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<title>Issue 3476: thread and jthread constructors require that the parameters be move-constructible 
but never move construct the parameters</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="3476"><a href="lwg-defects.html#3476">3476</a>. <code>thread</code> and <code>jthread</code> constructors require that the parameters be move-constructible 
but never move construct the parameters</h3>
<p><b>Section:</b> 32.4.3.3 <a href="https://wg21.link/thread.thread.constr">[thread.thread.constr]</a>, 32.4.4.2 <a href="https://wg21.link/thread.jthread.cons">[thread.jthread.cons]</a>, 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Billy O'Neal III <b>Opened:</b> 2020-08-18 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#thread.thread.constr">active issues</a> in [thread.thread.constr].</p>
<p><b>View all other</b> <a href="lwg-index.html#thread.thread.constr">issues</a> in [thread.thread.constr].</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>
I think this was upgraded to <i>Mandates</i> because C++17 and earlier had "<code>F</code> and each <code>T<sub><i>i</i></sub></code> 
in <code>Args</code> shall satisfy the <i>Cpp17MoveConstructible</i> requirements." And for those, I think the requirement 
was attempting to make the subsequent <code><i>decay-copy</i></code> valid. However, the 'Mandating the standard library' 
papers added <code>is_constructible</code> requirements which already serve that purpose; <code>std::(j)thread</code> has no 
reason to move the elements after they have been <code><i>decay-copy</i></code>'d to transfer to the launched thread.
</p>

<p><i>[2020-08-26; Reflector discussion]</i></p>

<p>
Jonathan noticed that the wording for <code>std::async</code> is affected by exactly the same unnecessary move-constructible 
requirements. The proposed wording has been updated to cope for that as well.
</p>

<p><i>[2020-09-02; Reflector prioritization]</i></p>

<p>
Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3476"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 32.4.3.3 <a href="https://wg21.link/thread.thread.constr">[thread.thread.constr]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F, class... Args&gt; explicit thread(F&amp;&amp; f, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-3- <i>Constraints:</i> <code>remove_cvref_t&lt;F&gt;</code> is not the same type as <code>thread</code>.
<p/>
-4- <i>Mandates:</i> The following are all <code>true</code>:
</p>
<ol style="list-style-type: none">
<li><p>(4.1) &mdash; <code>is_constructible_v&lt;decay_t&lt;F&gt;, F&gt;</code>,</p></li>
<li><p>(4.2) &mdash; <code>(is_constructible_v&lt;decay_t&lt;Args&gt;, Args&gt; &amp;&amp; ...)</code>, <ins>and</ins></p></li>
<li><p><del>(4.3) &mdash; <code>is_move_constructible_v&lt;decay_t&lt;F&gt;&gt;</code>,</del></p></li>
<li><p><del>(4.4) &mdash; <code>(is_move_constructible_v&lt;decay_t&lt;Args&gt;&gt; &amp;&amp; ...)</code>, and</del></p></li>
<li><p>(4.5) &mdash; <code>is_invocable_v&lt;decay_t&lt;F&gt;, decay_t&lt;Args&gt;...&gt;</code>.</p></li>
</ol>
<p>
<del>-5- <i>Preconditions:</i> <code>decay_t&lt;F&gt;</code> and each type in <code>decay_t&lt;Args&gt;</code> meet the 
<i>Cpp17MoveConstructible</i> requirements.</del>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 32.4.4.2 <a href="https://wg21.link/thread.jthread.cons">[thread.jthread.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F, class... Args&gt; explicit jthread(F&amp;&amp; f, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-3- <i>Constraints:</i> <code>remove_cvref_t&lt;F&gt;</code> is not the same type as <code>jthread</code>.
<p/>
-4- <i>Mandates:</i> The following are all <code>true</code>:
</p>
<ol style="list-style-type: none">
<li><p>(4.1) &mdash; <code>is_constructible_v&lt;decay_t&lt;F&gt;, F&gt;</code>,</p></li>
<li><p>(4.2) &mdash; <code>(is_constructible_v&lt;decay_t&lt;Args&gt;, Args&gt; &amp;&amp; ...)</code>, <ins>and</ins></p></li>
<li><p><del>(4.3) &mdash; <code>is_move_constructible_v&lt;decay_t&lt;F&gt;&gt;</code>,</del></p></li>
<li><p><del>(4.4) &mdash; <code>(is_move_constructible_v&lt;decay_t&lt;Args&gt;&gt; &amp;&amp; ...)</code>, and</del></p></li>
<li><p>(4.5) &mdash; <code>is_invocable_v&lt;decay_t&lt;F&gt;, decay_t&lt;Args&gt;...&gt; ||
is_invocable_v&lt;decay_t&lt;F&gt;, stop_token, decay_t&lt;Args&gt;...&gt;</code>.</p></li>
</ol>
<p>
<del>-5- <i>Preconditions:</i> <code>decay_t&lt;F&gt;</code> and each type in <code>decay_t&lt;Args&gt;</code> meet the 
<i>Cpp17MoveConstructible</i> requirements.</del>
</p>
</blockquote>
</blockquote>

</li>


<li><p>Modify 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F, class... Args&gt;
  [[nodiscard]] future&lt;invoke_result_t&lt;decay_t&lt;F&gt;, decay_t&lt;Args&gt;...&gt;&gt;
    async(F&amp;&amp; f, Args&amp;&amp;... args);
template&lt;class F, class... Args&gt;
  [[nodiscard]] future&lt;invoke_result_t&lt;decay_t&lt;F&gt;, decay_t&lt;Args&gt;...&gt;&gt;
    async(launch policy, F&amp;&amp; f, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-2- <i>Mandates:</i> The following are all <code>true</code>:
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; <code>is_constructible_v&lt;decay_t&lt;F&gt;, F&gt;</code>,</p></li>
<li><p>(2.2) &mdash; <code>(is_constructible_v&lt;decay_t&lt;Args&gt;, Args&gt; &amp;&amp; ...)</code>, <ins>and</ins></p></li>
<li><p><del>(2.3) &mdash; <code>is_move_constructible_v&lt;decay_t&lt;F&gt;&gt;</code>,</del></p></li>
<li><p><del>(2.4) &mdash; <code>(is_move_constructible_v&lt;decay_t&lt;Args&gt;&gt; &amp;&amp; ...)</code>, and</del></p></li>
<li><p>(2.5) &mdash; <code>is_invocable_v&lt;decay_t&lt;F&gt;, decay_t&lt;Args&gt;...&gt;</code>.</p></li>
</ol>
<p>
<del>-3- <i>Preconditions:</i> <code>decay_t&lt;F&gt;</code> and each type in <code>decay_t&lt;Args&gt;</code> meet the 
<i>Cpp17MoveConstructible</i> requirements.</del>
</p>
</blockquote>
</blockquote>

</li>

</ol>





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