  constexpr int& operator()(int x) { return ++x; }
+};
+
+template<class T>
+concept BeginInvocable = requires(T t) { t.begin(); };
+
+template<class T>
+concept EndInvocable = requires(T t) { t.end(); };
+
+template<class T>
+concept EndIsIter = requires(T t) { ++t.end(); };
+
+template<class T>
+concept SizeInvocable = requires(T t) { t.size(); };
+
+constexpr void testViewMembers() {
+  {
+    // transform_view::base
+    std::ranges::transform_view<View, PlusPlus> transformView1;
+    auto base1 = std::move(transformView1).base();
+    assert(std::ranges::begin(base1) == globalBuff);
+
+    std::ranges::transform_view<CopyableView, PlusPlus> transformView2;
+    auto base2 = transformView2.base();
+    assert(std::ranges::begin(base2) == globalBuff);
+    auto base3 = std::move(transformView2).base();
+    assert(std::ranges::begin(base3) == globalBuff);
+
+    const std::ranges::transform_view<CopyableView, PlusPlus> transformView3;
+    const auto base4 = transformView3.base();
+    assert(std::ranges::begin(base4) == globalBuff);
+    const auto base5 = std::move(transformView3).base();
+    assert(std::ranges::begin(base5) == globalBuff);
+  }
+
+  {
+    // transform_view::begin
+    int buff[8] = {0, 1, 2, 3, 4, 5, 6, 7};
+    std::ranges::transform_view transformView1(View{buff}, PlusPlus{});
+    assert(transformView1.begin().base() == buff);
+    assert(*transformView1.begin() == 1);
+
+    std::ranges::transform_view transformView2(ForwardView{buff}, PlusPlus{});
+    assert(transformView2.begin().base().base() == buff);
+    assert(*transformView2.begin() == 1);
+
+    std::ranges::transform_view transformView3(InputView{buff}, PlusPlus{});
+    assert(transformView3.begin().base().base() == buff);
+    assert(*transformView3.begin() == 1);
+
+    const std::ranges::transform_view transformView4(View{buff}, PlusPlusConst{});
+    assert(*transformView4.begin() == 1);
+
+    static_assert(!BeginInvocable<const std::ranges::transform_view<View, PlusPlus>>);
+  }
+
+  {
+    // transform_view::end
+    std::ranges::transform_view transformView1(View{}, PlusPlus{});
+    assert(transformView1.end().base() == globalBuff + 8);
+
+    std::ranges::transform_view transformView2(ForwardView{}, PlusPlus{});
+    assert(transformView2.end().base().base() == globalBuff + 8);
+
+    std::ranges::transform_view transformView3(InputView{}, PlusPlus{});
+    assert(transformView3.end().base() == globalBuff + 8);
+
+    const std::ranges::transform_view transformView4(View{}, PlusPlusConst{});
+    assert(transformView4.end().base() == globalBuff + 8);
+
+    static_assert(!EndInvocable<const std::ranges::transform_view<View, PlusPlus>>);
+    static_assert( EndInvocable<      std::ranges::transform_view<View, PlusPlus>>);
+    static_assert( EndInvocable<const std::ranges::transform_view<View, PlusPlusConst>>);
+    static_assert(!EndInvocable<const std::ranges::transform_view<InputView, PlusPlus>>);
+    static_assert( EndInvocable<      std::ranges::transform_view<InputView, PlusPlus>>);
+    static_assert( EndInvocable<const std::ranges::transform_view<InputView, PlusPlusConst>>);
+
+    static_assert(!EndIsIter<const std::ranges::transform_view<InputView, PlusPlusConst>>);
+    static_assert(!EndIsIter<      std::ranges::transform_view<InputView, PlusPlus>>);
+    static_assert( EndIsIter<const std::ranges::transform_view<View, PlusPlusConst>>);
+    static_assert( EndIsIter<      std::ranges::transform_view<View, PlusPlus>>);
+  }
+
+  {
+    // transform_view::size
+    std::ranges::transform_view transformView1(View{}, PlusPlus{});
+    assert(transformView1.size() == 8);
+
+    const std::ranges::transform_view transformView2(View{globalBuff, 4}, PlusPlus{});
+    assert(transformView2.size() == 4);
+
+    static_assert(!SizeInvocable<std::ranges::transform_view<ForwardView, PlusPlus>>);
+
+    static_assert(SizeInvocable<std::ranges::transform_view<SizedSentinelNotConstView, PlusPlus>>);
+    static_assert(!SizeInvocable<const std::ranges::transform_view<SizedSentinelNotConstView, PlusPlus>>);
+  }
+}
+
+template<class V, class F>
+concept HasIterConcept = requires { typename std::ranges::transform_view<V, F>::iterator_category; };
+
+template<class V, class F>
+concept BaseInvocable = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter.base();
+};
+
+constexpr void testIterator() {
+  // Member typedefs.
