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<title>Issue 3726: reverse_iterator::operator-&gt; is underconstrained for non-pointer iterators</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#NAD">NAD</a> status.</em></p>
<h3 id="3726"><a href="lwg-closed.html#3726">3726</a>. <code>reverse_iterator::operator-&gt;</code> is underconstrained for non-pointer iterators</h3>
<p><b>Section:</b> 24.5.1.6 <a href="https://wg21.link/reverse.iter.elem">[reverse.iter.elem]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-06-27 <b>Last modified:</b> 2022-11-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#reverse.iter.elem">issues</a> in [reverse.iter.elem].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
For non-pointer types, <code>reverse_iterator::operator-&gt;</code> only requires the expression 
<code>i.operator-&gt;()</code> to be well-formed.
<p/>
Since the return type of this function is explicitly specified as <code>pointer</code>, 
this will cause a hard error in the function body when the return type of <code>i.operator-&gt;()</code> 
cannot be converted to <code>pointer</code>.
<p/>
We should add a return type constraint for this.
</p>

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

<p>
<code>pointer</code> is <code>iterator_traits&lt;Iterator&gt;::pointer</code>,
which is required to name <code>decltype(i.operator-&gt;())</code>
(24.3.2.3 <a href="https://wg21.link/iterator.traits">[iterator.traits]</a>/1)
so the postulated problem simply does not arise in valid code.
</p>

<p><i>[2022-11-30 LWG telecon. Status changed: Tentatively NAD &rarr; NAD.]</i></p>



<p id="res-3726"><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 24.5.1.6 <a href="https://wg21.link/reverse.iter.elem">[reverse.iter.elem]</a> as indicated:</p>

<blockquote>
<pre>
constexpr pointer operator-&gt;() const
  requires (is_pointer_v&lt;Iterator&gt; ||
            requires(const Iterator i) { <ins>{</ins> i.operator-&gt;() <ins>} -&gt; convertible_to&lt;pointer&gt;</ins>; });
</pre>
<blockquote>
<p>
-2- <i>Effects</i>:
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; If <code>Iterator</code> is a pointer type, equivalent to: <code>return prev(current);</code></p></li>
<li><p>(2.2) &mdash; Otherwise, equivalent to: <code>return prev(current).operator-&gt;();</code></p>
</li>
</ol>
</blockquote>
</blockquote>
</li>

</ol>





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