[libc++][ranges] LWG4035: single_view should provide empty (#98371)

Implements: https://wg21.link/LWG4035

- https://eel.is/c++draft/range.single.view

Author: H-G-Hristov

libcxx/docs/Status/Cxx2cIssues.csv

@@ -53,7 +53,7 @@
 "`4025 <https://wg21.link/LWG4025>`__","Move assignment operator of ``std::expected<cv void, E>`` should not be conditionally deleted","Tokyo March 2024","","",""
 "`4030 <https://wg21.link/LWG4030>`__","Clarify whether arithmetic expressions in ``[numeric.sat.func]`` are mathematical or C++","Tokyo March 2024","|Nothing To Do|","",""
 "`4031 <https://wg21.link/LWG4031>`__","``bad_expected_access<void>`` member functions should be ``noexcept``","Tokyo March 2024","|Complete|","16.0",""
-"`4035 <https://wg21.link/LWG4035>`__","``single_view`` should provide ``empty``","Tokyo March 2024","","","|ranges|"
+"`4035 <https://wg21.link/LWG4035>`__","``single_view`` should provide ``empty``","Tokyo March 2024","|Complete|","19.0","|ranges|"
 "`4036 <https://wg21.link/LWG4036>`__","``__alignof_is_defined`` is only implicitly specified in C++ and not yet deprecated","Tokyo March 2024","","",""
 "`4037 <https://wg21.link/LWG4037>`__","Static data members of ``ctype_base`` are not yet required to be usable in constant expressions","Tokyo March 2024","","",""
 "`4038 <https://wg21.link/LWG4038>`__","``std::text_encoding::aliases_view`` should have constexpr iterators","Tokyo March 2024","","",""

libcxx/include/__ranges/single_view.h

@@ -70,6 +70,8 @@ class _LIBCPP_ABI_LLVM18_NO_UNIQUE_ADDRESS single_view : public view_interface<s
 
   _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* end() const noexcept { return data() + 1; }
 
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool empty() noexcept { return false; }
+
   _LIBCPP_HIDE_FROM_ABI static constexpr size_t size() noexcept { return 1; }
 
   _LIBCPP_HIDE_FROM_ABI constexpr _Tp* data() noexcept { return __value_.operator->(); }

libcxx/test/std/ranges/range.factories/range.single.view/empty.pass.cpp

@@ -0,0 +1,64 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// static constexpr bool empty() noexcept;
+
+#include <cassert>
+#include <concepts>
+#include <ranges>
+#include <utility>
+
+#include "test_macros.h"
+
+struct Empty {};
+struct BigType {
+  char buffer[64] = {10};
+};
+
+template <typename T>
+constexpr void test_empty(T value) {
+  using SingleView = std::ranges::single_view<T>;
+
+  {
+    std::same_as<bool> decltype(auto) result = SingleView::empty();
+    assert(result == false);
+    static_assert(noexcept(SingleView::empty()));
+  }
+
+  {
+    SingleView sv{value};
+
+    std::same_as<bool> decltype(auto) result = std::ranges::empty(sv);
+    assert(result == false);
+    static_assert(noexcept(std::ranges::empty(sv)));
+  }
+  {
+    const SingleView sv{value};
+
+    std::same_as<bool> decltype(auto) result = std::ranges::empty(sv);
+    assert(result == false);
+    static_assert(noexcept(std::ranges::empty(std::as_const(sv))));
+  }
+}
+
+constexpr bool test() {
+  test_empty<int>(92);
+  test_empty<Empty>(Empty{});
+  test_empty<BigType>(BigType{});
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}