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<title>Issue 3516: thread::id spaceship may be inconsistent with equality</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="3516"><a href="lwg-active.html#3516">3516</a>. <code>thread::id</code> spaceship may be inconsistent with equality</h3>
<p><b>Section:</b> 32.4.3.2 <a href="https://wg21.link/thread.thread.id">[thread.thread.id]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2021-01-26 <b>Last modified:</b> 2021-03-12</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#thread.thread.id">issues</a> in [thread.thread.id].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
32.4.3.2 <a href="https://wg21.link/thread.thread.id">[thread.thread.id]</a>/5-7 specify the behavior of <code>==</code> and <code>&lt;=&gt;</code> for 
<code>std::thread::id</code>:
</p>
<blockquote>
<pre>
bool operator==(thread::id x, thread::id y) noexcept;
</pre>
<blockquote>
<p>
-5- <i>Returns:</i> <code>true</code> only if <code>x</code> and <code>y</code> represent the same thread of execution 
or neither <code>x</code> nor <code>y</code> represents a thread of execution.
</p>
</blockquote>
<pre>
strong_ordering operator&lt;=&gt;(thread::id x, thread::id y) noexcept;
</pre>
<blockquote>
<p>
-6- Let <code><i>P</i>(x, y)</code> be an unspecified total ordering over <code>thread::id</code> as described in 25.8.
<p/>
-7- <i>Returns:</i> <code>strong_ordering::less</code> if <code><i>P</i>(x, y)</code> is <code>true</code>. Otherwise, 
<code>strong_ordering::greater</code> if <code><i>P</i>(y, x)</code> is <code>true</code>. Otherwise, <code>strong_ordering::equal</code>.
</p>
</blockquote>
</blockquote>
<p>
"Unspecified total ordering" provides too much freedom, since it does not require that <code>!P(x, y)</code> 
holds when <code>x</code> and <code>y</code> both represent the same thread of execution or both represent no 
thread of execution. A conforming implementation could return <code>strong_ordering::equal</code> from 
<code>&lt;=&gt;</code> for a pair of <code>thread::id</code> values for which <code>==</code> returns <code>false</code>. 
We should guarantee consistency of <code>==</code> and <code>&lt;=&gt;</code> for <code>thread::id</code> to 
preserve sanity of the programming model.
</p>

<p><i>[2021-03-12; Reflector poll]</i></p>

<p>
Set priority to 3 following reflector poll.
</p>



<p id="res-3516"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4878">N4878</a>. 
</p>

<ol>
<li><p>Modify 32.4.3.2 <a href="https://wg21.link/thread.thread.id">[thread.thread.id]</a> as indicated:</p>

<blockquote>
<pre>
strong_ordering operator&lt;=&gt;(thread::id x, thread::id y) noexcept;
</pre>
<blockquote>
<p>
-6- Let <code><i>P</i>(x, y)</code> be <del>an unspecified</del><ins>a</ins> total ordering over <code>thread::id</code> 
as described in 26.8 <a href="https://wg21.link/alg.sorting">[alg.sorting]</a><ins>, unspecified except that <code><i>P</i>(x, y)</code> is <code>false</code> 
when <code>x</code> and <code>y</code> both represent the same thread of execution, or when neither represents a thread 
of execution</ins>.
<p/>
-7- <i>Returns:</i> <code>strong_ordering::less</code> if <code><i>P</i>(x, y)</code> is <code>true</code>. Otherwise, 
<code>strong_ordering::greater</code> if <code><i>P</i>(y, x)</code> is <code>true</code>. Otherwise, <code>strong_ordering::equal</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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