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<title>Issue 3823: Unnecessary precondition for is_aggregate</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="3823"><a href="lwg-defects.html#3823">3823</a>. Unnecessary precondition for <code>is_aggregate</code></h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tomasz Kamiński <b>Opened:</b> 2022-11-09 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#meta.unary.prop">active issues</a> in [meta.unary.prop].</p>
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<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>
This is resolution of <b><a href="https://github.com/cplusplus/nbballot/issues/449">GB-090</a></b>
(21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> Unnecessary precondition for <code>is_aggregate</code>).
</p>

<p>
The precondition for <code>is_aggregate</code> is "<code>remove_all_extents_t&lt;T&gt;</code> shall be a complete type 
or <i>cv</i> <code>void</code>." This means that <code>is_aggregate_v&lt;Incomplete[2]&gt;</code> is undefined, but 
an array is always an aggregate, we don't need a complete element type to know that.	
</p>

<p>
Historically the <code>is_aggregate</code> was introduced by LWG <a href="lwg-defects.html#2911" title="An is_aggregate type trait is needed (Status: C++17)">2911</a><sup><a href="https://cplusplus.github.io/LWG/issue2911" title="Latest snapshot">(i)</a></sup> as part of the resolution of the NB comments.
The comment proposed to introduce this trait with requirement "<code>remove_all_extents_t&lt;T&gt;</code> shall 
be a complete type, an array type, or (possibly <i>cv</i>-qualified) <code>void</code>.",
that is close to resolution proposed in this issue.
According to notes this was simplified during review, after realizing that <code>remove_all_extents_t&lt;T&gt;</code> 
is never an array type.
</p>

<p><i>[Kona 2022-11-09; Move to Immediate]</i></p>


<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3823"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 21.3.3 <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a>, Table 46 ([tab:meta.unary.prop]) &mdash;
"Type property predicates" &mdash; as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 48: Type property predicates [tab:meta.unary.prop]</caption>
<tr>
<th align="center">Template</th>
<th align="center">Condition</th>
<th align="center">Preconditions</th>
</tr>

<tr>
<td colspan="3" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>template&lt;class T, class U&gt;<br/>
struct is_aggregate;</code>
</td>

<td>
<code>T</code> is an aggregate type (9.5.2 <a href="https://wg21.link/dcl.init.aggr">[dcl.init.aggr]</a>)	
</td>

<td>
<code><del>remove_all_extents_t&lt;</del>T<del>&gt;</del></code> shall be <ins>an array type,</ins> 
a complete type<ins>,</ins> or <i>cv</i> <code>void</code>.
</td>
</tr>

</table>
</blockquote>

</li>
</ol>





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