[libc++][ranges] P3137R3: views::to_input

Implements: https://wg21.link/P3137R3

Closes #127873

https://wg21.link/range.to.input

Author: H-G-Hristov

libcxx/docs/FeatureTestMacroTable.rst

@@ -488,6 +488,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_ranges_concat``                                *unimplemented*
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_ranges_to_input``                              ``202502L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_ratio``                                        ``202306L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_rcu``                                          *unimplemented*

libcxx/docs/ReleaseNotes/22.rst

@@ -39,6 +39,7 @@ Implemented Papers
 ------------------
 
 - P2321R2: ``zip`` (`Github <https://github.com/llvm/llvm-project/issues/105169>`__) (The paper is partially implemented. ``zip_transform_view`` is implemented in this release)
+- P3137R3: ``views::to_input`` (`Github <https://github.com/llvm/llvm-project/issues/127873>`__)
 
 Improvements and New Features
 -----------------------------

libcxx/docs/Status/Cxx2cPapers.csv

@@ -101,7 +101,7 @@
 "`P2900R14 <https://wg21.link/P2900R14>`__","Contracts for C++","2025-02 (Hagenberg)","","",""
 "`P3475R2 <https://wg21.link/P3475R2>`__","Defang and deprecate ``memory_order::consume``","2025-02 (Hagenberg)","","",""
 "`P2786R13 <https://wg21.link/P2786R13>`__","Trivial Relocatability For C++26","2025-02 (Hagenberg)","","",""
-"`P3137R3 <https://wg21.link/P3137R3>`__","``views::to_input``","2025-02 (Hagenberg)","","",""
+"`P3137R3 <https://wg21.link/P3137R3>`__","``views::to_input``","2025-02 (Hagenberg)","|Complete|","22",""
 "`P0472R3 <https://wg21.link/P0472R3>`__","Put ``std::monostate`` in ``<utility>``","2025-02 (Hagenberg)","|Complete|","21",""
 "`P3349R1 <https://wg21.link/P3349R1>`__","Converting contiguous iterators to pointers","2025-02 (Hagenberg)","","",""
 "`P3372R3 <https://wg21.link/P3372R3>`__","constexpr containers and adaptors","2025-02 (Hagenberg)","|In Progress|","",""

libcxx/include/CMakeLists.txt

@@ -735,6 +735,7 @@ set(files
   __ranges/take_view.h
   __ranges/take_while_view.h
   __ranges/to.h
+  __ranges/to_input_view.h
   __ranges/transform_view.h
   __ranges/view_interface.h
   __ranges/views.h

