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<title>Issue 4150: The preconditions on run_loop::run() are too strict</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#Resolved">Resolved</a> status.</em></p>
<h3 id="4150"><a href="lwg-defects.html#4150">4150</a>. The preconditions on <code>run_loop::run()</code> are too strict</h3>
<p><b>Section:</b> 33.12.1.4 <a href="https://wg21.link/exec.run.loop.members">[exec.run.loop.members]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2024-09-05 <b>Last modified:</b> 2025-02-07</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
From <a href="https://github.com/cplusplus/sender-receiver/issues/280">sender-receiver/issues #280</a>:
<p/>
The precondition on <code>run_loop::run()</code> is that the <code><i>state</i></code> is <i>starting</i>. 
Given that <code>run_loop::finish()</code> puts the loop in the <i>finishing</i> state, the implication 
is that one cannot call <code>finish()</code> before calling <code>run()</code>.
<p/>
However, in most cases <code>sync_wait</code> is calling <code>finish()</code> on its <code>run_loop</code> 
before calling <code>run()</code>, violating this precondition. <code>sync_wait</code> does the following:
</p>
<blockquote><pre>
run_loop loop;
auto op = connect(sndr, sync_wait_receiver{&amp;loop});
start(op);

loop.run();
</pre></blockquote>
<p>
If <code>sndr</code> completes synchronously, the <code>sync_wait_receiver</code> will call <code>finish()</code> 
on the loop before <code>loop.run()</code> is called, violating <code>run</code>'s precondition.
</p>
<p><i>[2025-02-07 Status changed: New &rarr; Resolved.]</i></p>

<p>Resolved by <a href="https://wg21.link/P3396R1" title=" std::execution wording fixes">P3396R1</a>.</p>



<p id="res-4150"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4988" title=" Working Draft, Programming Languages — C++">N4988</a>.
</p>

<ol>
<li><p>Modify 33.12.1.4 <a href="https://wg21.link/exec.run.loop.members">[exec.run.loop.members]</a> as indicated:</p>

<blockquote><pre>
void run();
</pre>
<blockquote>
<p>
-5- <i>Preconditions</i>: <code><i>state</i></code> is <i>starting</i> <ins>or <i>finishing</i></ins>.
<p/>
-6- <i>Effects</i>: Sets the <code><i>state</i></code> to <i>running</i>. Then, equivalent to:
</p>
<blockquote><pre>
while (auto* op = <i>pop-front</i>()) {
  op-&gt;<i>execute</i>();
}
</pre></blockquote>
<p>
-7- <i>Remarks</i>: When <code><i>state</i></code> changes, it does so without introducing data races.
</p>
</blockquote>
</blockquote>
</li>

</ol>






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