[libc++] Implement P2446R2 (views::as_rvalue)

Reviewed By: ldionne, var-const, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D137637

Author: philnik777

libcxx/docs/ReleaseNotes.rst

@@ -68,6 +68,7 @@ Implemented Papers
 - P0323R12 - ``std::expected``
 - P1035R7 - Input Range Adaptors
 - P2325R3 - Views should not be required to be default constructible
+- P2446R2 - ``views::as_rvalue``
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx2bPapers.csv

@@ -73,7 +73,7 @@
 "`P2419R2 <https://wg21.link/P2419R2>`__","LWG","Clarify handling of encodings in localized formatting of chrono types","July 2022","",""
 "`P2438R2 <https://wg21.link/P2438R2>`__","LWG","``std::string::substr() &&``","July 2022","|Complete|","16.0"
 "`P2445R1 <https://wg21.link/P2445R1>`__","LWG","``forward_like``","July 2022","|Complete|","16.0"
-"`P2446R2 <https://wg21.link/P2446R2>`__","LWG","``views::as_rvalue``","July 2022","","","|ranges|"
+"`P2446R2 <https://wg21.link/P2446R2>`__","LWG","``views::as_rvalue``","July 2022","|Complete|","16.0","|ranges|"
 "`P2460R2 <https://wg21.link/P2460R2>`__","LWG","Relax requirements on ``wchar_t`` to match existing practices","July 2022","",""
 "`P2465R3 <https://wg21.link/P2465R3>`__","LWG","Standard Library Modules ``std`` and ``std.compat``","July 2022","",""
 "`P2467R1 <https://wg21.link/P2467R1>`__","LWG","Support exclusive mode for ``fstreams``","July 2022","",""

libcxx/docs/Status/RangesViews.csv

@@ -33,5 +33,5 @@ C++23,`slide <https://wg21.link/P2442R1>`_,Unassigned,No patch yet,Not started
 C++23,`chunk <https://wg21.link/P2442R1>`_,Unassigned,No patch yet,Not started
 C++23,`chunk_by <https://wg21.link/P2443R1>`_,Unassigned,No patch yet,Not started
 C++23,`as_const <https://wg21.link/P2278R4>`_,Unassigned,No patch yet,Not started
-C++23,`as_rvalue <https://wg21.link/P2446R2>`_,Nikolas Klauser,`D137637 <https://llvm.org/D137637>`_,Under review
+C++23,`as_rvalue <https://wg21.link/P2446R2>`_,Nikolas Klauser,`D137637 <https://llvm.org/D137637>`_,Complete
 C++23,`stride <https://wg21.link/P1899R3>`_,Unassigned,No patch yet,Not started

libcxx/include/CMakeLists.txt

@@ -499,6 +499,7 @@ set(files
   __random/weibull_distribution.h
   __ranges/access.h
   __ranges/all.h
+  __ranges/as_rvalue_view.h
   __ranges/common_view.h
   __ranges/concepts.h
   __ranges/copyable_box.h

libcxx/include/__iterator/move_sentinel.h

@@ -50,6 +50,8 @@ class _LIBCPP_TEMPLATE_VIS move_sentinel
     _Sent __last_ = _Sent();
 };
 
+_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(move_sentinel);
+
 #endif // _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/__ranges/as_rvalue_view.h