libcxx/include/__ranges/to_input_view.h

@@ -0,0 +1,211 @@
+// -*- 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_TO_INPUT_VIEW_H
+#define _LIBCPP___RANGES_TO_INPUT_VIEW_H
+
+#include <__concepts/constructible.h>
+#include <__config>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/view_interface.h>
+#include <__type_traits/maybe_const.h>
+#include <__utility/move.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 _LIBCPP_STD_VER >= 26
+
+namespace ranges {
+
+// [range.to.input.view
+
+template <input_range _View>
+  requires view<_View>
+class to_input_view : public view_interface<to_input_view<_View>> {
+  _View __base_ = _View(); // exposition only
+
+  // [range.to.input.iterator], class template to_input_view​::​iterator
+  template <bool Const>
+  class iterator; // exposition only
+
+public:
+  to_input_view()
+    requires default_initializable<_View>
+  = default;
+  constexpr explicit to_input_view(_View __base) : __base_(std::move(__base)) {}
+
+  constexpr _View base() const&
+    requires copy_constructible<_View>
+  {
+    return __base_;
+  }
+  constexpr _View base() && { return std::move(__base_); }
+
+  constexpr auto begin()
+    requires(!__simple_view<_View>)
+  {
+    return iterator<false>{ranges::begin(__base_)};
+  }
+  constexpr auto begin() const
+    requires range<const _View>
+  {
+    return iterator<true>{ranges::begin(__base_)};
+  }
+
+  constexpr auto end()
+    requires(!__simple_view<_View>)
+  {
+    return ranges::end(__base_);
+  }
+  constexpr auto end() const
+    requires range<const _View>
+  {
+    return 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_);
+  }
+
+  // TODO: Implement when P2846R6 is available.
+  // constexpr auto reserve_hint()
+  //   requires approximately_sized_range<_View>
+  // {
+  //   return ranges::reserve_hint(__base_);
+  // }
+  // constexpr auto reserve_hint() const
+  //   requires approximately_sized_range<const _View>
+  // {
+  //   return ranges::reserve_hint(__base_);
+  // }
+};
+
+template <class _Range>
+to_input_view(_Range&&) -> to_input_view<views::all_t<_Range>>;
+
+// [range.to.input.iterator]
+
+template <input_range _View>
+  requires view<_View>
+template <bool _Const>
+class to_input_view<_View>::iterator {
+  using _Base = __maybe_const<_Const, _View>; // exposition only
+
+  iterator_t<_Base> __current_ = iterator_t<_Base>(); // exposition only
+
+  constexpr explicit iterator(iterator_t<_Base> __current) : __current_(std::move(__current)) {} // exposition only
+
+  friend class to_input_view<_View>;
+
+public:
+  using difference_type  = range_difference_t<_Base>;
+  using value_type       = range_value_t<_Base>;
+  using iterator_concept = input_iterator_tag;
+
+  iterator()
+    requires default_initializable<iterator_t<_Base>>
+  = default;
+
+  iterator(iterator&&)            = default;
+  iterator& operator=(iterator&&) = default;
+
+  constexpr iterator(iterator<!_Const> __i)
+    requires _Const && convertible_to<iterator_t<_View>, iterator_t<_Base>>
+      : __current_(std::move(__i.__current_)) {}
+
+  constexpr iterator_t<_Base> base() && { return std::move(__current_); }
+  constexpr const iterator_t<_Base>& base() const& noexcept { return __current_; }
+
+  constexpr decltype(auto) operator*() const { return *__current_; }
+
+  constexpr iterator& operator++() {
+    ++__current_;
+    return *this;
+  }
+  constexpr void operator++(int) { ++*this; }
+
+  friend constexpr bool operator==(const iterator& __x, const sentinel_t<_Base>& __y) { return __x.__current_ == __y; }
+
+  friend constexpr difference_type operator-(const sentinel_t<_Base>& __y, const iterator& __x)
+    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base>>
+  {
+    return __y - __x.__current_;
+  }
+  friend constexpr difference_type operator-(const iterator& __x, const sentinel_t<_Base>& __y)
+    requires sized_sentinel_for<sentinel_t<_Base>, iterator_t<_Base>>
+  {
+    return __x.__current_ - __y;
+  }
+
+  friend constexpr range_rvalue_reference_t<_Base>
+  iter_move(const iterator& __i) noexcept(noexcept(ranges::iter_move(__i.__current_))) {
+    return ranges::iter_move(__i.__current_);
+  }
+
+  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 <class _View>
+constexpr bool enable_borrowed_range<to_input_view<_View>> = enable_borrowed_range<_View>;
+
+namespace views {
+namespace __to_input_view {
+
+struct __fn : __range_adaptor_closure<__fn> {
+  template <class _Range>
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto
+  operator()(_Range&& __range) noexcept(noexcept(/**/ to_input_view(std::forward<_Range>(__range))))
+      -> decltype(/*--*/ to_input_view(std::forward<_Range>(__range))) {
+    return /*---------*/ to_input_view(std::forward<_Range>(__range));
+  }
+
+  template <class _Range>
+    requires(!common_range<_Range> && !forward_range<_Range>)
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static constexpr auto
+  operator()(_Range&& __range) noexcept(noexcept(views::all(std::forward<_Range>(__range))))
+      -> decltype(/*--------------------------*/ views::all(std::forward<_Range>(__range))) {
+    return /*---------------------------------*/ views::all(std::forward<_Range>(__range));
+  }
+};
+
+} // namespace __to_input_view
+
+inline namespace __cpo {
+inline constexpr auto to_input = __to_input_view::__fn{};
+} // namespace __cpo
+} // namespace views
+} // namespace ranges
+
+#endif // _LIBCPP_STD_VER >= 26
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___RANGES_TO_INPUT_VIEW_H

libcxx/include/module.modulemap.in

@@ -1929,6 +1929,9 @@ module std [system] {
       header "__ranges/to.h"
       export std.functional.bind_back
     }
+    module to {
+      header "__ranges/to_input_view.h"
+    }
     module transform_view {
       header "__ranges/transform_view.h"
       export std.functional.bind_back

libcxx/include/ranges

@@ -362,6 +362,17 @@ namespace std::ranges {
   class chunk_by_view;                                                      // C++23
 
   namespace views { inline constexpr unspecified chunk_by = unspecified; }  // C++23
+
+  // [range.to.input], to input view
+  template<input_range V>
+    requires view<V>
+  class to_input_view;                                                      // C++26
+
+  template<class V>
+    constexpr bool enable_borrowed_range<to_input_view<V>> =
+      enable_borrowed_range<V>;                                             // C++26
+
+  namespace views { inline constexpr unspecified to_input = unspecified; }  // C++26
 }
 
 namespace std {
@@ -453,6 +464,10 @@ namespace std {
 #    include <__ranges/zip_view.h>
 #  endif
 
