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<title>Issue 2958: Moves improperly defined as deleted</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++20">C++20</a> status.</em></p>
<h3 id="2958"><a href="lwg-defects.html#2958">2958</a>. Moves improperly defined as deleted</h3>
<p><b>Section:</b> 22.3.2 <a href="https://wg21.link/pairs.pair">[pairs.pair]</a>, 22.4.4.3 <a href="https://wg21.link/tuple.assign">[tuple.assign]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2017-05-05 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#pairs.pair">active issues</a> in [pairs.pair].</p>
<p><b>View all other</b> <a href="lwg-index.html#pairs.pair">issues</a> in [pairs.pair].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
LWG <a href="lwg-defects.html#2729" title="Missing SFINAE on std::pair::operator= (Status: C++17)">2729</a><sup><a href="https://cplusplus.github.io/LWG/issue2729" title="Latest snapshot">(i)</a></sup> constrained many <code>pair</code> &amp; <code>tuple</code> assignment operators to "not participate in overload resolution" and "be defined as deleted if." As discussed when LWG reviewed <a href="lwg-defects.html#2756" title="C++ WP optional&lt;T&gt; should 'forward' T's implicit conversions (Status: C++17)">2756</a><sup><a href="https://cplusplus.github.io/LWG/issue2756" title="Latest snapshot">(i)</a></sup>, it is undesirable to require 
that a move constructor or assignment operator be "defined as deleted," since it is unclear whether that operation should be 
<em>implicitly</em> deleted, and therefore not participate in overload resolution, or <em>explicitly</em> deleted and therefore 
impede overload resolution for usages that would otherwise find copies.
</p>

<p><i>[2017-07 Toronto Wed Issue Prioritization]</i></p>

<p>Priority 2</p>

<p><i>[2017-11 Albuquerque Wednesday issue processing]</i></p>

<p>Move to Immediate</p>


<p id="res-2958"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.</p>

<ol>
<li><p>Edit 22.3.2 <a href="https://wg21.link/pairs.pair">[pairs.pair]</a> as indicated:</p>

<blockquote>
<pre>
pair&amp; operator=(pair&amp;&amp; p) noexcept(<i>see below</i>);
</pre>
<blockquote>
<p>
-21- <i>Effects:</i> Assigns to <code>first</code> with <code>std::forward&lt;first_type&gt;(p.first)</code> and to 
<code>second</code> with <code>std::forward&lt;second_type&gt;(p.second)</code>.
<p/>
-22- <i>Remarks:</i> This operator shall <del>be defined as deleted</del><ins>not participate in overload resolution</ins> 
unless <code>is_move_assignable_v&lt;first_type&gt;</code> is <code>true</code> and <code>is_move_assignable_v&lt;second_type&gt;</code> 
is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Edit 22.4.4.3 <a href="https://wg21.link/tuple.assign">[tuple.assign]</a> as indicated:</p>

<blockquote>
<pre>
tuple&amp; operator=(tuple&amp;&amp; u) noexcept(<i>see below</i>);
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> For all <code><i>i</i></code>, assigns <code>std::forward&lt;T<sub><i>i</i></sub>&gt;(get&lt;<i>i</i>&gt;(u))</code> 
to <code>get&lt;<i>i</i>&gt;(*this)</code>.
<p/>
-6- <i>Remarks:</i> This operator shall <del>be defined as deleted</del><ins>not participate in overload resolution</ins> unless 
<code>is_move_assignable_v&lt;T<sub><i>i</i></sub>&gt;</code> is <code>true</code> for all <code><i>i</i></code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





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