[libcxx-commits] [libcxx] [libcxx] cbegin should always return a constant iterator (PR #99915)

[A. Jiang via libcxx-commits]

================
@@ -0,0 +1,139 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++20
+
+// std::basic_const_iterator
+
+#include <iterator>
+#include <list>
+#include <ranges>
+#include <vector>
+#include "test_macros.h"
+#include "test_iterators.h"
+#include "type_algorithms.h"
+
+template <class T>
+concept has_iterator_category = requires { typename T::iterator_category; };
+
+template <class It>
+constexpr bool check_category_and_concept() {
+  using ConstIt = std::basic_const_iterator<It>;
+
+  if constexpr (std::contiguous_iterator<It>) {
+    ASSERT_SAME_TYPE(typename ConstIt::iterator_concept, std::contiguous_iterator_tag);
+    static_assert(std::contiguous_iterator<ConstIt>);
+  } else if constexpr (std::random_access_iterator<It>) {
+    ASSERT_SAME_TYPE(typename ConstIt::iterator_concept, std::random_access_iterator_tag);
+    static_assert(!std::contiguous_iterator<ConstIt>);
+    static_assert(std::random_access_iterator<ConstIt>);
+  } else if constexpr (std::bidirectional_iterator<It>) {
+    ASSERT_SAME_TYPE(typename ConstIt::iterator_concept, std::bidirectional_iterator_tag);
+    static_assert(!std::random_access_iterator<ConstIt>);
+    static_assert(std::bidirectional_iterator<ConstIt>);
+  } else if constexpr (std::forward_iterator<It>) {
+    ASSERT_SAME_TYPE(typename ConstIt::iterator_concept, std::forward_iterator_tag);
+    static_assert(!std::bidirectional_iterator<ConstIt>);
+    static_assert(std::forward_iterator<ConstIt>);
+  } else {
+    ASSERT_SAME_TYPE(typename ConstIt::iterator_concept, std::input_iterator_tag);
+    static_assert(!std::forward_iterator<ConstIt>);
+    static_assert(std::input_iterator<ConstIt>);
+  }
+
+  if constexpr (std::forward_iterator<It>) {
+    ASSERT_SAME_TYPE(typename ConstIt::iterator_category, typename std::iterator_traits<It>::iterator_category);
+  } else {
+    static_assert(!has_iterator_category<ConstIt>);
+  }
+
+  return true;
+}
+
+constexpr bool test_p2836r1() {
+  auto f = [](std::vector<int>::const_iterator) {};
+
+  auto v = std::vector<int>();
+  {
+    auto i1 = std::ranges::cbegin(v);
+    f(i1);
+  }
+
+  auto t = v | std::views::take_while([](int const x) { return x < 100; });
+  {
+    auto i2 = std::ranges::cbegin(t);
+    f(i2);
+  }
+
+  return true;
+}
+
+constexpr bool test_basic_operations() {
----------------
frederick-vs-ja wrote:

Changed.

https://github.com/llvm/llvm-project/pull/99915