[libc++][ranges] LWG4001: iota_view should provide empty (#79687)

Implements: https://wg21.link/LWG4001
- https://eel.is/c++draft/range.iota.view

---------

Co-authored-by: Zingam <[email protected]>
Co-authored-by: Will Hawkins <[email protected]>

libcxx/docs/Status/Cxx2cIssues.csv

@@ -38,7 +38,7 @@
 "`3974 <https://wg21.link/LWG3974>`__","``mdspan::operator[]`` should not copy ``OtherIndexTypes``","Kona November 2023","","",""
 "`3987 <https://wg21.link/LWG3987>`__","Including ``<flat_foo>`` doesn't provide ``std::begin``/``end``","Kona November 2023","","","|flat_containers|"
 "`3990 <https://wg21.link/LWG3990>`__","Program-defined specializations of ``std::tuple`` and ``std::variant`` can't be properly supported","Kona November 2023","","",""
-"`4001 <https://wg21.link/LWG4001>`__","``iota_view`` should provide ``empty``","Kona November 2023","","","|ranges|"
+"`4001 <https://wg21.link/LWG4001>`__","``iota_view`` should provide ``empty``","Kona November 2023","|Complete|","19.0","|ranges|"
 "","","","","",""
 "`3767 <https://wg21.link/LWG3767>`__","``codecvt<charN_t, char8_t, mbstate_t>`` incorrectly added to locale","Tokyo March 2024","","",""
 "`3919 <https://wg21.link/LWG3919>`__","``enumerate_view`` may invoke UB for sized common non-forward underlying ranges","Tokyo March 2024","","","|ranges|"

libcxx/include/__ranges/iota_view.h

@@ -344,6 +344,8 @@ class iota_view : public view_interface<iota_view<_Start, _BoundSentinel>> {
     return __iterator{__bound_sentinel_};
   }
 
+  _LIBCPP_HIDE_FROM_ABI constexpr bool empty() const { return __value_ == __bound_sentinel_; }
+
   _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
     requires(same_as<_Start, _BoundSentinel> && __advanceable<_Start>) ||
             (integral<_Start> && integral<_BoundSentinel>) || sized_sentinel_for<_BoundSentinel, _Start>

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

@@ -0,0 +1,117 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// constexpr bool empty() const;
+
+#include <cassert>
+#include <concepts>
+#include <ranges>
+#include <utility>
+#include <vector>
+
+#include "types.h"
+
+template <typename R>
+concept HasEmpty = requires(const R r) {
+  std::ranges::empty(r);
+  { r.empty() } -> std::same_as<bool>;
+};
+
+constexpr void test_empty_iota_sfinae() {
+  std::vector<int> ev;
+
+  auto iv = std::views::iota(std::ranges::begin(ev), std::ranges::end(ev));
+
+  static_assert(HasEmpty<decltype(iv)>);
+  static_assert(HasEmpty<decltype(std::as_const(iv))>);
+}
+
+constexpr void test_nonempty_iota_sfinae() {
+  // Default ctr
+  {
+    std::ranges::iota_view<Int42<DefaultTo42>> iv;
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+  // Value pass
+  {
+    std::ranges::iota_view<SomeInt> iv(SomeInt(94));
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+
+  {
+    std::vector<char> v;
+    auto it = std::back_inserter(v);
+    auto iv = std::views::iota(it);
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+  {
+    std::vector<char> v{'b', 'a', 'b', 'a', 'z', 'm', 't'};
+    auto it = std::back_inserter(v);
+    auto iv = std::views::iota(it);
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+}
+
+constexpr void test_empty_iota() {
+  std::vector<int> ev;
+
+  auto iv = std::views::iota(std::ranges::begin(ev), std::ranges::end(ev));
+
+  assert(iv.empty());
+  assert(std::as_const(iv).empty());
+}
+
+constexpr void test_nonempty_iota() {
+  // Default ctr
+  {
+    std::ranges::iota_view<Int42<DefaultTo42>> iv;
+
+    assert(!iv.empty());
+  }
+  // Value pass
+  {
+    std::ranges::iota_view<SomeInt> iv(SomeInt(94));
+
+    assert(!iv.empty());
+  }
+
+  {
+    std::vector<char> v;
+    auto it = std::back_inserter(v);
+    auto iv = std::views::iota(it);
+
+    assert(!iv.empty());
+  }
+  {
+    std::vector<char> v{'b', 'a', 'b', 'a', 'z', 'm', 't'};
+    auto it = std::back_inserter(v);
+    auto iv = std::views::iota(it);
+
+    assert(!iv.empty());
+  }
+}
+
+constexpr bool test() {
+  test_empty_iota();
+  test_nonempty_iota();
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}