[libcxx-commits] [libcxx] [libc++][ranges] P2164R9: Implements `views::enumerate` (PR #73617)

[A. Jiang via libcxx-commits]

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@@ -0,0 +1,77 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++23
+
+// <ranges>
+
+// class enumerate_view
+
+// class enumerate_view::iterator
+
+// constexpr auto operator[](difference_type n) const
+//   requires random_access_range<Base>;
+
+#include <array>
+#include <cassert>
+#include <ranges>
+#include <tuple>
+
+#include "test_iterators.h"
+
+template <class T, class U>
+concept HasSubscriptOperator = requires(T t, U u) { t[u]; };
+
+template <class BaseRange>
+using EnumerateIterator = std::ranges::iterator_t<std::ranges::enumerate_view<BaseRange>>;
+
+using Subrange = std::ranges::subrange<int*>;
+static_assert(HasSubscriptOperator<EnumerateIterator<Subrange>, int>);
+
+using BidirectionalRange = std::ranges::subrange<bidirectional_iterator<int*>>;
+static_assert(!HasSubscriptOperator<EnumerateIterator<BidirectionalRange>, int>);
+
+constexpr bool test() {
+  // Reference
+  {
+    std::array ts = {90, 1, 2, 84};
+    auto view     = ts | std::views::enumerate;
+    auto it       = view.begin();
+
+    using DifferenceT = std::iter_difference_t<std::iter_difference_t<decltype(it)>>;
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frederick-vs-ja wrote:

Why is `std::iter_difference_t` doubly used? Ditto below.

https://github.com/llvm/llvm-project/pull/73617