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<title>Issue 3265: move_iterator's conversions are more broken after P1207</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="3265"><a href="lwg-defects.html#3265">3265</a>. <code>move_iterator</code>'s conversions are more broken after P1207</h3>
<p><b>Section:</b> 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2019-08-23 <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#move.iter.cons">issues</a> in [move.iter.cons].</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>
The converting constructor and assignment operator specified in 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> 
were technically broken before P1207:
</p>
<ul>
<li><p>24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> para 3 (and 5 for that matter) is an instance of LWG 
<a href="lwg-active.html#3105" title="T1 is convertible to T2 (Status: New)">3105</a><sup><a href="https://cplusplus.github.io/LWG/issue3105" title="Latest snapshot">(i)</a></sup>; it should instead mandate that <code>u.base()</code> is convertible to <code>Iterator</code>.</p></li>
<li><p>24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> para 5 uses "is convertible" to guard an assignment operation 
instead of a conversion; it should instead mandate.</p></li>
</ul>
<p>
After applying <a href="https://wg21.link/p1207r4">P1207R4</a> "Movability of Single-pass Iterators", 
<code>u.base()</code> is not always well-formed, exacerbating the problem. These operations must ensure 
that <code>u.base()</code> is well-formed.
<p/>
Drive-by:
</p>
<ul>
<li><p>Let's burninate "Constructs a <code>move_iterator</code>" while we're touching this subclause.</p></li>
<li><p>We'll also burninate "Iterator operations applied..." since the requirement it wants to impose 
is covered (and indeed must be covered) by the specifications of those other operations.</p></li>
</ul>

<p><i>[2019-09-14 Priority set to 2 based on reflector discussion]</i></p>


<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> as indicated:</p>

<blockquote>
<pre>
constexpr move_iterator();
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> <del>Constructs a <code>move_iterator</code>, 
v</del><ins>V</ins>alue-initializ<ins>es</ins><del>ing</del> <code>current</code>. <del>Iterator operations applied to the
resulting iterator have defined behavior if and only if the corresponding operations are defined on a
value-initialized iterator of type <code>Iterator</code>.</del>
</p>
</blockquote>
<pre>
constexpr explicit move_iterator(Iterator i);
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> <del>Constructs a move_iterator, i</del><ins>I</ins>nitializ<ins>es</ins><del>ing</del> 
<code>current</code> with <code>std::move(i)</code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr move_iterator(const move_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
-3- <i>Mandates:</i> <code><del>U</del><ins>u.base()</ins></code> is <ins>well-formed and</ins> convertible 
to <code>Iterator</code>.
<p/>
-4- <i>Effects:</i> <del>Constructs a <code>move_iterator</code>, i</del><ins>I</ins>nitializ<ins>es</ins><del>ing</del> 
<code>current</code> with <code>u.base()</code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr move_iterator&amp; operator=(const move_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
-5- <i>Mandates:</i> <del><code>U</code> is convertible to <code>Iterator</code></del><ins><code>u.base()</code> is 
well-formed and <code>is_assignable_v&lt;Iterator&amp;, const U&amp;&gt;</code> is <code>true</code></ins>.
<p/>
-6- <i>Effects:</i> Assigns <code>u.base()</code> to <code>current</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2020-02-14; Prague]</i></p>

<p>
LWG Review. Some wording improvements have been made and lead to revised wording.
</p>

<p><i>[2020-02-16; Prague]</i></p>

<p>Reviewed revised wording and moved to Ready for Varna.</p>

<p><i>[2020-07-17; superseded by <a href="lwg-defects.html#3435" title="three_way_comparable_with&lt;reverse_iterator&lt;int*&gt;, reverse_iterator&lt;const int*&gt;&gt; (Status: C++23)">3435</a><sup><a href="https://cplusplus.github.io/LWG/issue3435" title="Latest snapshot">(i)</a></sup>]</i></p>

<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3265"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4849">N4849</a>.</p>

<ol>
<li><p>Modify 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> as indicated:</p>

<blockquote>
<pre>
constexpr move_iterator();
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> <del>Constructs a <code>move_iterator</code>, 
v</del><ins>V</ins>alue-initializ<ins>es</ins><del>ing</del> <code>current</code>. <del>Iterator operations applied to the
resulting iterator have defined behavior if and only if the corresponding operations are defined on a
value-initialized iterator of type <code>Iterator</code>.</del>
</p>
</blockquote>
<pre>
constexpr explicit move_iterator(Iterator i);
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> <del>Constructs a move_iterator, i</del><ins>I</ins>nitializ<ins>es</ins><del>ing</del> 
<code>current</code> with <code>std::move(i)</code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr move_iterator(const move_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
-3- <i>Mandates:</i> <code><del>U</del><ins>u.base()</ins></code> is <ins>well-formed and</ins> convertible 
to <code>Iterator</code>.
<p/>
-4- <i>Effects:</i> <del>Constructs a <code>move_iterator</code>, i</del><ins>I</ins>nitializ<ins>es</ins><del>ing</del> 
<code>current</code> with <code>u.base()</code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr move_iterator&amp; operator=(const move_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
-5- <i>Mandates:</i> <del><code>U</code> is convertible to <code>Iterator</code></del><ins><code>u.base()</code> is 
well-formed and <code>is_assignable_v&lt;Iterator&amp;, U&gt;</code> is <code>true</code></ins>.
<p/>
-6- <i>Effects:</i> Assigns <code>u.base()</code> to <code>current</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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