[libc++] add global variable template std::views::empty

[libc++] add global variable template std::views::empty
Note it is neither a range adaptor, nor a CPO. It is simplify a global variable template.

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D122996

libcxx/include/__ranges/empty_view.h

@@ -36,6 +36,13 @@ namespace ranges {
 
   template<class _Tp>
   inline constexpr bool enable_borrowed_range<empty_view<_Tp>> = true;
+
+  namespace views {
+
+  template <class _Tp>
+  inline constexpr empty_view<_Tp> empty{};
+
+  } // namespace views
 } // namespace ranges
 
 #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)

libcxx/include/ranges

@@ -120,6 +120,14 @@ namespace std::ranges {
     requires is_object_v<T>
   class empty_view;
 
+  template<class T>
+    inline constexpr bool enable_borrowed_range<empty_view<T>> = true;
+
+  namespace views {
+    template<class T>
+      inline constexpr empty_view<T> empty{};
+  }
+
   // [range.all], all view
   namespace views {
     inline constexpr unspecified all = unspecified;

libcxx/test/std/ranges/range.adaptors/range.empty/views.empty.pass.cpp

@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+// 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-has-no-incomplete-ranges
+
+// template <class _Tp>
+// inline constexpr empty_view<_Tp> empty{};
+
+#include <ranges>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class T>
+constexpr void testType() {
+  ASSERT_SAME_TYPE(decltype(std::views::empty<T>), const std::ranges::empty_view<T>);
+  ASSERT_SAME_TYPE(decltype((std::views::empty<T>)), const std::ranges::empty_view<T>&);
+
+  auto v = std::views::empty<T>;
+  assert(std::ranges::empty(v));
+}
+
+struct Empty {};
+struct BigType {
+  char buff[8];
+};
+
+constexpr bool test() {
+
+  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;
+}