[libc++][ranges] P2387R3: Pipe support for user-defined range adaptors (#89148)

This patch finalizes the std::ranges::range_adaptor_closure
class template from https://wg21.link/P2387R3.

  // [range.adaptor.object], range adaptor objects
  template<class D>
    requires is_class_v<D> && same_as<D, remove_cv_t<D>>
  class range_adaptor_closure { };

The current implementation of __range_adaptor_closure was introduced
in ee44dd8062a26541808fc0d3fd5c6703e19f6016 and has served as the
foundation for the range adaptors in libc++ for a while. This patch
keeps its implementation, with the exception of the following changes:

- __range_adaptor_closure now includes the missing constraints
  `is_class_v<D> && same_as<D, remove_cv_t<D>>` to restrict the 
  type of class that can inherit from it. (https://eel.is/c++draft/ranges.syn)
- The operator| of __range_adaptor_closure no longer requires its
  first argument to model viewable_range. (https://eel.is/c++draft/range.adaptor.object#1)
- The _RangeAdaptorClosure concept is refined to exclude cases where
  T models range or where T has base classes of type range_adaptor_closure<U>
  for another type U. (https://eel.is/c++draft/range.adaptor.object#2)

7 files changed, 186 insertions, 20 deletions

diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index b466b4cd8140..8724f321a9d1 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -49,6 +49,7 @@ Implemented Papers
 - P2302R4 - ``std::ranges::contains``
 - P1659R3 - ``std::ranges::starts_with`` and ``std::ranges::ends_with``
 - P3029R1 - Better ``mdspan``'s CTAD
+- P2387R3 - Pipe support for user-defined range adaptors
 
 Improvements and New Features
 -----------------------------
diff --git a/libcxx/docs/Status/Cxx23.rst b/libcxx/docs/Status/Cxx23.rst
index b19ff4fdc0f7..23d30c8128d7 100644
--- a/libcxx/docs/Status/Cxx23.rst
+++ b/libcxx/docs/Status/Cxx23.rst
@@ -43,7 +43,6 @@ Paper Status
    .. [#note-P0533R9] P0533R9: ``isfinite``, ``isinf``, ``isnan`` and ``isnormal`` are implemented.
    .. [#note-P1413R3] P1413R3: ``std::aligned_storage_t`` and ``std::aligned_union_t`` are marked deprecated, but
       clang doesn't issue a diagnostic for deprecated using template declarations.
-   .. [#note-P2387R3] P2387R3: ``bind_back`` only
    .. [#note-P2520R0] P2520R0: Libc++ implemented this paper as a DR in C++20 as well.
    .. [#note-P2711R1] P2711R1: ``join_with_view`` hasn't been done yet since this type isn't implemented yet.
    .. [#note-P2770R0] P2770R0: ``join_with_view`` hasn't been done yet since this type isn't implemented yet.
diff --git a/libcxx/docs/Status/Cxx23Papers.csv b/libcxx/docs/Status/Cxx23Papers.csv
index 065db97a0b0b..f75dd288304b 100644
--- a/libcxx/docs/Status/Cxx23Papers.csv
+++ b/libcxx/docs/Status/Cxx23Papers.csv
@@ -45,7 +45,7 @@
 "`P1413R3 <https://wg21.link/P1413R3>`__","LWG","Deprecate ``std::aligned_storage`` and ``std::aligned_union``","February 2022","|Complete| [#note-P1413R3]_",""
 "`P2255R2 <https://wg21.link/P2255R2>`__","LWG","A type trait to detect reference binding to temporary","February 2022","",""
 "`P2273R3 <https://wg21.link/P2273R3>`__","LWG","Making ``std::unique_ptr`` constexpr","February 2022","|Complete|","16.0"
-"`P2387R3 <https://wg21.link/P2387R3>`__","LWG","Pipe support for user-defined range adaptors","February 2022","|Partial| [#note-P2387R3]_","","|ranges|"
+"`P2387R3 <https://wg21.link/P2387R3>`__","LWG","Pipe support for user-defined range adaptors","February 2022","|Complete|","19.0","|ranges|"
 "`P2440R1 <https://wg21.link/P2440R1>`__","LWG","``ranges::iota``, ``ranges::shift_left`` and ``ranges::shift_right``","February 2022","","","|ranges|"
 "`P2441R2 <https://wg21.link/P2441R2>`__","LWG","``views::join_with``","February 2022","|In Progress|","","|ranges|"
 "`P2442R1 <https://wg21.link/P2442R1>`__","LWG","Windowing range adaptors: ``views::chunk`` and ``views::slide``","February 2022","","","|ranges|"
diff --git a/libcxx/docs/Status/RangesMajorFeatures.csv b/libcxx/docs/Status/RangesMajorFeatures.csv
index c0bec8d924e8..d00fbce9edf4 100644
--- a/libcxx/docs/Status/RangesMajorFeatures.csv
+++ b/libcxx/docs/Status/RangesMajorFeatures.csv
@@ -1,5 +1,5 @@
 Standard,Name,Assignee,CL,Status
 C++23,`ranges::to <https://wg21.link/P1206R7>`_,Konstantin Varlamov,`D142335 <https://reviews.llvm.org/D142335>`_,Complete
-C++23,`Pipe support for user-defined range adaptors <https://wg21.link/P2387R3>`_,Unassigned,No patch yet,Not started
+C++23,`Pipe support for user-defined range adaptors <https://wg21.link/P2387R3>`_,"Louis Dionne, Jakub Mazurkiewicz, and Xiaoyang Liu",Various,Complete
 C++23,`Formatting Ranges <https://wg21.link/P2286R8>`_,Mark de Wever,Various,Complete
 C++20,`Stashing stashing iterators for proper flattening <https://wg21.link/P2770R0>`_,Jakub Mazurkiewicz,Various,In progress
diff --git a/libcxx/include/__ranges/range_adaptor.h b/libcxx/include/__ranges/range_adaptor.h
index 726b7eda019e..2da246f24e1d 100644
--- a/libcxx/include/__ranges/range_adaptor.h
+++ b/libcxx/include/__ranges/range_adaptor.h
@@ -19,6 +19,7 @@
 #include <__functional/invoke.h>
 #include <__ranges/concepts.h>
 #include <__type_traits/decay.h>
+#include <__type_traits/is_class.h>
 #include <__type_traits/is_nothrow_constructible.h>
 #include <__type_traits/remove_cvref.h>
 #include <__utility/forward.h>
@@ -35,12 +36,15 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
+namespace ranges {
+
 // CRTP base that one can derive from in order to be considered a range adaptor closure
 // by the library. When deriving from this class, a pipe operator will be provided to
 // make the following hold:
 // - `x | f` is equivalent to `f(x)`
 // - `f1 | f2` is an adaptor closure `g` such that `g(x)` is equivalent to `f2(f1(x))`
 template <class _Tp>
+  requires is_class_v<_Tp> && same_as<_Tp, remove_cv_t<_Tp>>
 struct __range_adaptor_closure;
 