+  {
+    // Member typedefs for contiguous iterator.
+    static_assert(std::same_as<std::iterator_traits<int*>::iterator_concept, std::contiguous_iterator_tag>);
+    static_assert(std::same_as<std::iterator_traits<int*>::iterator_category, std::random_access_iterator_tag>);
+
+    using TView = std::ranges::transform_view<View, PlusPlusRef>;
+    using TIter = std::ranges::iterator_t<TView>;
+    static_assert(std::same_as<typename TIter::iterator_concept, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename TIter::iterator_category, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename TIter::value_type, int>);
+    static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
+  }
+  {
+    // Member typedefs for random access iterator.
+    using TView = std::ranges::transform_view<RandomAccessView, PlusPlusRef>;
+    using TIter = std::ranges::iterator_t<TView>;
+    static_assert(std::same_as<typename TIter::iterator_concept, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename TIter::iterator_category, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename TIter::value_type, int>);
+    static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
+  }
+  {
+    // Member typedefs for random access iterator/not-lvalue-ref.
+    using TView = std::ranges::transform_view<RandomAccessView, PlusPlus>;
+    using TIter = std::ranges::iterator_t<TView>;
+    static_assert(std::same_as<typename TIter::iterator_concept, std::random_access_iterator_tag>);
+    static_assert(std::same_as<typename TIter::iterator_category, std::input_iterator_tag>); // Note: this is now input_iterator_tag.
+    static_assert(std::same_as<typename TIter::value_type, int>);
+    static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
+  }
+  {
+    // Member typedefs for bidirectional iterator.
+    using TView = std::ranges::transform_view<BidirectionalView, PlusPlusRef>;
+    using TIter = std::ranges::iterator_t<TView>;
+    static_assert(std::same_as<typename TIter::iterator_concept, std::bidirectional_iterator_tag>);
+    static_assert(std::same_as<typename TIter::iterator_category, std::bidirectional_iterator_tag>);
+    static_assert(std::same_as<typename TIter::value_type, int>);
+    static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
+  }
+  {
+    // Member typedefs for forward iterator.
+    using TView = std::ranges::transform_view<ForwardView, PlusPlusRef>;
+    using TIter = std::ranges::iterator_t<TView>;
+    static_assert(std::same_as<typename TIter::iterator_concept, std::forward_iterator_tag>);
+    static_assert(std::same_as<typename TIter::iterator_category, std::forward_iterator_tag>);
+    static_assert(std::same_as<typename TIter::value_type, int>);
+    static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
+  }
+  {
+    // Member typedefs for input iterator.
+    using TView = std::ranges::transform_view<InputView, PlusPlusRef>;
+    using TIter = std::ranges::iterator_t<TView>;
+    static_assert(std::same_as<typename TIter::iterator_concept, std::input_iterator_tag>);
+    static_assert(!HasIterConcept<InputView, PlusPlusRef>);
+    static_assert(std::same_as<typename TIter::value_type, int>);
+    static_assert(std::same_as<typename TIter::difference_type, std::ptrdiff_t>);
+  }
+
+  // Members
+  {
+    // __iterator::base
+    std::ranges::transform_view<View, PlusPlus> transformView1;
+    auto iter1 = std::move(transformView1).begin();
+    assert(iter1.base() == globalBuff);
+    assert(std::move(iter1).base() == globalBuff);
+
+    std::ranges::transform_view<InputView, PlusPlus> transformView2;
+    auto iter2 = transformView2.begin();
+    assert(std::move(iter2).base() == globalBuff);
+    static_assert(!BaseInvocable<InputView, PlusPlus>);
+  }
+  // Note: __iterator::operator* tested elsewhere.
+  // Note: member operator requirements tested at the end of the file.
+  {
+    // __iterator::operator{++,--,+=,-=}
+    std::ranges::transform_view<View, PlusPlus> transformView1;
+    auto iter1 = std::move(transformView1).begin();
+    assert((++iter1).base() == globalBuff + 1);
+
+    assert((iter1++).base() == globalBuff + 1);
+    assert(iter1.base() == globalBuff + 2);
+
+    assert((--iter1).base() == globalBuff + 1);
+    assert((iter1--).base() == globalBuff + 1);
+    assert(iter1.base() == globalBuff);
+
+    // Check that decltype(InputIter++) == void.
