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<title>Issue 3221: Result of year_month arithmetic with months is ambiguous</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="3221"><a href="lwg-defects.html#3221">3221</a>. Result of <code>year_month</code> arithmetic with <code>months</code> is ambiguous</h3>
<p><b>Section:</b> 30.8.13.3 <a href="https://wg21.link/time.cal.ym.nonmembers">[time.cal.ym.nonmembers]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Tomasz Kami&nacute;ski <b>Opened:</b> 2019-06-16 <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 addition of <code>year_month</code> and
<code>months</code> does not define a unique result value.
<p/>
To illustrate, both  <code>year(2019)/month(1)</code> and <code>year(2018)/month(13)</code>
are valid results of <code>year(2018)/month(12) + months(1)</code> addition,
according to the spec in 30.8.13.3 <a href="https://wg21.link/time.cal.ym.nonmembers">[time.cal.ym.nonmembers]</a>.
</p>

<p><i>[2019-06-24; LWG discussion]</i></p>

<p>
During discussions on the LWG reflector there was a preference to add "is <code>true</code>" at the
end of the modified <i>Returns:</i> element. This additional edit has been applied to Tomasz'
original wording below.
</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-3221"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4810">N4810</a>.</p>

<ol>
<li><p>Modify 30.8.13.3 <a href="https://wg21.link/time.cal.ym.nonmembers">[time.cal.ym.nonmembers]</a> as indicated:</p>

<blockquote>
<pre>
constexpr year_month operator+(const year_month&amp; ym, const months&amp; dm) noexcept;
</pre>
<blockquote>
<p>
-3- <i>Returns:</i> A <code>year_month</code> value <code>z</code> such that <code><ins>z.ok() &amp;&amp;</ins>
z - ym == dm</code> <ins>is <code>true</code></ins>.
<p/>
<i>Complexity:</i> <code>&#x1d4aa;(1)</code> with respect to the value of <code>dm</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>




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