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<title>Issue 2871: User specializations of type traits should be ill-formed</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#NAD">NAD</a> status.</em></p>
<h3 id="2871"><a href="lwg-closed.html#2871">2871</a>. User specializations of type traits should be ill-formed</h3>
<p><b>Section:</b> 21.3.3 <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#meta.type.synop">active issues</a> in [meta.type.synop].</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.type.synop">issues</a> in [meta.type.synop].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses US 10</b></p>

<p>A user specialization of any type trait should produce 
an ill-formed program, not merely one whose behavior is unspecified.
See also the related comment re 26.3.6.2 <a href="https://wg21.link/execpol.type">[execpol.type]</a>.</p>

<p>
Proposed change:
</p>
<p>
Reword the paragraph as follows:
</p>
<blockquote>
<p>
Unless otherwise specified, a program that adds 
specializations for any of the templates defined in 
this subclause is ill-formed; no diagnostic required.
</p>
</blockquote>

<p>
See also <a href="lwg-closed.html#2909" title="User specializations of is_execution_policy should be ill-formed (Status: NAD)">2909</a><sup><a href="https://cplusplus.github.io/LWG/issue2909" title="Latest snapshot">(i)</a></sup>
</p>

<p><i>[2017-07 Toronto Thurs Issue Prioritization]</i></p>

<p>NAD; what the standard says is that this is "undefined", not "unspecified".</p>


<p id="res-2871"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4618">N4618</a>.</p>

<ol>
<li><p>Modify 21.3.3 <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a> as indicated:</p>
<blockquote>
<p>
-1- <del>The behavior of a program that adds specializations for any of the templates defined in this subclause is
undefined unless otherwise specified</del><ins>Unless otherwise specified, a program that adds 
specializations for any of the templates defined in this subclause is ill-formed; no diagnostic required</ins>.
</p>
</blockquote>
</li>
</ol>






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