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<title>Issue 2729: Missing SFINAE on std::pair::operator=</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++17">C++17</a> status.</em></p>
<h3 id="2729"><a href="lwg-defects.html#2729">2729</a>. Missing SFINAE on <code>std::pair::operator=</code></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++17">C++17</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2016-06-07 <b>Last modified:</b> 2017-07-30</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++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>std::is_copy_assignable&lt;std::pair&lt;int, std::unique_ptr&lt;int&gt;&gt;&gt;::value</code> is <code>true</code>, and 
should be <code>false</code>. We're missing a "shall not participate in overload resolution unless" for <code>pair</code>'s 
<code>operator=</code>, and likewise for <code>tuple</code>.
</p>

<p><i>[2016-08-03 Chicago LWG]</i></p>

<p>
Inspired by Eric Fiselier and Ville, Walter and Nevin provide initial Proposed Resolution.
</p>

<p><i>[2016-08 - Chicago]</i></p>

<p>Thurs PM: Moved to Tentatively Ready</p>
<p>Lots of discussion, but no one had a better idea.</p>


<p id="res-2729"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4606.</p>

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

<blockquote>
<pre>
pair&amp; operator=(const pair&amp; p);
</pre>
<blockquote>
<p>
-15- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall be defined as deleted unless</ins> 
<code>is_copy_assignable_v&lt;first_type&gt;</code> is <code>true</code> and 
<code>is_copy_assignable_v&lt;second_type&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class U, class V&gt; pair&amp; operator=(const pair&lt;U, V&gt;&amp; p);
</pre>
<blockquote>
<p>
-18- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall not participate in overload resolution 
unless</ins> <code>is_assignable_v&lt;first_type&amp;, const U&amp;&gt;</code> is <code>true</code> and 
<code>is_assignable_v&lt;second_type&amp;, const V&amp;&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
pair&amp; operator=(pair&amp;&amp; p) noexcept(<i>see below</i>);
</pre>
<blockquote>
<p>
-21- <i>Remarks:</i> The expression inside <code>noexcept</code> is equivalent to:
</p>
<blockquote><pre>
is_nothrow_move_assignable_v&lt;T1&gt; &amp;&amp; is_nothrow_move_assignable_v&lt;T2&gt;
</pre></blockquote>
<p>
-22- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall be defined as deleted unless</ins> 
<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>
<pre>
template&lt;class U, class V&gt; pair&amp; operator=(pair&lt;U, V&gt;&amp;&amp; p);
</pre>
<blockquote>
<p>
-25- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall not participate in overload resolution 
unless</ins> <code>is_assignable_v&lt;first_type&amp;, U&amp;&amp;&gt;</code> is <code>true</code> and 
<code>is_assignable_v&lt;second_type&amp;, V&amp;&amp;&gt;</code> is <code>true</code>.
</p>
</blockquote>
</blockquote>
</li>

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

<blockquote>
<pre>
tuple&amp; operator=(const tuple&amp; u);
</pre>
<blockquote>
<p>
-2- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall be defined as deleted unless</ins> 
<code>is_copy_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>
<pre>
tuple&amp; operator=(tuple&amp;&amp; u) noexcept(<i>see below</i>);
</pre>
<blockquote>
<p>
-5- <i>Remark:</i> The expression inside <code>noexcept</code> is equivalent to the logical AND of the following expressions:
</p>
<blockquote><pre>
is_nothrow_move_assignable_v&lt;T<sub><i>i</i></sub>&gt;
</pre></blockquote>
<p>
where <code>T<sub><i>i</i></sub></code> is the <code><i>i<sup>th</sup></i></code> type in <code>Types</code>.
<p/>
-6- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall be defined as deleted unless</ins>
<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>
<pre>
template &lt;class... UTypes&gt;
  tuple&amp; operator=(const tuple&lt;UTypes...&gt;&amp; u);
</pre>
<blockquote>
<p>
-9- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall not participate in overload resolution 
unless</ins> <code>sizeof...(Types) == sizeof...(UTypes)</code> and 
<code>is_assignable_v&lt;T<sub><i>i</i></sub>&amp;, const U<sub><i>i</i></sub>&amp;&gt;</code> is <code>true</code> 
for all <code><i>i</i></code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class... UTypes&gt;
  tuple&amp; operator=(tuple&lt;UTypes...&gt;&amp;&amp; u);
</pre>
<blockquote>
<p>
-12- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall not participate in overload resolution 
unless</ins> <code>is_assignable_v&lt;T<sub><i>i</i></sub>&amp;, U<sub><i>i</i></sub>&amp;&amp;&gt; == true</code> 
for all <code><i>i</i></code><del>.</del> <ins>and</ins> <code>sizeof...(Types) == sizeof...(UTypes)</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class U1, class U2&gt; tuple&amp; operator=(const pair&lt;U1, U2&gt;&amp; u);
</pre>
<blockquote>
<p>
-15- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall not participate in overload resolution 
unless</ins> <code>sizeof...(Types) == 2</code><del>.</del> <ins>and</ins> <code>is_assignable_v&lt;T<sub><i>0</i></sub>&amp;, 
const U<sub><i>1</i></sub>&amp;&gt;</code> is <code>true</code> for the first type <code>T<sub><i>0</i></sub></code> in 
<code>Types</code> and <code>is_assignable_v&lt;T<sub><i>1</i></sub>&amp;, const U<sub><i>2</i></sub>&amp;&gt;</code> 
is <code>true</code> for the second type <code>T<sub><i>1</i></sub></code> in <code>Types</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class U1, class U2&gt; tuple&amp; operator=(pair&lt;U1, U2&gt;&amp;&amp; u);
</pre>
<blockquote>
<p>
-18- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This operator shall not participate in overload resolution 
unless</ins> <code>sizeof...(Types) == 2</code><del>.</del> <ins>and</ins> <code>is_assignable_v&lt;T<sub><i>0</i></sub>&amp;, 
U<sub><i>1</i></sub>&amp;&amp;&gt;</code> is <code>true</code> for the first type <code>T<sub><i>0</i></sub></code> in 
<code>Types</code> and <code>is_assignable_v&lt;T<sub><i>1</i></sub>&amp;, U<sub><i>2</i></sub>&amp;&amp;&gt;</code> 
is <code>true</code> for the second type <code>T<sub><i>1</i></sub></code> in <code>Types</code>.
</p>
</blockquote>
</blockquote>
</li>

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