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<title>Issue 3125: duration streaming precondition should be a SFINAE condition</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="3125"><a href="lwg-defects.html#3125">3125</a>. <code>duration</code> streaming precondition should be a SFINAE condition</h3>
<p><b>Section:</b> 30.5.11 <a href="https://wg21.link/time.duration.io">[time.duration.io]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Johel Ernesto Guerrero Pe&ntilde;a <b>Opened:</b> 2018-06-23 <b>Last modified:</b> 2020-11-09</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#time.duration.io">issues</a> in [time.duration.io].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
30.5.11 <a href="https://wg21.link/time.duration.io">[time.duration.io]</a>/1 states:
</p>
<blockquote>
<pre>
template&lt;class charT, class traits, class Rep, class Period&gt;
  basic_ostream&lt;charT, traits&gt;&amp;
    operator&lt;&lt;(basic_ostream&lt;charT, traits&gt;&amp; os, const duration&lt;Rep, Period&gt;&amp; d);
</pre>
<blockquote>
<p>
<i>Requires:</i> <code>Rep</code> is an integral type whose integer conversion rank (6.9.6 <a href="https://wg21.link/conv.rank">[conv.rank]</a>) is greater 
than or equal to that of <code>short</code>, or a floating point type. <code>charT</code> is <code>char</code> or <code>wchar_t</code>.
</p>
</blockquote>
</blockquote>
<p>
I think the intention was to make this a compile-time error, since all the information to make it so is available 
at compile-time. But the wording doesn't suggest that.
</p>
<p><i>[2018-06-29; Daniel comments]</i></p>

<p>
The wording will be significantly simplified by the application of the new Library element <i>Constraints:</i> 
introduced by <a href="https://wg21.link/p0788r3">P0788R3</a> and available with the post-Rapperswil working draft.
</p>

<p><i>[2018-07-20 Priority set to 2 after reflector discussion. NAD and P0 were also mentioned.]</i></p>


<p><i>[2018-08-23 Batavia Issues processing]</i></p>

<p>Marshall to talk to Howard about his intent. Status to Open</p>

<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4750">N4750</a>.
</p>

<p>Change 30.5.11 <a href="https://wg21.link/time.duration.io">[time.duration.io]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> The suggested wording changes include the insertion of two bullets into the existing running text
to improve clarification of the logic arm of the "unless"].
</p>
</blockquote>

<blockquote>
<pre>
template&lt;class charT, class traits, class Rep, class Period&gt;
  basic_ostream&lt;charT, traits&gt;&amp;
    operator&lt;&lt;(basic_ostream&lt;charT, traits&gt;&amp; os, const duration&lt;Rep, Period&gt;&amp; d);
</pre>
<blockquote>
<p>
-1- <i><del>Requires</del><ins>Remarks</ins>:</i> <ins>This function shall not participate in overload resolution 
unless:</ins>
</p>
<ul>
<li><p><code>Rep</code> is an integral type whose integer conversion rank (6.9.6 <a href="https://wg21.link/conv.rank">[conv.rank]</a>) is 
greater than or equal to that of <code>short</code>, or a floating point type<del>.</del><ins>, and</ins></p></li>
<li><p><code>charT</code> is <code>char</code> or <code>wchar_t</code></p></li>
</ul>
<p>
[&hellip;]
</p>
</blockquote>
</blockquote>
</blockquote>

<p><i>[2020-02-13, Prague]</i></p>

<p>
Will be resolved by LWG <a href="lwg-defects.html#3317" title="Incorrect operator&lt;&lt; for floating-point durations (Status: C++20)">3317</a><sup><a href="https://cplusplus.github.io/LWG/issue3317" title="Latest snapshot">(i)</a></sup>.
</p>
<p><i>[2020-11-09 Resolved for C++20. Status changed: Tentatively Resolved &rarr; Resolved.]</i></p>



<p id="res-3125"><b>Proposed resolution:</b></p>
<p>
</p>





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