@@ -0,0 +1,137 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_AS_RVALUE_H
+#define _LIBCPP___RANGES_AS_RVALUE_H
+
+#include <__concepts/constructible.h>
+#include <__concepts/same_as.h>
+#include <__config>
+#include <__iterator/move_iterator.h>
+#include <__iterator/move_sentinel.h>
+#include <__ranges/access.h>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/enable_borrowed_range.h>
+#include <__ranges/range_adaptor.h>
+#include <__ranges/size.h>
+#include <__ranges/view_interface.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+template <view _View>
+  requires input_range<_View>
+class as_rvalue_view : public view_interface<as_rvalue_view<_View>> {
+  _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
+
+public:
+  _LIBCPP_HIDE_FROM_ABI as_rvalue_view()
+    requires default_initializable<_View>
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit as_rvalue_view(_View __base) : __base_(std::move(__base)) {}
+
+  _LIBCPP_HIDE_FROM_ABI constexpr _View base() const&
+    requires copy_constructible<_View>
+  {
+    return __base_;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin()
+    requires(!__simple_view<_View>)
+  {
+    return move_iterator(ranges::begin(__base_));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+    requires range<const _View>
+  {
+    return move_iterator(ranges::begin(__base_));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+    requires(!__simple_view<_View>)
+  {
+    if constexpr (common_range<_View>) {
+      return move_iterator(ranges::end(__base_));
+    } else {
+      return move_sentinel(ranges::end(__base_));
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+    requires range<const _View>
+  {
+    if constexpr (common_range<const _View>) {
+      return move_iterator(ranges::end(__base_));
+    } else {
+      return move_sentinel(ranges::end(__base_));
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+    requires sized_range<_View>
+  {
+    return ranges::size(__base_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+    requires sized_range<const _View>
+  {
+    return ranges::size(__base_);
+  }
+};
+
+template <class _Range>
+as_rvalue_view(_Range&&) -> as_rvalue_view<views::all_t<_Range>>;
+
+template <class _View>
+inline constexpr bool enable_borrowed_range<as_rvalue_view<_View>> = enable_borrowed_range<_View>;
+
+namespace views {
+namespace __as_rvalue {
+struct __fn : __range_adaptor_closure<__fn> {
+  template <class _Range>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Range&& __range) const
+      noexcept(noexcept(/**/ as_rvalue_view(std::forward<_Range>(__range))))
+          -> decltype(/*--*/ as_rvalue_view(std::forward<_Range>(__range))) {
+    return /*-------------*/ as_rvalue_view(std::forward<_Range>(__range));
+  }
+
+  template <class _Range>
+    requires same_as<range_rvalue_reference_t<_Range>, range_reference_t<_Range>>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Range&& __range) const
+      noexcept(noexcept(/**/ views::all(std::forward<_Range>(__range))))
+          -> decltype(/*--*/ views::all(std::forward<_Range>(__range))) {
+    return /*-------------*/ views::all(std::forward<_Range>(__range));
+  }
+};
+} // namespace __as_rvalue
+
+inline namespace __cpo {
+constexpr auto as_rvalue = __as_rvalue::__fn{};
+} // namespace __cpo
+} // namespace views
+} // namespace ranges
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 23
+
+#endif // _LIBCPP___RANGES_AS_RVALUE_H

libcxx/include/module.modulemap.in

@@ -1221,6 +1221,7 @@ module std [system] {
         export functional.__functional.compose
         export functional.__functional.perfect_forward
       }
+      module as_rvalue_view         { private header "__ranges/as_rvalue_view.h" }
       module common_view            { private header "__ranges/common_view.h" }
       module concepts               { private header "__ranges/concepts.h" }
       module copyable_box           { private header "__ranges/copyable_box.h" }

libcxx/include/ranges

@@ -292,6 +292,13 @@ namespace std::ranges {
       (enable_borrowed_range<Views> && ...);
 
   namespace views { inline constexpr unspecified zip = unspecified; }    // C++2b
+
+  // [range.as.rvalue]
+  template <view V>
+    requires input_range<V>
+  class as_rvalue_view; // since C++23
+
+  namespace views { inline constexpr unspecified as_rvalue ) unspecified; } // since C++23
 }
 
 namespace std {
@@ -330,6 +337,7 @@ namespace std {
 #include <__config>
 #include <__ranges/access.h>
 #include <__ranges/all.h>
+#include <__ranges/as_rvalue_view.h>
 #include <__ranges/common_view.h>
 #include <__ranges/concepts.h>
 #include <__ranges/counted.h>

libcxx/test/libcxx/diagnostics/view_adaptors.nodiscard_extensions.verify.cpp

@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Check that view adaptors are marked [[nodiscard]] as a conforming extension
+
+// UNSUPPORTED: c++03, c++11, c++14 ,c++17, c++20
+
+#include <ranges>
+#include <vector>
+
+void func() {
+  std::vector<int> range;
+
+  auto rvalue_view = std::views::as_rvalue(range);
+  std::views::as_rvalue(range);         // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::as_rvalue(rvalue_view);   // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

libcxx/test/libcxx/private_headers.verify.cpp

@@ -530,6 +530,7 @@ END-SCRIPT
 #include <__random/weibull_distribution.h> // expected-error@*:* {{use of private header from outside its module: '__random/weibull_distribution.h'}}
 #include <__ranges/access.h> // expected-error@*:* {{use of private header from outside its module: '__ranges/access.h'}}
 #include <__ranges/all.h> // expected-error@*:* {{use of private header from outside its module: '__ranges/all.h'}}
+#include <__ranges/as_rvalue_view.h> // expected-error@*:* {{use of private header from outside its module: '__ranges/as_rvalue_view.h'}}
 #include <__ranges/common_view.h> // expected-error@*:* {{use of private header from outside its module: '__ranges/common_view.h'}}
 #include <__ranges/concepts.h> // expected-error@*:* {{use of private header from outside its module: '__ranges/concepts.h'}}
 #include <__ranges/copyable_box.h> // expected-error@*:* {{use of private header from outside its module: '__ranges/copyable_box.h'}}

libcxx/test/std/iterators/predef.iterators/move.iterators/move.sentinel/ctad.compile.pass.cpp