+    ASSERT_SAME_TYPE(decltype(
+      std::declval<std::ranges::iterator_t<std::ranges::transform_view<InputView, PlusPlus>>>()++),
+      void);
+
+    assert((iter1 += 4).base() == globalBuff + 4);
+    assert((iter1 -= 3).base() == globalBuff + 1);
+  }
+  {
+    // __iterator::operator[]
+    int buff[8] = {0, 1, 2, 3, 4, 5, 6, 7};
+    std::ranges::transform_view transformView1(View{buff}, PlusPlus{});
+    auto iter1 = std::move(transformView1).begin() + 1;
+    assert(iter1[0] == 2);
+    assert(iter1[4] == 6);
+  }
+  {
+    // __iterator::operator{<,>,<=,>=}
+    std::ranges::transform_view<View, PlusPlus> transformView1;
+    auto iter1 = std::move(transformView1).begin();
+    std::ranges::transform_view<View, PlusPlus> transformView2;
+    auto iter2 = std::move(transformView2).begin();
+    assert(iter1 == iter2);
+    assert(iter1 + 1 != iter2);
+    assert(iter1 + 1 == iter2 + 1);
+
+    assert(iter1 < iter1 + 1);
+    assert(iter1 + 1 > iter1);
+    assert(iter1 <= iter1 + 1);
+    assert(iter1 <= iter2);
+    assert(iter1 + 1 >= iter2);
+    assert(iter1     >= iter2);
+    // TODO: operator<=> when three_way_comparable is implemented.
+  }
+  {
+    // __iterator::operator{+,-}
+    std::ranges::transform_view<View, PlusPlus> transformView1;
+    auto iter1 = std::move(transformView1).begin();
+    std::ranges::transform_view<View, PlusPlus> transformView2;
+    auto iter2 = std::move(transformView2).begin();
+    iter1 += 4;
+    assert((iter1 + 1).base() == globalBuff + 5);
+    assert((1 + iter1).base() == globalBuff + 5);
+    assert((iter1 - 1).base() == globalBuff + 3);
+    assert(iter1 - iter2 == 4);
+  }
+  // TODO: I'm not going to test iter_move until the TODOs in transform_view.h are addressed.
+  {
+    // __iterator::iter_swap
+    int buff[8] = {0, 1, 2, 3, 4, 5, 6, 7};
+    std::ranges::transform_view transformView1(View{buff}, PlusPlus{});
+    auto iter1 = std::move(transformView1).begin();
+    std::ranges::transform_view transformView2(View{buff}, PlusPlus{});
+    auto iter2 = std::move(transformView2).begin();
+    iter1 += 4;
+    std::ranges::iter_swap(iter1, iter2);
+    assert(*iter1 == 1);
+    assert(*(buff + 4) == 0);
+    assert(*iter2 == 5);
+    assert(*buff == 4);
+    assert(*transformView1.begin() == 5);
+  }
+}
+
+constexpr void testSentinel() {
+  std::ranges::transform_view<SentinelView, PlusPlusConst> transformView1;
+  // Going to const and back.
+  auto sent1 = transformView1.end();
+  std::ranges::sentinel_t<const std::ranges::transform_view<SentinelView, PlusPlusConst>> sent2{sent1};
+  std::ranges::sentinel_t<const std::ranges::transform_view<SentinelView, PlusPlusConst>> sent3{sent2};
+  (void)sent3;
+
+  std::ranges::transform_view transformView2(SentinelView{4}, PlusPlusConst());
+  auto sent4 = transformView2.end();
+
+  static_assert(!EndIsIter<decltype(sent1)>);
+  static_assert(!EndIsIter<decltype(sent2)>);
+  assert(sent1.base() == globalBuff + 8);
+
+  auto iter1 = transformView1.begin();
+  assert(iter1 != sent1);
+  assert(iter1 != sent2);
+  assert(iter1 != sent4);
+  assert(iter1 + 8 == sent1);
+  assert(iter1 + 8 == sent2);
+  assert(iter1 + 4 == sent4);
+
+  assert(sent1 - iter1 == 8);
+  assert(sent4 - iter1 == 4);
+  assert(iter1 - sent1 == -8);
+  assert(iter1 - sent4 == -4);
+}
+
+// CTAD
+static_assert(std::same_as<decltype(std::ranges::transform_view(InputView(), PlusPlus())),
+                           std::ranges::transform_view<InputView, PlusPlus>>);
+static_assert(std::same_as<decltype(std::ranges::transform_view(std::declval<ForwardRange&>(), PlusPlus())),
+                           std::ranges::transform_view<std::ranges::ref_view<ForwardRange>, PlusPlus>>);
+static_assert(std::same_as<decltype(std::ranges::transform_view(BorrowableRange(), PlusPlus())),
+                           std::ranges::transform_view<std::ranges::subrange<int*>, PlusPlus>>);
+
+constexpr bool test() {
+  testViewMembers();
+  testIterator();
+  testSentinel();
+
+  return true;
+}
+
+template<std::ranges::range R>
+auto toUpper(R range) {
+  return std::ranges::transform_view(range, [](char c) { return std::toupper(c); });
+}
+
+template<std::ranges::range R, class Fn = std::plus<std::iter_value_t<R>>>
+auto withRandom(R range, Fn func = Fn()) {
