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<title>Issue 3660: iterator_traits&lt;common_iterator&gt;::pointer should conform to &sect;[iterator.traits]</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="3660"><a href="lwg-defects.html#3660">3660</a>. <code>iterator_traits&lt;common_iterator&gt;::pointer</code> should conform to &sect;[iterator.traits]</h3>
<p><b>Section:</b> 24.5.5.2 <a href="https://wg21.link/common.iter.types">[common.iter.types]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2022-01-20 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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>
24.3.2.3 <a href="https://wg21.link/iterator.traits">[iterator.traits]</a>/1 says:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
[&hellip;] In addition, the types
</p>
<blockquote><pre>
iterator_traits&lt;I&gt;::pointer
iterator_traits&lt;I&gt;::reference
</pre></blockquote>
<p>
shall be defined as the iterator's pointer and reference types; that is, for an iterator object <code>a</code> of class
type, the same type as <code>decltype(a.operator-&gt;())</code> and <code>decltype(*a)</code>, respectively. The type 
<code>iterator_traits&lt;I&gt;::pointer</code> shall be <code>void</code> for an iterator of class type <code>I</code> 
that does not support <code>operator-&gt;</code>. [&hellip;]
</p>
</blockquote>
<p>
24.5.5.2 <a href="https://wg21.link/common.iter.types">[common.iter.types]</a>/1 slightly contradicts this:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
The nested <i>typedef-name</i>s of the specialization of <code>iterator_traits</code> for 
<code>common_iterator&lt;I, S&gt;</code> are defined as follows.
</p>
<ol style="list-style-type:none">
<li><p>[&hellip;]</p></li>
<li><p>(1.3) &mdash; If the expression <code>a.operator-&gt;()</code> is well-formed, where <code>a</code> is an lvalue of 
type <code>const common_iterator&lt;I, S&gt;</code>, then <code>pointer</code> denotes the type of that expression. Otherwise, 
<code>pointer</code> denotes <code>void</code>.</p></li>
</ol>
</blockquote>
<p>
"The type of <code>a.operator-&gt;()</code>" is not necessarily the same as <code>decltype(a.operator-&gt;())</code>: 
when the expression is an lvalue or xvalue of type <code>T</code>, "the type of <code>a.operator-&gt;()</code>" is <code>T</code> 
but <code>decltype(a.operator-&gt;())</code> is either <code>T&amp;</code> or <code>T&amp;&amp;</code>. An implementation 
therefore cannot conform to the requirements of both cited paragraphs for some specializations of <code>common_iterator</code>.
<p/>
The most likely explanation for this contradiction is that the writer of the phrase 
"type of <code>a.operator-&gt;()</code>" was not cognizant of the difference in meaning and intended to actually write 
<code>decltype(a.operator-&gt;())</code>.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3660"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>. 
</p>

<blockquote class="note">
<p>
[<i>Drafting Note:</i> The wording change below includes an additional drive-by fix that ensures that the last
sentence "Otherwise, <code>pointer</code> denotes <code>void</code>" cannot be misinterpreted (due to the leading "If") 
to apply also for a situation when the outcome of the expression <code>a.operator-&gt;()</code> for a non-<code>const</code> 
<code>common_iterator&lt;I, S&gt;</code> is reflected upon.] 
</p>
</blockquote>


<ol>
<li><p>Modify 24.5.5.2 <a href="https://wg21.link/common.iter.types">[common.iter.types]</a> as indicated:</p>

<blockquote>
<p>
-1- The nested <i>typedef-name</i>s of the specialization of <code>iterator_traits</code> for 
<code>common_iterator&lt;I, S&gt;</code> are defined as follows.
</p>
<ol style="list-style-type:none">
<li><p>[&hellip;]</p></li>
<li><p>(1.3) &mdash; <ins>Let <code>a</code> denote an lvalue of type <code>const common_iterator&lt;I, 
S&gt;</code>.</ins> If the expression <code>a.operator-&gt;()</code> is well-formed, <del>where <code>a</code> 
is an lvalue of type <code>const common_iterator&lt;I, S&gt;</code>,</del> then <code>pointer</code> denotes 
<ins><code>decltype(a.operator-&gt;())</code></ins><del>the type of that expression</del>. Otherwise, 
<code>pointer</code> denotes <code>void</code>.</p></li>
</ol>
</blockquote>
</li>
</ol>





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