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<title>Issue 3433: subrange::advance(n) has UB when n &lt; 0</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#C++23">C++23</a> status.</em></p>
<h3 id="3433"><a href="lwg-defects.html#3433">3433</a>. <code>subrange::advance(n)</code> has UB when <code>n &lt; 0</code></h3>
<p><b>Section:</b> 25.5.4.3 <a href="https://wg21.link/range.subrange.access">[range.subrange.access]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-04-21 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.subrange.access">issues</a> in [range.subrange.access].</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>
<code>subrange::advance(n)</code> is specified to call <code>ranges::advance(<i>begin_</i>, n, <i>end_</i>)</code> 
in both <code><i>StoreSize</i></code> and <code>!<i>StoreSize</i></code> cases (25.5.4.3 <a href="https://wg21.link/range.subrange.access">[range.subrange.access]</a>/9). 
Unfortunately, <code>ranges::advance(<i>begin_</i>, n, <i>end_</i>)</code> has undefined behavior when 
<code>n &lt; 0</code> unless <code>[<i>end_</i>, <i>begin_</i>)</code> denotes a valid range 
(24.4.4.2 <a href="https://wg21.link/range.iter.op.advance">[range.iter.op.advance]</a>/5). This would all be perfectly fine &mdash; the UB is exposed to 
the caller via effects-equivalent-to &mdash; were it not the design intent that <code>subrange::advance(-n)</code> 
be usable to reverse the effects of <code>subrange::advance(n)</code> when the <code>subrange</code> has a 
bidirectional iterator type. That is, however, clearly the design intent: <code>subrange::prev()</code>, 
for example, is equivalent to <code>subrange::advance(-1)</code>.
</p>

<p><i>[2020-05-11; Reflector prioritization]</i></p>

<p>
Set priority to 2 after reflector discussions.
</p>

<p><i>[2021-01-15; Issue processing telecon]</i></p>

<p>
Set status to Tentatively Ready after discussion and poll.
<table>
<tr><th>F</th><th>A</th><th>N</th></tr>
<tr><td>7</td><td>0</td><td>3</td></tr>
</table>
</p>
<p><i>[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3433"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 25.5.4.3 <a href="https://wg21.link/range.subrange.access">[range.subrange.access]</a> as indicated:</p>

<blockquote>
<pre>
constexpr subrange&amp; advance(iter_difference_t&lt;I&gt; n);
</pre>
<blockquote>
<p>
-9- <i>Effects:</i> Equivalent to:
<ol style="list-style-type: none">
<li><p><del>(9.1) &mdash; If <code><i>StoreSize</i></code> is <code>true</code>,</del></p>
<blockquote><pre>
<del>auto d = n - ranges::advance(<i>begin_</i>, n, <i>end_</i>);
if (d &gt;= 0)
  <i>size_</i> -= <i>to-unsigned-like</i>(d);
else
  <i>size_</i> += <i>to-unsigned-like</i>(-d);
return *this;</del>
</pre></blockquote>
</li>
<li><p><del>(9.2) &mdash; Otherwise,</del></p>
<blockquote><pre>
<del>ranges::advance(<i>begin_</i>, n, <i>end_</i>);
return *this;</del>
</pre></blockquote>
</li>
</ol>
<blockquote><pre>
<ins>if constexpr (bidirectional_iterator&lt;I&gt;) {
  if (n &lt; 0) {
    ranges::advance(<i>begin_</i>, n);
    if constexpr (<i>StoreSize</i>)
      <i>size_</i> += <i>to-unsigned-like</i>(-n);
    return *this;
  }
}

auto d = n - ranges::advance(<i>begin_</i>, n, <i>end_</i>);
if constexpr (<i>StoreSize</i>)
  <i>size_</i> -= <i>to-unsigned-like</i>(d);
return *this;</ins>
</pre></blockquote>
</p>
</blockquote>
</blockquote>
</li>
</ol>




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