+  return std::ranges::transform_view(range, std::bind_front(func, std::rand()));
+}
+
+template<class E1, class E2, size_t N, class Join = std::plus<E1>>
+auto joinArrays(E1 (&a)[N], E2 (&b)[N], Join join = Join()) {
+  return std::ranges::transform_view(a, [&a, &b, join](auto& x) {
+    auto idx = (&x) - a;
+    return join(x, b[idx]);
+  });
+}
+
+template<>
+constexpr bool std::ranges::enable_view<std::vector<int>> = true;
+
+template<>
+constexpr bool std::ranges::enable_view<std::string_view> = true;
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  {
+    std::vector vec = {1,2,3,4};
+    auto sortOfRandom = withRandom(vec);
+    std::vector check = {16808, 16809, 16810, 16811};
+    assert(std::equal(sortOfRandom.begin(), sortOfRandom.end(), check.begin(), check.end()));
+  }
+
+  {
+    int a[4] = {1, 2, 3, 4};
+    int b[4] = {4, 3, 2, 1};
+    auto out = joinArrays(a, b);
+    int check[4] = {5, 5, 5, 5};
+    assert(std::equal(out.begin(), out.end(), check));
+  }
+
+  {
+    std::string_view str = "Hello, World.";
+    auto upp = toUpper(str);
+    std::string_view check = "HELLO, WORLD.";
+    assert(std::equal(upp.begin(), upp.end(), check.begin(), check.end()));
+  }
+
+  return 0;
+}
+
+// The requirements for __iterator:
+template<class V, class F>
+concept IterMinusMinus = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter--;
+};
+static_assert(IterMinusMinus<BidirectionalView, PlusPlusConst>);
+static_assert(!IterMinusMinus<ForwardView, PlusPlusConst>);
+
+template<class V, class F>
+concept MinusMinusIter = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  --iter;
+};
+static_assert(MinusMinusIter<BidirectionalView, PlusPlusConst>);
+static_assert(!MinusMinusIter<ForwardView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterPlusEquals = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter += 1;
+};
+static_assert(IterPlusEquals<RandomAccessView, PlusPlusConst>);
+static_assert(!IterPlusEquals<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterMinusEquals = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter -= 1;
+};
+static_assert(IterMinusEquals<RandomAccessView, PlusPlusConst>);
+static_assert(!IterMinusEquals<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterSubscript = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter[0];
+};
+static_assert(IterSubscript<RandomAccessView, PlusPlusConst>);
+static_assert(!IterSubscript<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterLess = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter < iter;
+};
+static_assert(IterLess<RandomAccessView, PlusPlusConst>);
+static_assert(!IterLess<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterGreater = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter > iter;
+};
+static_assert(IterGreater<RandomAccessView, PlusPlusConst>);
+static_assert(!IterGreater<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterLessEq = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter <= iter;
+};
+static_assert(IterLessEq<RandomAccessView, PlusPlusConst>);
+static_assert(!IterLessEq<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterGreaterEq = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter >= iter;
+};
+static_assert(IterGreaterEq<RandomAccessView, PlusPlusConst>);
+static_assert(!IterGreaterEq<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterPlusN = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter + 1;
+};
+static_assert(IterPlusN<RandomAccessView, PlusPlusConst>);
+static_assert(!IterPlusN<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept NPlusIter = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  std::declval<typename decltype(iter)::difference_type>() + iter;
+};
+static_assert(NPlusIter<RandomAccessView, PlusPlusConst>);
+static_assert(!NPlusIter<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterMinusN = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter - 1;
+};
+static_assert(IterMinusN<RandomAccessView, PlusPlusConst>);
+static_assert(!IterMinusN<BidirectionalView, PlusPlusConst>);
+
+template<class V, class F>
+concept IterMinusIter = requires(std::ranges::iterator_t<std::ranges::transform_view<V, F>> iter) {
+  iter - iter;
+};
+static_assert(IterMinusIter<RandomAccessView, PlusPlusConst>);
+static_assert(!IterMinusIter<BidirectionalView, PlusPlusConst>);