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<title>Issue 3465: compare_partial_order_fallback requires F &lt; E</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++23">C++23</a> status.</em></p>
<h3 id="3465"><a href="lwg-defects.html#3465">3465</a>. <code>compare_partial_order_fallback</code> requires <code>F &lt; E</code></h3>
<p><b>Section:</b> 17.12.6 <a href="https://wg21.link/cmp.alg">[cmp.alg]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2020-07-18 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#cmp.alg">active issues</a> in [cmp.alg].</p>
<p><b>View all other</b> <a href="lwg-index.html#cmp.alg">issues</a> in [cmp.alg].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>compare_partial_order_fallback</code> uses three expressions, but requires only two. The decayed types 
of <code>E</code> and <code>F</code> are required to be identical, but variations in constness might make a difference.
</p>

<p><i>[2020-07-26; Reflector prioritization]</i></p>

<p>
Set priority to 0 and status to Tentatively Ready after seven votes in favour during reflector discussions.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3465"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>. 
</p>

<ol>
<li><p>Modify 17.12.6 <a href="https://wg21.link/cmp.alg">[cmp.alg]</a> as indicated:</p>

<blockquote>
<p>
-6- The name <code>compare_partial_order_fallback</code> denotes a customization point object 
(16.3.3.3.5 <a href="https://wg21.link/customization.point.object">[customization.point.object]</a>). Given subexpressions <code>E</code> and <code>F</code>, 
the expression <code>compare_partial_order_fallback(E, F)</code> is expression-equivalent
( [defns.expression-equivalent]) to:
</p>
<ol style="list-style-type: none">
<li><p>(6.1) &mdash; If the decayed types of <code>E</code> and <code>F</code> differ, 
<code>compare_partial_order_fallback(E, F)</code> is ill-formed.</p></li>
<li><p>(6.2) &mdash; Otherwise, <code>partial_order(E, F)</code> if it is a well-formed expression.</p></li>
<li><p>(6.3) &mdash; Otherwise, if the expressions <code>E == F</code><ins>,</ins> <del>and</del> 
<code>E &lt; F</code><ins>, and <code>F &lt; E</code></ins> are <ins>all</ins><del>both</del> well-formed 
and convertible to <code>bool</code>,</p>
<blockquote>
<pre>
E == F ? partial_ordering::equivalent :
E &lt; F  ? partial_ordering::less :
F &lt; E  ? partial_ordering::greater :
         partial_ordering::unordered
</pre>
</blockquote>
<p>
except that <code>E</code> and <code>F</code> are evaluated only once.
</p>
</li>
<li><p>(6.4) &mdash; Otherwise, <code>compare_partial_order_fallback(E, F)</code> is ill-formed.</p></li></ol>
</blockquote>
</li>
</ol>





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