 // Type that wraps an arbitrary function object and makes it into a range adaptor closure,
@@ -52,27 +56,42 @@ struct __range_adaptor_closure_t : _Fn, __range_adaptor_closure<__range_adaptor_
 _LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__range_adaptor_closure_t);
 
 template <class _Tp>
-concept _RangeAdaptorClosure = derived_from<remove_cvref_t<_Tp>, __range_adaptor_closure<remove_cvref_t<_Tp>>>;
+_Tp __derived_from_range_adaptor_closure(__range_adaptor_closure<_Tp>*);
 
 template <class _Tp>
-struct __range_adaptor_closure {
-  template <ranges::viewable_range _View, _RangeAdaptorClosure _Closure>
-    requires same_as<_Tp, remove_cvref_t<_Closure>> && invocable<_Closure, _View>
-  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr decltype(auto)
-  operator|(_View&& __view, _Closure&& __closure) noexcept(is_nothrow_invocable_v<_Closure, _View>) {
-    return std::invoke(std::forward<_Closure>(__closure), std::forward<_View>(__view));
-  }
-
-  template <_RangeAdaptorClosure _Closure, _RangeAdaptorClosure _OtherClosure>
-    requires same_as<_Tp, remove_cvref_t<_Closure>> && constructible_from<decay_t<_Closure>, _Closure> &&
-             constructible_from<decay_t<_OtherClosure>, _OtherClosure>
-  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr auto operator|(_Closure&& __c1, _OtherClosure&& __c2) noexcept(
-      is_nothrow_constructible_v<decay_t<_Closure>, _Closure> &&
-      is_nothrow_constructible_v<decay_t<_OtherClosure>, _OtherClosure>) {
-    return __range_adaptor_closure_t(std::__compose(std::forward<_OtherClosure>(__c2), std::forward<_Closure>(__c1)));
-  }
+concept _RangeAdaptorClosure = !ranges::range<remove_cvref_t<_Tp>> && requires {
+  // Ensure that `remove_cvref_t<_Tp>` is derived from `__range_adaptor_closure<remove_cvref_t<_Tp>>` and isn't derived
+  // from `__range_adaptor_closure<U>` for any other type `U`.
+  { ranges::__derived_from_range_adaptor_closure((remove_cvref_t<_Tp>*)nullptr) } -> same_as<remove_cvref_t<_Tp>>;
 };
 
