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<title>Issue 3584: Clarify common comparison category conversions</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#New">New</a> status.</em></p>
<h3 id="3584"><a href="lwg-active.html#3584">3584</a>. Clarify common comparison category conversions</h3>
<p><b>Section:</b> 17.12.3 <a href="https://wg21.link/cmp.common">[cmp.common]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Peter Brett <b>Opened:</b> 2021-08-23 <b>Last modified:</b> 2021-09-20</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
17.12.3 <a href="https://wg21.link/cmp.common">[cmp.common]</a>/1 says:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;"><p>
The type <code>common_comparison_category</code> provides an alias for the
strongest comparison category to which all of the template arguments
can be converted.
</p></blockquote>
<p>
A naive reader like me might interpret this as meaning that (1) you
attempt to convert the template arguments to comparison categories and
then (2) obtain the strongest among them.
<p/>
However, the intent is in fact to realize the common comparison type
notion from 11.10.3 <a href="https://wg21.link/class.spaceship">[class.spaceship]</a>/4. To obtain a non-<code>void</code>
result, all the template arguments must be comparison categories, rather than
convertible to comparison categories.
<p/>
17.12.3 <a href="https://wg21.link/cmp.common">[cmp.common]</a>/2 mildly contradicts the first paragraph:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;"><p>
<i>Remarks:</i> The member <i>typedef-name</i> <code>type</code> denotes the common comparison
type (11.10.3 <a href="https://wg21.link/class.spaceship">[class.spaceship]</a>) of <code>Ts...</code>, the expanded parameter pack, or
<code>void</code> if any element of <code>Ts</code> is not a comparison category type.
</p></blockquote>
<p>
It more precisely states the behaviour, cross-references
11.10.3 <a href="https://wg21.link/class.spaceship">[class.spaceship]</a>, and uses the correct core terminology for the
metafunction that the template represents.
<p/>
Suggested resolution;
<p/>
Delete 17.12.3 <a href="https://wg21.link/cmp.common">[cmp.common]</a>/1, because it does not provide any information not
already more precisely included in 17.12.3 <a href="https://wg21.link/cmp.common">[cmp.common]</a>/2.
</p>

<p><i>[2021-09-20; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>

<p><i>[2021-09-20; Reflector poll]</i></p>

<p>
Jens suggests alternative wording.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 17.12.3 <a href="https://wg21.link/cmp.common">[cmp.common]</a> as indicated:</p>

<blockquote>
<p>
<del>-1- The type <code>common_comparison_category</code> provides an alias for the strongest comparison category to which
all of the template arguments can be converted.
[<i>Note 1</i>: A comparison category type is stronger than another if they are distinct types and an instance of the former
can be converted to an instance of the latter. &mdash; <i>end note</i>]</del>
</p>
<pre>
template&lt;class... Ts&gt;
struct common_comparison_category {
  using type = <i>see below</i>;
};
</pre>
<p>
-2- <i>Remarks:</i> The member <i>typedef-name</i> <code>type</code> denotes the common comparison type (11.10.3 <a href="https://wg21.link/class.spaceship">[class.spaceship]</a>) 
of <code>Ts...</code>, the expanded parameter pack, or <code>void</code> if any element of <code>Ts</code> is not a comparison category type.
</p>
</blockquote>
</li>
</ol>

</blockquote>



<p id="res-3584"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 17.12.3 <a href="https://wg21.link/cmp.common">[cmp.common]</a> as indicated:</p>

<blockquote>
<p>
-1- The type <code>common_comparison_category</code> provides an alias for the strongest comparison category <del>to which
all of the template arguments can be converted</del>
<ins>among all the template arguments</ins>.
[<i>Note 1</i>: A comparison category type is stronger than another if they are distinct types and an instance of the former
can be converted to an instance of the latter. &mdash; <i>end note</i>]
</p>
<pre>
template&lt;class... Ts&gt;
struct common_comparison_category {
  using type = <i>see below</i>;
};
</pre>
<p>
-2- <i>Remarks:</i> The member <i>typedef-name</i> <code>type</code> denotes the common comparison type (11.10.3 <a href="https://wg21.link/class.spaceship">[class.spaceship]</a>)
of <code>Ts...</code>, the expanded parameter pack, or <code>void</code> if any element of <code>Ts</code> is not a comparison category type.
</p>
</blockquote>
</li>
</ol>





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