+#  if _LIBCPP_STD_VER >= 26
+#    include <__ranges/to_input_view.h>
+#  endif
+
 #  include <version>
 
 // standard-mandated includes

libcxx/include/version

@@ -211,6 +211,7 @@ __cpp_lib_ranges_repeat                                 202207L <ranges>
 __cpp_lib_ranges_slide                                  202202L <ranges>
 __cpp_lib_ranges_starts_ends_with                       202106L <algorithm>
 __cpp_lib_ranges_to_container                           202202L <ranges>
+__cpp_lib_ranges_to_input                               202502L <ranges>
 __cpp_lib_ranges_zip                                    202110L <ranges> <tuple> <utility>
 __cpp_lib_ratio                                         202306L <ratio>
 __cpp_lib_raw_memory_algorithms                         201606L <memory>
@@ -590,6 +591,7 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # define __cpp_lib_out_ptr                              202311L
 // # define __cpp_lib_philox_engine                        202406L
 // # define __cpp_lib_ranges_concat                        202403L
+# define __cpp_lib_ranges_to_input                      202502L
 # define __cpp_lib_ratio                                202306L
 // # define __cpp_lib_rcu                                  202306L
 # define __cpp_lib_reference_wrapper                    202403L

libcxx/modules/std/ranges.inc

@@ -94,6 +94,10 @@ export namespace std {
     // [range.utility.conv], range conversions
     using std::ranges::to;
 #endif
+#if _LIBCPP_STD_VER >= 26
+    // [range.to.input], to input view
+    using std::ranges::to_input_view;
+#endif
 
     // [range.empty], empty view
     using std::ranges::empty_view;

libcxx/test/std/language.support/support.limits/support.limits.general/ranges.version.compile.pass.cpp

@@ -64,6 +64,10 @@
 #    error "__cpp_lib_ranges_to_container should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_ranges_to_input
+#    error "__cpp_lib_ranges_to_input should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_ranges_zip
 #    error "__cpp_lib_ranges_zip should not be defined before c++23"
 #  endif
@@ -114,6 +118,10 @@
 #    error "__cpp_lib_ranges_to_container should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_ranges_to_input
+#    error "__cpp_lib_ranges_to_input should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_ranges_zip
 #    error "__cpp_lib_ranges_zip should not be defined before c++23"
 #  endif
@@ -164,6 +172,10 @@
 #    error "__cpp_lib_ranges_to_container should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_ranges_to_input
+#    error "__cpp_lib_ranges_to_input should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_ranges_zip
 #    error "__cpp_lib_ranges_zip should not be defined before c++23"
 #  endif
@@ -217,6 +229,10 @@
 #    error "__cpp_lib_ranges_to_container should not be defined before c++23"
 #  endif
 
+#  ifdef __cpp_lib_ranges_to_input
+#    error "__cpp_lib_ranges_to_input should not be defined before c++26"
+#  endif
+
 #  ifdef __cpp_lib_ranges_zip
 #    error "__cpp_lib_ranges_zip should not be defined before c++23"
 #  endif
@@ -312,6 +328,10 @@
 #    error "__cpp_lib_ranges_to_container should have the value 202202L in c++23"
 #  endif
 
+#  ifdef __cpp_lib_ranges_to_input
+#    error "__cpp_lib_ranges_to_input should not be defined before c++26"
+#  endif
+
 #  if !defined(_LIBCPP_VERSION)
 #    ifndef __cpp_lib_ranges_zip
 #      error "__cpp_lib_ranges_zip should be defined in c++23"
@@ -434,6 +454,13 @@
 #    error "__cpp_lib_ranges_to_container should have the value 202202L in c++26"
 #  endif
 
+#  ifndef __cpp_lib_ranges_to_input
+#    error "__cpp_lib_ranges_to_input should be defined in c++26"
+#  endif
+#  if __cpp_lib_ranges_to_input != 202502L
+#    error "__cpp_lib_ranges_to_input should have the value 202502L in c++26"
+#  endif
+
 #  if !defined(_LIBCPP_VERSION)
 #    ifndef __cpp_lib_ranges_zip
 #      error "__cpp_lib_ranges_zip should be defined in c++26"