+template <ranges::range _Range, _RangeAdaptorClosure _Closure>
+  requires invocable<_Closure, _Range>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto)
+operator|(_Range&& __range, _Closure&& __closure) noexcept(is_nothrow_invocable_v<_Closure, _Range>) {
+  return std::invoke(std::forward<_Closure>(__closure), std::forward<_Range>(__range));
+}
+
+template <_RangeAdaptorClosure _Closure, _RangeAdaptorClosure _OtherClosure>
+  requires constructible_from<decay_t<_Closure>, _Closure> && constructible_from<decay_t<_OtherClosure>, _OtherClosure>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto operator|(_Closure&& __c1, _OtherClosure&& __c2) noexcept(
+    is_nothrow_constructible_v<decay_t<_Closure>, _Closure> &&
+    is_nothrow_constructible_v<decay_t<_OtherClosure>, _OtherClosure>) {
+  return __range_adaptor_closure_t(std::__compose(std::forward<_OtherClosure>(__c2), std::forward<_Closure>(__c1)));
+}
+
+template <class _Tp>
+  requires is_class_v<_Tp> && same_as<_Tp, remove_cv_t<_Tp>>
+struct __range_adaptor_closure {};
+
+#  if _LIBCPP_STD_VER >= 23
+template <class _Tp>
+  requires is_class_v<_Tp> && same_as<_Tp, remove_cv_t<_Tp>>
+class range_adaptor_closure : public __range_adaptor_closure<_Tp> {};
+#  endif // _LIBCPP_STD_VER >= 23
+
+} // namespace ranges
+
 #endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/include/ranges b/libcxx/include/ranges
index 167d2137eaf4..07a525ed8641 100644
--- a/libcxx/include/ranges
+++ b/libcxx/include/ranges
@@ -93,6 +93,11 @@ namespace std::ranges {
   template<class T>
   concept viewable_range = see below;
 
