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<title>Issue 3522: Missing requirement on InputIterator template parameter for priority_queue constructors</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="3522"><a href="lwg-defects.html#3522">3522</a>. Missing requirement on <code>InputIterator</code> template parameter for <code>priority_queue</code> constructors</h3>
<p><b>Section:</b> 23.6.4 <a href="https://wg21.link/priority.queue">[priority.queue]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-02-17 <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#priority.queue">issues</a> in [priority.queue].</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>
There is nothing in 23.6.4 <a href="https://wg21.link/priority.queue">[priority.queue]</a> or more generally 23.6 <a href="https://wg21.link/container.adaptors">[container.adaptors]</a>
saying that <code>InputIterator</code> in the following constructor templates has to be an input iterator.
</p>
<blockquote><pre>
template&lt;class InputIterator&gt;
    priority_queue(InputIterator first, InputIterator last, const Compare&amp; x,
                    const Container&amp;);
template&lt;class InputIterator&gt;
    priority_queue(InputIterator first, InputIterator last,
                    const Compare&amp; x = Compare(), Container&amp;&amp; = Container());
</pre></blockquote>
<p>
The second constructor template above therefore accepts
</p>
<blockquote><pre>
std::priority_queue&lt;int&gt; x = {1, 2};
</pre></blockquote>
<p>
to produce a <code>priority_queue</code> that contains a single element <code>2</code>. This behavior seems extremely questionable.
</p>

<p><i>[2021-02-26; LWG telecon]</i></p>

<p>
Set status to Tentatively Ready after discussion and poll.
<table>
<tr><th>F</th><th>A</th><th>N</th></tr>
<tr><td>11</td><td>0</td><td>0</td></tr>
</table>
</p>

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3522"><b>Proposed resolution:</b></p>
<blockquote class="note">
<p>
[<i>Drafting note</i>: Because <a href="https://wg21.link/P1425">an upcoming paper</a>
provides iterator-pair constructors for other container adaptors, the wording below
adds the restriction to 23.6.1 <a href="https://wg21.link/container.adaptors.general">[container.adaptors.general]</a> so that it also
covers the constructors that will be added by that paper.
&mdash; <i>end drafting note</i>]
</p>
</blockquote>
<p>
This wording is relative to <a href="https://wg21.link/n4878">N4878</a>.
</p>

<ol>
<li><p>Add the following paragraph to 23.6.1 <a href="https://wg21.link/container.adaptors.general">[container.adaptors.general]</a> after p3:</p>
<blockquote>
<p>
<ins>-?- A constructor template of a container adaptor shall not participate in
overload resolution if it has an <code>InputIterator</code> template parameter and a
type that does not qualify as an input iterator is deduced for that parameter.</ins>
</p>
<p>
-4- A deduction guide for a container adaptor shall not participate in overload resolution if any of the following are true:
</p>
<ol style="list-style-type: none">
<li><p>(4.1) &mdash; It has an <code>InputIterator</code> template parameter and a type that does not qualify as an input iterator is deduced for that parameter.</p></li>
<li><p>[&hellip;]</p></li>
</ol>
</blockquote>
</li>
</ol>





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