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<title>Issue 2582: &sect;[res.on.functions]/2's prohibition against incomplete types shouldn't apply to type traits</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++17">C++17</a> status.</em></p>
<h3 id="2582"><a href="lwg-defects.html#2582">2582</a>. &sect;[res.on.functions]/2's prohibition against incomplete types shouldn't apply to type traits</h3>
<p><b>Section:</b> 21 <a href="https://wg21.link/meta">[meta]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2016-01-07 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#meta">active issues</a> in [meta].</p>
<p><b>View all other</b> <a href="lwg-index.html#meta">issues</a> in [meta].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a>/2.5 says that the behavior is undefined "if an incomplete type is used as a template argument 
when instantiating a template component, unless specifically allowed for that component."
<p/>
This rule should not apply to type traits &mdash; a literal application would make <code>is_same&lt;void, void&gt;</code> 
undefined behavior, since nothing in 21 <a href="https://wg21.link/meta">[meta]</a> (or elsewhere) "specifically allows" instantiating <code>is_same</code> 
with incomplete types.
<p/>
Traits that require complete types are already explicitly specified as such, so the proposed wording below simply negates 
16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a>/2.5 for 21 <a href="https://wg21.link/meta">[meta]</a>.
</p>

<p><i>[2016-02, Issues Telecon]</i></p>

<p>
P0; move to Tentatively Ready.
</p>


<p id="res-2582"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N4567.
</p>

<ol>
<li><p>Insert a new paragraph after 21.3.3 <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a>/1:</p>

<blockquote>
<p>
<ins>-?- Unless otherwise specified, an incomplete type may be used to instantiate a template in this subclause.</ins>
</p>
</blockquote>
</li>
</ol>





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