[libcxx][ranges] Add `ranges::empty_view`.

Differential Revision: https://reviews.llvm.org/D103208

libcxx/docs/OneRangesProposalStatus.csv

@@ -53,7 +53,7 @@ bidirectional_range: `D100278 <https://llvm.org/D100278>`_",
 [range.iota],iota_view,[range.all],Louis Dionne,,
 [range.take],take_view,[range.all],Zoe Carver,,
 [range.join],join_view,[range.all],Christopher Di Bella,,
-[range.empty],empty_view,[view.interface],Zoe Carver,,
+[range.empty],empty_view,[view.interface],Zoe Carver,,✅
 [range.single],single_view,[view.interface],,,
 [range.split],split_view,[range.all],,,
 [range.counted],view::counted,[range.subrange],,,

libcxx/include/CMakeLists.txt

@@ -122,6 +122,7 @@ set(files
   __ranges/concepts.h
   __ranges/data.h
   __ranges/empty.h
+  __ranges/empty_view.h
   __ranges/enable_borrowed_range.h
   __ranges/view_interface.h
   __ranges/view.h

libcxx/include/__ranges/empty_view.h

@@ -0,0 +1,45 @@
+// -*- 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_EMPTY_VIEW_H
+#define _LIBCPP___RANGES_EMPTY_VIEW_H
+
+#include <__config>
+#include <__ranges/view_interface.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)
+
+namespace ranges {
+  template<class _Tp>
+    requires is_object_v<_Tp>
+  class empty_view : public view_interface<empty_view<_Tp>> {
+  public:
+    static constexpr _Tp* begin() noexcept { return nullptr; }
+    static constexpr _Tp* end() noexcept { return nullptr; }
+    static constexpr _Tp* data() noexcept { return nullptr; }
+    static constexpr size_t size() noexcept { return 0; }
+    static constexpr bool empty() noexcept { return true; }
+  };
+} // namespace ranges
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___RANGES_EMPTY_VIEW_H

libcxx/include/ranges

@@ -83,6 +83,11 @@ namespace std::ranges {
   template<class D>
     requires is_class_v<D> && same_as<D, remove_cv_t<D>>
   class view_interface;
+
+  // [range.empty], empty view
+  template<class T>
+    requires is_object_v<T>
+  class empty_view;
 }
 
 */
@@ -92,6 +97,7 @@ namespace std::ranges {
 #include <__ranges/concepts.h>
 #include <__ranges/data.h>
 #include <__ranges/empty.h>
+#include <__ranges/empty_view.h>
 #include <__ranges/enable_borrowed_range.h>
 #include <__ranges/size.h>
 #include <__ranges/view_interface.h>

libcxx/test/std/ranges/range.adaptors/range.empty/empty_view.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<class T>
+// class empty_view;
+
+#include <ranges>
+#include <cassert>
+
+#include "test_macros.h"
+
+template<class T>
+constexpr void testType() {
+  static_assert(std::ranges::range<std::ranges::empty_view<T>>);
+  static_assert(std::ranges::range<const std::ranges::empty_view<T>>);
+  static_assert(std::ranges::view<std::ranges::empty_view<T>>);
+
+  std::ranges::empty_view<T> empty;
+
+  assert(empty.begin() == nullptr);
+  assert(empty.end() == nullptr);
+  assert(empty.data() == nullptr);
+  assert(empty.size() == 0);
+  assert(empty.empty() == true);
+
+  assert(std::ranges::begin(empty) == nullptr);
+  assert(std::ranges::end(empty) == nullptr);
+  assert(std::ranges::data(empty) == nullptr);
+  assert(std::ranges::size(empty) == 0);
+  assert(std::ranges::empty(empty) == true);
+}
+
+struct Empty {};
+struct BigType { char buff[8]; };
+
+template<class T>
+concept ValidEmptyView = requires { typename std::ranges::empty_view<T>; };
+
+constexpr bool test() {
+  // Not objects:
+  static_assert(!ValidEmptyView<int&>);
+  static_assert(!ValidEmptyView<void>);
+
+  testType<int>();
+  testType<const int>();
+  testType<int*>();
+  testType<Empty>();
+  testType<const Empty>();
+  testType<BigType>();
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}