+  // [range.adaptor.object], range adaptor objects
+  template<class D>
+    requires is_class_v<D> && same_as<D, remove_cv_t<D>>
+  class range_adaptor_closure { };  // Since c++23
+
   // [view.interface], class template view_interface
   template<class D>
     requires is_class_v<D> && same_as<D, remove_cv_t<D>>
diff --git a/libcxx/test/std/ranges/range.adaptors/range.adaptor.object/range_adaptor_closure.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.adaptor.object/range_adaptor_closure.pass.cpp
new file mode 100644
index 000000000000..9d1eb1243458
--- /dev/null
+++ b/libcxx/test/std/ranges/range.adaptors/range.adaptor.object/range_adaptor_closure.pass.cpp
@@ -0,0 +1,142 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++20
+
+// std::ranges::range_adaptor_closure;
+
+#include <ranges>
+
+#include <algorithm>
+#include <vector>
+
+#include "test_range.h"
+
+template <class T>
+concept CanDeriveFromRangeAdaptorClosure = requires { typename std::ranges::range_adaptor_closure<T>; };
+static_assert(!CanDeriveFromRangeAdaptorClosure<int>);
+
+struct Foo {};
+static_assert(CanDeriveFromRangeAdaptorClosure<Foo>);
+static_assert(!CanDeriveFromRangeAdaptorClosure<Foo&>);
+static_assert(!CanDeriveFromRangeAdaptorClosure<const Foo>);
+static_assert(!CanDeriveFromRangeAdaptorClosure<volatile Foo>);
+static_assert(!CanDeriveFromRangeAdaptorClosure<const volatile Foo&&>);
+
+struct incomplete_t;
+static_assert(CanDeriveFromRangeAdaptorClosure<incomplete_t>);
+
+using range_t = std::vector<int>;
+
+template <class T>
+concept RangeAdaptorClosure =
+    CanBePiped<range_t, T&> && CanBePiped<range_t, const T&> && CanBePiped<range_t, T&&> &&
+    CanBePiped<range_t, const T&&>;
+
+struct callable : std::ranges::range_adaptor_closure<callable> {
+  static void operator()(const range_t&) {}
+};
+static_assert(RangeAdaptorClosure<callable>);
+
+// `not_callable_1` doesn't have an `operator()`
+struct not_callable_1 : std::ranges::range_adaptor_closure<not_callable_1> {};
+static_assert(!RangeAdaptorClosure<not_callable_1>);
+
+// `not_callable_2` doesn't have an `operator()` that accepts a `range` argument
+struct not_callable_2 : std::ranges::range_adaptor_closure<not_callable_2> {
+  static void operator()() {}
+};
+static_assert(!RangeAdaptorClosure<not_callable_2>);
+
+// `not_derived_from_1` doesn't derive from `std::ranges::range_adaptor_closure`
+struct not_derived_from_1 {
+  static void operator()(const range_t&) {}
+};
+static_assert(!RangeAdaptorClosure<not_derived_from_1>);
+
+// `not_derived_from_2` doesn't publicly derive from `std::ranges::range_adaptor_closure`
+struct not_derived_from_2 : private std::ranges::range_adaptor_closure<not_derived_from_2> {
+  static void operator()(const range_t&) {}
+};
+static_assert(!RangeAdaptorClosure<not_derived_from_2>);
+
+// `not_derived_from_3` doesn't derive from the correct specialization of `std::ranges::range_adaptor_closure`
+struct not_derived_from_3 : std::ranges::range_adaptor_closure<callable> {
+  static void operator()(const range_t&) {}
+};
+static_assert(!RangeAdaptorClosure<not_derived_from_3>);
+
+// `not_derived_from_4` doesn't derive from exactly one specialization of `std::ranges::range_adaptor_closure`
+struct not_derived_from_4
+    : std::ranges::range_adaptor_closure<not_derived_from_4>,
+      std::ranges::range_adaptor_closure<callable> {
+  static void operator()(const range_t&) {}
+};
+static_assert(!RangeAdaptorClosure<not_derived_from_4>);
+
+// `is_range` models `range`
+struct is_range : std::ranges::range_adaptor_closure<is_range> {
+  static void operator()(const range_t&) {}
+  int* begin() const { return nullptr; }
+  int* end() const { return nullptr; }
+};
+static_assert(std::ranges::range<is_range> && std::ranges::range<const is_range>);
+static_assert(!RangeAdaptorClosure<is_range>);
+
+// user-defined range adaptor closure object
+struct negate_fn : std::ranges::range_adaptor_closure<negate_fn> {
+  template <std::ranges::range Range>
+  static constexpr decltype(auto) operator()(Range&& range) {
+    return std::forward<Range>(range) | std::views::transform([](auto element) { return -element; });
+  }
+};
+static_assert(RangeAdaptorClosure<negate_fn>);
+constexpr auto negate = negate_fn{};
+
+// user-defined range adaptor closure object
+struct plus_1_fn : std::ranges::range_adaptor_closure<plus_1_fn> {
+  template <std::ranges::range Range>
+  static constexpr decltype(auto) operator()(Range&& range) {
+    return std::forward<Range>(range) | std::views::transform([](auto element) { return element + 1; });
+  }
+};
+static_assert(RangeAdaptorClosure<plus_1_fn>);
+constexpr auto plus_1 = plus_1_fn{};
+
+constexpr bool test() {
+  const std::vector<int> n{1, 2, 3, 4, 5};
+  const std::vector<int> n_negate{-1, -2, -3, -4, -5};
+
+  assert(std::ranges::equal(n | negate, n_negate));
+  assert(std::ranges::equal(negate(n), n_negate));
+
+  assert(std::ranges::equal(n | negate | negate, n));
+  assert(std::ranges::equal(n | (negate | negate), n));
+  assert(std::ranges::equal((n | negate) | negate, n));
+  assert(std::ranges::equal(negate(n) | negate, n));
+  assert(std::ranges::equal(negate(n | negate), n));
+  assert(std::ranges::equal((negate | negate)(n), n));
+  assert(std::ranges::equal(negate(negate(n)), n));
+
+  const std::vector<int> n_plus_1_negate{-2, -3, -4, -5, -6};
+  assert(std::ranges::equal(n | plus_1 | negate, n_plus_1_negate));
+  assert(std::ranges::equal(
+      n | plus_1 | std::views::transform([](auto element) { return element; }) | negate, n_plus_1_negate));
+
+  const std::vector<int> n_negate_plus_1{0, -1, -2, -3, -4};
+  assert(std::ranges::equal(n | negate | plus_1, n_negate_plus_1));
+  assert(std::ranges::equal(n | std::views::reverse | negate | plus_1 | std::views::reverse, n_negate_plus_1));
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}