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<title>Issue 4288: The Constraints: element in [time.clock.system.members] is probably wrong</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="4288"><a href="lwg-active.html#4288">4288</a>. The <i>Constraints:</i> element in [time.clock.system.members] is probably wrong</h3>
<p><b>Section:</b> 30.7.2.2 <a href="https://wg21.link/time.clock.system.members">[time.clock.system.members]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2025-06-19 <b>Last modified:</b> 2025-08-26</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, the <i>Constraints:</i> element in 30.7.2.2 <a href="https://wg21.link/time.clock.system.members">[time.clock.system.members]</a> for the member typedef
<code class='backtick'>system_clock::rep</code> imposes a requirement for the implementation, without establishing any condition for user 
code. Perhaps it's wrong to use a <i>Constraints:</i> element there.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>

<li><p>Modify 30.7.2.2 <a href="https://wg21.link/time.clock.system.members">[time.clock.system.members]</a> as indicated:</p>

<blockquote>
<pre>
using system_clock::rep = <i>unspecified</i>;
</pre>
<blockquote>
<p>
-1- <del><i>Constraints:</i></del><ins>The implementation shall ensure that</ins> 
<code>system_clock::duration::min() &lt; system_clock::duration::zero()</code> is <code>true</code>.
[<i>Note 1</i>: This implies that <code class='backtick'>rep</code> is a signed type. &mdash; <i>end note</i>]
</p>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2025-07-15; Reflector discussion]</i></p>

<p>
The discussion revealed a preference to not insert the additional 
"The implementation shall ensure that", because its not really needed, 
since this is just a normal implementation requirement that falls out of
the specification. 
</p>

<p><i>[2025-08-21; Reflector poll]</i></p>

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



<p id="res-4288"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>

<li><p>Modify 30.7.2.2 <a href="https://wg21.link/time.clock.system.members">[time.clock.system.members]</a> as indicated:</p>

<blockquote>
<pre>
using system_clock::rep = <i>unspecified</i>;
</pre>
<blockquote>
<p>
-1- <del><i>Constraints:</i></del> <code>system_clock::duration::min() &lt; system_clock::duration::zero()</code> 
is <code>true</code>.
[<i>Note 1</i>: This implies that <code class='backtick'>rep</code> is a signed type. &mdash; <i>end note</i>]
</p>
</blockquote>
</blockquote>
</li>

</ol>





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