[libcxx-commits] [libcxx] [libc++] Implement ranges::iota (PR #68494)

[Konstantin Varlamov via libcxx-commits]

================
@@ -0,0 +1,54 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___NUMERIC_RANGES_IOTA_H
+#define _LIBCPP___NUMERIC_RANGES_IOTA_H
+
+#include <__algorithm/out_value_result.h>
+#include <__config>
+#include <__ranges/concepts.h>
+#include <__utility/as_const.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+namespace ranges {
+template <typename _Out, typename _Tp>
+using iota_result = ranges::out_value_result<_Out, _Tp>;
+
+struct __iota_fn {
+  template <input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
+    requires indirectly_writable<_Out, const _Tp&>
+  constexpr iota_result<_Out, _Tp> operator()(_Out __first, _Sent __last, _Tp __value) const {
+    while (__first != __last) {
+      *__first = std::as_const(__value);
+      ++__first;
+      ++__value;
+    }
+    return {std::move(__first), std::move(__value)};
+  }
+
+  template <weakly_incrementable _Tp, ranges::output_range<const _Tp&> _Range>
+  constexpr iota_result<ranges::borrowed_iterator_t<_Range>, _Tp> operator()(_Range&& __r, _Tp __value) const {
+    return (*this)(ranges::begin(__r), ranges::end(__r), std::move(__value));
----------------
var-const wrote:

We have "robust" tests to make sure we're not copying or moving comparators and projections, but I don't think we do that for "values" as is the case here. Whether it's an omission or not is a separate question, but as it stands, I think the original form of this code (forwarding directly from the ranges overload to the iterator overload) should work -- can you please check that, @jamesETsmith?

https://github.com/llvm/llvm-project/pull/68494