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<title>Issue 3104: Fixing duration division</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++20">C++20</a> status.</em></p>
<h3 id="3104"><a href="lwg-defects.html#3104">3104</a>. Fixing <code>duration</code> division</h3>
<p><b>Section:</b> 30.5.6 <a href="https://wg21.link/time.duration.nonmember">[time.duration.nonmember]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Johel Ernesto Guerrero Pe&ntilde;a <b>Opened:</b> 2018-04-17 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#time.duration.nonmember">issues</a> in [time.duration.nonmember].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
[time.duration.nonmember]/1 states
</p>
<blockquote><p>
In the function descriptions that follow, <code>CD</code> represents the return type of the function.
</p></blockquote>
<p>
From what I could find, many definitions of <code>CD</code> in the paragraphs of [time.duration.nonmember] were lifted to 
[time.duration.nonmember]/1 as cited above. That works for all other paragraphs, but not for [time.duration.nonmember]/10, 
which the change rendered ill-formed:
</p>
<blockquote>
<pre>
template&lt;class Rep1, class Period1, class Rep2, class Period2&gt;
  constexpr common_type_t&lt;Rep1, Rep2&gt;
    operator/(const duration&lt;Rep1, Period1&gt;&amp; lhs, const duration&lt;Rep2, Period2&gt;&amp; rhs);
</pre>
<blockquote>
<i>Returns:</i> <code>CD(lhs).count() / CD(rhs).count()</code>.
</blockquote>
</blockquote>
<p>
In this case, we want <code>CD</code> to mean <code>common_type_t&lt;duration&lt;Rep1, Period1&gt;, duration&lt;Rep2, Period2&gt;&gt;</code>. 
That way, the division has the expected semantics of dividing two quantities of the same dimension.
</p>

<p><i>[
2018-04-24 Moved to Tentatively Ready after 6 positive votes on c++std-lib.
]</i></p>


<p><i>[2018-06 Rapperswil: Adopted]</i></p>



<p id="res-3104"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4741">N4741</a>.</p>

<blockquote>
<ol>
<li>
<p>Edit 30.5.6 <a href="https://wg21.link/time.duration.nonmember">[time.duration.nonmember]</a> as indicated:</p>
<blockquote>
<p>
-1- In the function descriptions that follow, <ins>unless stated otherwise, let</ins> <code>CD</code> represent<del>s</del> 
the return type of the function.
<p/>
[&hellip;]
</p>
<pre>
template&lt;class Rep1, class Period1, class Rep2, class Period2&gt;
  constexpr common_type_t&lt;Rep1, Rep2&gt;
    operator/(const duration&lt;Rep1, Period1&gt;&amp; lhs, const duration&lt;Rep2, Period2&gt;&amp; rhs);
</pre>
<blockquote>
<p>
<ins>Let <code>CD</code> be <code>common_type_t&lt;duration&lt;Rep1, Period1&gt;, duration&lt;Rep2, Period2&gt;&gt;</code>.</ins>
<p/>
-10- <i>Returns:</i> <code>CD(lhs).count() / CD(rhs).count()</code>.
</p>
</blockquote>
</blockquote>
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</blockquote>





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