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<title>Issue 3919: enumerate_view may invoke UB for sized common non-forward underlying ranges</title>
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<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#WP">WP</a> status.</em></p>
<h3 id="3919"><a href="lwg-defects.html#3919">3919</a>. <code>enumerate_view</code> may invoke UB for sized common non-forward underlying ranges</h3>
<p><b>Section:</b> 25.7.24 <a href="https://wg21.link/range.enumerate">[range.enumerate]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Patrick Palka <b>Opened:</b> 2023-04-07 <b>Last modified:</b> 2024-04-02</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
For a sized common range, <code>enumerate_view::end()</code> is specified to call
<code>ranges::distance</code>. But <code>ranges::distance</code> is not necessarily well-defined
for a sized non-forward range after calling <code>ranges::begin</code> (according to
25.4.4 <a href="https://wg21.link/range.sized">[range.sized]</a>).
<p/>
So for a sized common non-forward underlying range, it seems calling
<code>enumerate_view::begin()</code> followed by <code>enumerate_view::end()</code> may invoke UB
and thus make <code>enumerate_view</code> potentially unusable for such ranges.
<p/>
I suppose we might need to instead call and cache the result of
<code>ranges::distance</code> from <code>enumerate_view::begin()</code> for such ranges.
</p>

<p><i>[2022-04-12; Patrick Palka provides wording]</i></p>

<p>
The proposed wording follows the suggestion provided by Tim Song, to simply make <code>enumerate</code> non-common for this case.
</p>

<p><i>[2023-05-24; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>

<p><i>[Kona 2023-11-10; move to Ready]</i></p>


<p><i>[Tokyo 2024-03-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3919"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4944" title=" Working Draft, Standard for Programming Language C++">N4944</a>.
</p>

<ol>
<li>
<p>Modify 25.7.24.2 <a href="https://wg21.link/range.enumerate.view">[range.enumerate.view]</a>, class template class <code>enumerate_view</code> synopsis, as indicated:</p>

<blockquote>
<blockquote><pre>
[&hellip;]
constexpr auto end() requires (!<i>simple-view</i>&lt;V&gt;) {
  if constexpr (<ins>forward_range&lt;V&gt; &amp;&amp;</ins> common_range&lt;V&gt; &amp;&amp; sized_range&lt;V&gt;)
    return <i>iterator</i>&lt;false&gt;(ranges::end(<i>base_</i>), ranges::distance(<i>base_</i>));
  else
    return <i>sentinel</i>&lt;false&gt;(ranges::end(<i>base_</i>));
}
constexpr auto end() const requires <i>range-with-movable-references</i>&lt;const V&gt; {
  if constexpr (<ins>forward_range&lt;const V&gt; &amp;&amp;</ins> common_range&lt;const V&gt; &amp;&amp; sized_range&lt;const V&gt;)
    return <i>iterator</i>&lt;true&gt;(ranges::end(<i>base_</i>), ranges::distance(<i>base_</i>));
  else
    return <i>sentinel</i>&lt;true&gt;(ranges::end(<i>base_</i>));
}
[&hellip;]
</pre></blockquote>
</blockquote>
</li>

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