<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3736: move_iterator missing disable_sized_sentinel_for specialization</title>
<meta property="og:title" content="Issue 3736: move_iterator missing disable_sized_sentinel_for specialization">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3736.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++23">C++23</a> status.</em></p>
<h3 id="3736"><a href="lwg-defects.html#3736">3736</a>. <code>move_iterator</code> missing <code>disable_sized_sentinel_for</code> specialization</h3>
<p><b>Section:</b> 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-07-14 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.synopsis">issues</a> in [iterator.synopsis].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Since <code>reverse_iterator::operator-</code> is not constrained, the standard adds a 
<code>disable_sized_sentinel_for</code> specialization for it to avoid situations where the 
underlying iterator can be subtracted making <code>reverse_iterator</code> accidentally model 
<code>sized_sentinel_for</code>.
<p/>
However, given that <code>move_iterator::operator-</code> is also unconstrained and the standard 
does not have the <code>disable_sized_sentinel_for</code> specialization for it, this makes 
<code>subrange&lt;move_iterator&lt;I&gt;, move_iterator&lt;I&gt;&gt;</code> unexpectedly satisfy 
<code>sized_range</code> and incorrectly use <code>I::operator-</code> to get size when <code>I</code> 
can be subtracted but not modeled <code>sized_sentinel_for&lt;I&gt;</code>.
<p/>
In addition, since <a href="https://wg21.link/P2520" title=" move_iterator should be a random access iterator">P2520</a> makes <code>move_iterator</code> no longer just <code>input_iterator</code>, 
<code>ranges::size</code> can get the size of the range by subtracting two <code>move_iterator</code> pairs,
this also makes <code>r | views::as_rvalue</code> may satisfy <code>sized_range</code> when neither <code>r</code> nor 
<code>r | views::reverse</code> is <code>sized_range</code>.
<p/>
We should add a <code>move_iterator</code> version of the <code>disable_sized_sentinel_for</code> 
specialization to the standard to avoid the above situation.
</p>

<p><i>[2022-08-23; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six votes in favour during reflector poll.
</p>
<p>
"but I don't think the issue text is quite right - both
<code>move_iterator</code> and <code>reverse_iterator</code>'s
<code>operator-</code> are constrained on <code>x.base() - y.base()</code>
being valid."
</p>
<p>
Does anyone remember why we did this for <code>reverse_iterator</code>
and not <code>move_iterator</code>?
</p>

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3736"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>

<li><p>Modify 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a>, header <code>&lt;iterator&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;class Iterator&gt;
    constexpr move_iterator&lt;Iterator&gt; make_move_iterator(Iterator i);

  <ins>template&lt;class Iterator1, class Iterator2&gt;
      requires (!sized_sentinel_for&lt;Iterator1, Iterator2&gt;)
    inline constexpr bool disable_sized_sentinel_for&lt;move_iterator&lt;Iterator1&gt;,
                                                     move_iterator&lt;Iterator2&gt;&gt; = true;</ins>

  [&hellip;]
}
</pre>
</blockquote>
</li>
</ol>





</body>
</html>