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+#include <iterator>
+#include <type_traits>
+
+#include <test_iterators.h>
+
+static_assert(std::is_same_v<decltype(std::move_sentinel(std::default_sentinel_t{})),
+                             std::move_sentinel<std::default_sentinel_t>>);
+
+static_assert(
+    std::is_same_v<decltype(std::move_sentinel(sentinel_wrapper<int*>{})), std::move_sentinel<sentinel_wrapper<int*>>>);

libcxx/test/std/ranges/range.adaptors/range.as.rvalue/adaptor.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, c++20
+
+// std::views::as_rvalue
+
+#include <cassert>
+#include <functional>
+#include <ranges>
+#include <vector>
+
+#include "test_iterators.h"
+
+struct DefaultConstructibleView : std::ranges::view_base {
+  int i_;
+  int* begin();
+  int* end();
+};
+
+struct RValueView : std::ranges::view_base {};
+
+template <class View, class T>
+concept HasPipe = requires(View&& view, T&& t) {
+  { std::forward<View>(view) | std::forward<T>(t) };
+};
+
+struct NoView {};
+static_assert(std::is_invocable_v<decltype(std::views::as_rvalue), DefaultConstructibleView>);
+static_assert(!std::is_invocable_v<decltype(std::views::as_rvalue)>);
+static_assert(!std::is_invocable_v<decltype(std::views::as_rvalue), NoView>);
+static_assert(HasPipe<DefaultConstructibleView&, decltype(std::views::as_rvalue)>);
+static_assert(HasPipe<int (&)[10], decltype(std::views::as_rvalue)>);
+static_assert(!HasPipe<int (&&)[10], decltype(std::views::as_rvalue)>);
+static_assert(!HasPipe<NoView, decltype(std::views::as_rvalue)>);
+static_assert(std::is_same_v<decltype(std::views::as_rvalue), decltype(std::ranges::views::as_rvalue)>);
+
+struct move_iterator_range {
+  constexpr std::move_iterator<int*> begin() const { return {}; }
+  constexpr std::move_iterator<int*> end() const { return {}; }
+};
+
+static_assert(!std::ranges::view<move_iterator_range>);
+static_assert(std::ranges::range<move_iterator_range>);
+
+constexpr bool test() {
+  { // view | views::as_rvalue
+    DefaultConstructibleView v{{}, 3};
+    std::same_as<std::ranges::as_rvalue_view<DefaultConstructibleView>> decltype(auto) view = v | std::views::as_rvalue;
+    assert(view.base().i_ == 3);
+  }
+
+  { // adaptor | views::as_rvalue
+    DefaultConstructibleView v{{}, 3};
+    const auto partial = std::views::transform(std::identity{}) | std::views::as_rvalue;
+    std::same_as<std::ranges::as_rvalue_view<
+        std::ranges::transform_view<DefaultConstructibleView, std::identity>>> decltype(auto) view = partial(v);
+    assert(view.base().base().i_ == 3);
+  }
+
+  { // views::as_rvalue | adaptor
+    DefaultConstructibleView v{{}, 3};
+    const auto partial = std::views::as_rvalue | std::views::transform(std::identity{});
+    std::same_as<std::ranges::transform_view<std::ranges::as_rvalue_view<DefaultConstructibleView>,
+                                             std::identity>> decltype(auto) view = partial(v);
+    assert(view.base().base().i_ == 3);
+  }
+
+  { // rvalue-view | views::as_rvalue
+    int a[4] = {1, 2, 3, 4};
+    std::ranges::subrange range(rvalue_iterator{a}, rvalue_iterator{a + 4});
+    [[maybe_unused]] std::same_as<std::ranges::subrange<rvalue_iterator<int>>> decltype(auto) rval_range =
+        range | std::views::as_rvalue;
+  }
+
+  { // range | views::as_rvalue
+    [[maybe_unused]] std::same_as<std::ranges::as_rvalue_view<std::views::all_t<std::vector<int>>>> decltype(auto)
+        view = std::vector<int>{} | std::views::as_rvalue;
+  }
+
+  { // rvalue-range | views::as_rvalue
+    [[maybe_unused]] std::same_as<std::views::all_t<move_iterator_range>> decltype(auto) view =
+        move_iterator_range{} | std::views::as_rvalue;
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}