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<title>Issue 2057: time_point + duration semantics should be made constexpr conforming</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="2057"><a href="lwg-defects.html#2057">2057</a>. <code>time_point + duration</code> semantics should be made <code>constexpr</code> conforming</h3>
<p><b>Section:</b> 30.6.6 <a href="https://wg21.link/time.point.nonmember">[time.point.nonmember]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2011-05-21 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#time.point.nonmember">issues</a> in [time.point.nonmember].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It has been observed by LWG <a href="lwg-defects.html#2054" title="time_point constructors need to be constexpr (Status: Resolved)">2054</a><sup><a href="https://cplusplus.github.io/LWG/issue2054" title="Latest snapshot">(i)</a></sup> that the specification of some <code>time_point</code> member functions
already imply that <code>time_point</code> needs to be a literal type and suggests to specify the constructors
and the member function <code>time_since_epoch()</code> as <code>constexpr</code> functions at the
minimum necessary. Adding further <code>constexpr</code> specifier to other operations should
clearly be allowed and should probably be done as well. But to allow for further <code>constexpr</code> 
functions in the future requires that their semantics is compatible to operations allowed in <code>constexpr</code> 
functions. This is already fine for all operations, except this binary plus operator:
</p>
<blockquote><pre>
template &lt;class Clock, class Duration1, class Rep2, class Period2&gt;
time_point&lt;Clock, typename common_type&lt;Duration1, duration&lt;Rep2, Period2&gt;&gt;::type&gt;
operator+(const time_point&lt;Clock, Duration1&gt;&amp; lhs, const duration&lt;Rep2, Period2&gt;&amp; rhs);
</pre><blockquote><p>
-1- <i>Returns</i>: <code>CT(lhs) += rhs</code>, where <code>CT</code> is the type of the return value.
</p></blockquote></blockquote>
<p>
for similar reasons as those mentioned in <a href="lwg-defects.html#2020" title="Time utility arithmetic constexpr functions have invalid effects (Status: C++11)">2020</a><sup><a href="https://cplusplus.github.io/LWG/issue2020" title="Latest snapshot">(i)</a></sup>. The semantics should be fixed to allow
for making them <code>constexpr</code>. This issue should also be considered as a placeholder for a request
to make the remaining <code>time_point</code> operations similarly <code>constexpr</code> as had been done for 
<code>duration</code>.
</p>


<p><i>[2012, Portland]</i></p>

<p>
Resolved by adopting paper <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3469">n3469</a>.
</p>



<p id="res-2057"><b>Proposed resolution:</b></p>
<p>This wording is relative to the FDIS.</p>
<ol>
<li><p>In 30.6.6 <a href="https://wg21.link/time.point.nonmember">[time.point.nonmember]</a>, p.1 change the <i>Returns</i> element semantics as indicated:</p>
<blockquote><pre>
template &lt;class Clock, class Duration1, class Rep2, class Period2&gt;
time_point&lt;Clock, typename common_type&lt;Duration1, duration&lt;Rep2, Period2&gt;&gt;::type&gt;
operator+(const time_point&lt;Clock, Duration1&gt;&amp; lhs, const duration&lt;Rep2, Period2&gt;&amp; rhs);
</pre><blockquote><p>
-1- <i>Returns</i>: <code><del>CT(lhs) += rhs</del><ins>CT(lhs.time_since_epoch() + rhs)</ins></code>, where <code>CT</code> 
is the type of the return value.
</p></blockquote></blockquote>
</li>
</ol>





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