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<title>Issue 3206: year_month_day conversion to sys_days uses not-existing member function</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="3206"><a href="lwg-defects.html#3206">3206</a>. <code>year_month_day</code> conversion to <code>sys_days</code> uses not-existing member function</h3>
<p><b>Section:</b> 30.8.14.2 <a href="https://wg21.link/time.cal.ymd.members">[time.cal.ymd.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> 2019-05-19 <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</code> conversion function to <code>sys_days</code>,
uses the <code>day</code> member function on the <code>sys_days</code> (a.k.a. 
<code>time_point&lt;system_clock, days&gt;</code>), that does not exist.
<p/>
In 30.8.14.2 <a href="https://wg21.link/time.cal.ymd.members">[time.cal.ymd.members]</a> p18, the expression <code>sys_days{y_/m_/last}.day()</code> is 
ill-formed:
</p>
<blockquote><p>
[&hellip;] Otherwise, if <code>y_.ok() &amp;&amp; m_.ok()</code> is <code>true</code>, returns a
<code>sys_days</code> which is offset from <code>sys_days{y_/m_/last}</code>
by the number of days <code>d_</code> is offset from <code>sys_days{y_/m_/last}.day()</code>.
Otherwise the value returned is unspecified.
</p></blockquote>

<p><i>[2019-06-08; Priority to 0 and Status to Tentatively Ready after six positive votes on the reflector.]</i></p>



<p id="res-3206"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4810">N4810</a>.</p>

<ol>
<li><p>Modify 30.8.14.2 <a href="https://wg21.link/time.cal.ymd.members">[time.cal.ymd.members]</a> as indicated:</p>

<blockquote>
<pre>
constexpr operator sys_days() const noexcept;
</pre>
<blockquote>
<p>
-18- <i>Returns:</i> If <code>ok()</code>, returns a <code>sys_days</code> holding a count of days from 
the <code>sys_days</code> epoch to <code>*this</code> (a negative value if <code>*this</code> represents a date 
prior to the <code>sys_days</code> epoch). Otherwise, if <code>y_.ok() &amp;&amp; m_.ok()</code> is <code>true</code>, 
returns <ins><code>sys_days{y_/m_/1d} + (d_ - 1d)</code></ins><del>a <code>sys_days</code> which is offset from 
<code>sys_days{y_/m_/last}</code> by the number of days <code>d_</code> is offset from 
<code>sys_days{y_/m_/last}.day()</code></del>. Otherwise the value returned is unspecified.
</p>
</blockquote>
</blockquote>
</li>

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