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<title>Issue 3231: year_month_day_last::day specification does not cover !ok() values</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="3231"><a href="lwg-defects.html#3231">3231</a>. <code>year_month_day_last::day</code> specification does not cover <code>!ok()</code> values</h3>
<p><b>Section:</b> 30.8.15.2 <a href="https://wg21.link/time.cal.ymdlast.members">[time.cal.ymdlast.members]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Tomasz Kami&nacute;ski <b>Opened:</b> 2019-06-29 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</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>
The current specification of the <code>year_month_day_last::day</code>
function does not cover the behaviour in the situation when <code>year_month_day_last</code>
value is not <code>ok()</code>. To illustrate the sentence from
30.8.15.2 <a href="https://wg21.link/time.cal.ymdlast.members">[time.cal.ymdlast.members]</a> p13:
</p>
<blockquote><p>
A <code>day</code> representing the last day of the (<code>year</code>, <code>month</code>)
pair represented by <code>*this</code>.
</p></blockquote>
<p>
is unclear in the situation when <code>month</code> member has
<code>!ok</code> value, e.g. what is last day of 14th month of 2019.
<p/>
The proposed resolution makes the value of <code>ymdl.day()</code> (and by
consequence conversion to <code>sys_days</code>/<code>local_days</code>)
unspecified if <code>ymdl.ok()  </code> is <code>false</code>. This make is
consistent with rest of the library, that produces unspecified values in
similiar situation, e.g.: 30.8.14.2 <a href="https://wg21.link/time.cal.ymd.members">[time.cal.ymd.members]</a> p18,
30.8.16.2 <a href="https://wg21.link/time.cal.ymwd.members">[time.cal.ymwd.members]</a> p19,
30.8.17.2 <a href="https://wg21.link/time.cal.ymwdlast.members">[time.cal.ymwdlast.members]</a> p14.
</p>

<p><i>[2019-07 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready after five positive votes on the reflector.</p>


<p id="res-3231"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4820">N4820</a>.</p>

<ol>
<li><p>Modify 30.8.15.2 <a href="https://wg21.link/time.cal.ymdlast.members">[time.cal.ymdlast.members]</a> as indicated:</p>

<blockquote>
<pre>
constexpr chrono::day day() const noexcept;
</pre>
<blockquote>
<p>
-13- <i>Returns:</i> <ins>If <code>ok()</code> is <code>true</code>, returns a</ins><del>A</del> <code>day</code>
representing the last day of the (<code>year, month</code>) pair represented by <code>*this</code>.
<ins>Otherwise the returned value is unspecified.</ins>
<p/>
-14- [<i>Note:</i> This value may be computed on demand. &mdash; <i>end note</i>]
</p>
</blockquote>
</blockquote>

</li>
</ol>




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