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<title>Issue 3804: flat_foo missing some allocator-extended deduction guides</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#New">New</a> status.</em></p>
<h3 id="3804"><a href="lwg-active.html#3804">3804</a>. <code>flat_<i>foo</i></code> missing some allocator-extended deduction guides</h3>
<p><b>Section:</b> 23.6.8 <a href="https://wg21.link/flat.map">[flat.map]</a>, 23.6.9 <a href="https://wg21.link/flat.multimap">[flat.multimap]</a>, 23.6.11 <a href="https://wg21.link/flat.set">[flat.set]</a>, 23.6.12 <a href="https://wg21.link/flat.multiset">[flat.multiset]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Arthur O'Dwyer <b>Opened:</b> 2022-10-25 <b>Last modified:</b> 2022-11-04</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#flat.map">active issues</a> in [flat.map].</p>
<p><b>View all other</b> <a href="lwg-index.html#flat.map">issues</a> in [flat.map].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Tony Table:
</p>
<blockquote><pre>
std::vector&lt;int&gt; v;
std::flat_set s = std::flat_set(v, MyAllocator&lt;int&gt;());
std::flat_set s = std::flat_set(v, std::less(), MyAllocator&lt;int&gt;());
std::flat_set s = std::flat_set(v.begin(), v.end(), MyAllocator&lt;int&gt;());
std::flat_set s = std::flat_set(v.begin(), v.end(), std::less(), MyAllocator&lt;int&gt;());
// BEFORE: all fail to compile
// AFTER: all compile successfully
</pre></blockquote>
<p>
Contrast 23.6.11.3 <a href="https://wg21.link/flat.set.cons">[flat.set.cons]</a> with 23.6.4.2 <a href="https://wg21.link/priqueue.cons">[priqueue.cons]</a>, where most of these are okay:
</p>
<blockquote><pre>
std::vector&lt;int, MyAllocator&lt;int&gt;&gt; v;
std::priority_queue pq1 = std::priority_queue(v, std::less(), MyAllocator&lt;int&gt;());
std::priority_queue pq2 = std::priority_queue(v.begin(), v.end(), MyAllocator&lt;int&gt;());
std::priority_queue pq3 = std::priority_queue(v.begin(), v.end(), std::less(), MyAllocator&lt;int&gt;());
// BEFORE AND AFTER: pq1 compiles successfully
// BEFORE AND AFTER: pq2 and pq3 also compile successfully thanks to LWG <a href="lwg-defects.html#3506" title="Missing allocator-extended constructors for priority_queue (Status: C++23)">3506</a><sup><a href="https://cplusplus.github.io/LWG/issue3506" title="Latest snapshot">(i)</a></sup>
</pre></blockquote>

<p><i>[2022-11-04; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>



<p id="res-3804"><b>Proposed resolution:</b></p>





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