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<title>Issue 3601: common_iterator's postfix-proxy needs indirectly_readable</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="3601"><a href="lwg-defects.html#3601">3601</a>. <code>common_iterator</code>'s <code><i>postfix-proxy</i></code> needs <code>indirectly_readable</code></h3>
<p><b>Section:</b> 24.5.5.5 <a href="https://wg21.link/common.iter.nav">[common.iter.nav]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2021-09-24 <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#common.iter.nav">issues</a> in [common.iter.nav].</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>
It would appear that when <a href="https://wg21.link/p2259r1">P2259R1</a> added <code><i>postfix-proxy</i></code> to 
<code>common_iterator::operator++(int)</code> LWG missed a crucial difference between <code>operator++(int)</code> and 
<code>operator-&gt;</code> which uses a similar proxy: <code>operator-&gt;</code> requires the wrapped type to be 
<code>indirectly_readable</code>, but <code>operator++(int)</code> does not. Consequently, operations that read from the 
wrapped type for the <code><i>postfix-proxy</i></code> case in <code>operator++(int)</code> are not properly constrained 
to be valid.
</p>

<p><i>[2021-10-14; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six 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-3601"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 24.5.5.5 <a href="https://wg21.link/common.iter.nav">[common.iter.nav]</a> as indicated:</p>

<blockquote>
<pre>
decltype(auto) operator++(int);
</pre>
<blockquote>
<p>
-4- <i>Preconditions:</i> <code>holds_alternative&lt;I&gt;(v_)</code> is <code>true</code>.
<p/>
-5- <i>Effects:</i> If <code>I</code> models <code>forward_iterator</code>, equivalent to:
</p>
<blockquote><pre>
common_iterator tmp = *this;
++*this;
return tmp;
</pre></blockquote>
<p>
Otherwise, if <code>requires (I&amp; i) { { *i++ } -&gt; <i>can-reference</i>; }</code> is <code>true</code> 
or <code><ins>indirectly_readable&lt;I&gt; &amp;&amp;</ins> constructible_from&lt;iter_value_t&lt;I&gt;, 
iter_reference_t&lt;I&gt;&gt; &amp;&amp; move_constructible&lt;iter_value_t&lt;I&gt;&gt;</code> is <code>false</code>, 
equivalent to:
</p>
<blockquote><pre>
return get&lt;I&gt;(v_)++;
</pre></blockquote>
<p>
Otherwise, equivalent to: [&hellip;]
</p>
</blockquote>
</blockquote>
</li>

</ol>





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