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<title>Issue 2407: packaged_task(allocator_arg_t, const Allocator&amp;, F&amp;&amp;) should neither be constrained nor 
explicit</title>
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explicit">
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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="2407"><a href="lwg-defects.html#2407">2407</a>. <code>packaged_task(allocator_arg_t, const Allocator&amp;, F&amp;&amp;)</code> should neither be constrained nor 
<code>explicit</code></h3>
<p><b>Section:</b> 32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2014-06-14 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.task.members">issues</a> in [futures.task.members].</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>
LWG <a href="lwg-defects.html#2097" title="packaged_task constructors should be constrained (Status: C++14)">2097</a><sup><a href="https://cplusplus.github.io/LWG/issue2097" title="Latest snapshot">(i)</a></sup>'s resolution was slightly too aggressive. It constrained 
<code>packaged_task(allocator_arg_t, const Allocator&amp;, F&amp;&amp;)</code>, but that's unnecessary because 
<code>packaged_task</code> doesn't have any other three-argument constructors. Additionally, it's marked as 
<code>explicit</code> (going back to WP N2798 when <code>packaged_task</code> first appeared) which is unnecessary.
</p>

<p><i>[2015-02 Cologne]</i></p>

<p>
Handed over to SG1.
</p>

<p><i>[2015-05 Lenexa, SG1 response]</i></p>

<p>
Back to LWG; not an SG1 issue.
</p>

<p><i>[2015-05 Lenexa]</i></p>

<p>
STL improves proposed wording by restoring the constraint again.
</p>


<p id="res-2407"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3936.</p>

<ol>
<li><p>Change 32.10.10 <a href="https://wg21.link/futures.task">[futures.task]</a> p2, class template <code>packaged_task</code> as indicated:</p>

<blockquote>
<pre>
template &lt;class F&gt;
explicit packaged_task(F&amp;&amp; f);
template &lt;class F, class Allocator&gt;
<del>explicit</del> packaged_task(allocator_arg_t, const Allocator&amp; a, F&amp;&amp; f);
</pre>
</blockquote>
</li>

<li><p>Change 32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a> as indicated:</p>

<blockquote>
<pre>
template &lt;class F&gt;
packaged_task(F&amp;&amp; f);
template &lt;class F, class Allocator&gt;
<del>explicit</del> packaged_task(allocator_arg_t, const Allocator&amp; a, F&amp;&amp; f);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-3- <i>Remarks</i>: These constructors shall not participate in overload resolution if <code>decay_t&lt;F&gt;</code> 
is the same type as <code>std::packaged_task&lt;R(ArgTypes...)&gt;</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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