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<title>Issue 3293: move_iterator operator+() has incorrect constraints</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="3293"><a href="lwg-defects.html#3293">3293</a>. <code>move_iterator operator+()</code> has incorrect constraints</h3>
<p><b>Section:</b> 24.5.4.9 <a href="https://wg21.link/move.iter.nonmember">[move.iter.nonmember]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Bo Persson <b>Opened:</b> 2019-09-13 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#move.iter.nonmember">issues</a> in [move.iter.nonmember].</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>
Section 24.5.4.9 <a href="https://wg21.link/move.iter.nonmember">[move.iter.nonmember]</a>/2-3 says:
</p>
<blockquote>
<pre>
template&lt;class Iterator&gt;
  constexpr move_iterator&lt;Iterator&gt;
    operator+(iter_difference_t&lt;Iterator&gt; n, const move_iterator&lt;Iterator&gt;&amp; x);
</pre>
<blockquote>
<p>
<i>Constraints:</i> <code>x + n</code> is well-formed and has type <code>Iterator</code>.
<p/>
<i>Returns:</i> <code>x + n</code>.
</p>
</blockquote>
</blockquote>
<p>
However, the return type of this operator is <code>move_iterator&lt;Iterator&gt;</code>, so
the expression <code>x + n</code> ought to have that type. Also, there is no <code>operator+</code> 
that matches the constraints, so it effectively disables the addition.
</p>

<p><i>[2019-10-31 Issue Prioritization]</i></p>

<p>Priority to 3 after reflector discussion.</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Previous resolution [SUPERSEDED]: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 24.5.4.9 <a href="https://wg21.link/move.iter.nonmember">[move.iter.nonmember]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Iterator&gt;
  constexpr move_iterator&lt;Iterator&gt;
    operator+(iter_difference_t&lt;Iterator&gt; n, const move_iterator&lt;Iterator&gt;&amp; x);
</pre>
<blockquote>
<p>
-2- <i>Constraints:</i> <code>x + n</code> is well-formed and has type <code><ins>move_iterator&lt;</ins>Iterator<ins>&gt;</ins></code>.
<p/>
-3- <i>Returns:</i> <code>x + n</code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2019-11-04; Casey comments and provides revised wording]</i></p>

<p>
After applying the P/R the <i>Constraint</i> element requires <code>x + n</code> to be well-formed (it always is, 
since that operation is unconstrained) and requires <code>x + n</code> to have type 
<code>move_iterator&lt;Iterator&gt;</code> (which it always does). Consequently, this <i>Constraint</i> 
is always satisfied and it has no normative effect. The intent of the change in 
<a href="https://wg21.link/p0896r4">P0896R4</a> was that this operator be constrained to require 
addition on the base iterator to be well-formed and have type <code>Iterator</code>, which ensures that 
the other semantics of this operation are implementable.
</p>

<p><i>[2021-06-23; Reflector poll]</i></p>

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

<p><i>[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3293"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4835">N4835</a>.</p>

<ol>
<li><p>Modify 24.5.4.9 <a href="https://wg21.link/move.iter.nonmember">[move.iter.nonmember]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Iterator&gt;
  constexpr move_iterator&lt;Iterator&gt;
    operator+(iter_difference_t&lt;Iterator&gt; n, const move_iterator&lt;Iterator&gt;&amp; x);
</pre>
<blockquote>
<p>
-2- <i>Constraints:</i> <code>x<ins>.base()</ins> + n</code> is well-formed and has type <code>Iterator</code>.
<p/>
-3- <i>Returns:</i> <code>x + n</code>.
</p>
</blockquote>
</blockquote>

</li>

</ol>




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