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<title>Issue 2396: underlying_type doesn't say what to do for an incomplete enumeration type</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="2396"><a href="lwg-defects.html#2396">2396</a>. <code>underlying_type</code> doesn't say what to do for an incomplete enumeration type</h3>
<p><b>Section:</b> 21.3.9.7 <a href="https://wg21.link/meta.trans.other">[meta.trans.other]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2014-06-12 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.trans.other">issues</a> in [meta.trans.other].</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>
Consider:
</p>
<blockquote><pre>
enum E {
  e = std::underlying_type&lt;E&gt;::type(1)
};
</pre></blockquote>
<p>
Clearly this should be ill-formed, but the library section doesn't appear to ban it. Suggestion:
<p/>
Change in 21.3.9.7 <a href="https://wg21.link/meta.trans.other">[meta.trans.other]</a> Table 57:
</p>
<blockquote>
<p>
Template: <code>template&lt;class T&gt; struct underlying_type;</code>
<p/>
Condition: <code>T</code> shall be <ins>a complete</ins> <del>an</del> enumeration type (7.2)
<p/>
Comments: [&hellip;]
</p>
</blockquote>

<p><i>[2014-06-16 Rapperswil]</i></p>

<p>
Move to Ready
</p>



<p id="res-2396"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3936.</p>

<ol>
<li><p>Change Table 57 &mdash; "Other transformations" as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 3 &mdash; Other type transformations</caption>
<tr>
<th align="center">Template</th>
<th align="center">Condition</th>
<th align="center">Comments</th>
</tr>

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

<tr>
<td>
<code>template &lt;class T&gt;<br/>
struct underlying_type;</code>
</td>
<td>
T shall be <ins>a complete</ins> <del>an</del> enumeration type (7.2)
</td>
<td align="center">
[&hellip;]
</td>
</tr>

</table>
</blockquote>
</li>

</ol>





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