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<title>Issue 3177: Limit permission to specialize variable templates to program-defined types</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="3177"><a href="lwg-defects.html#3177">3177</a>. Limit permission to specialize variable templates to program-defined types</h3>
<p><b>Section:</b> 16.4.5.2.1 <a href="https://wg21.link/namespace.std">[namespace.std]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Johel Ernesto Guerrero Pe&ntilde;a <b>Opened:</b> 2018-12-11 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#namespace.std">active issues</a> in [namespace.std].</p>
<p><b>View all other</b> <a href="lwg-index.html#namespace.std">issues</a> in [namespace.std].</p>
<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>
The permission denoted by [namespace.std]/3 should be limited to program-defined types.
</p>

<p><i>[2018-12-21 Reflector prioritization]</i></p>

<p>Set Priority to 3</p>
<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4791">N4791</a>.</p>

<ol>
<li><p>Change 16.4.5.2.1 <a href="https://wg21.link/namespace.std">[namespace.std]</a> as indicated:</p>
<blockquote>
<p>
-2- Unless explicitly prohibited, a program may add a template specialization for any standard library class
template to namespace <code>std</code> provided that (a) the added declaration depends on at least one program-defined
type and (b) the specialization meets the standard library requirements for the original template.(footnote 174)
<p/>
-3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard
library variable template, except where explicitly permitted by the specification of that variable 
template<ins>, provided that the added declaration depends on at least one program-defined type</ins>.
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2022-08-24; LWG telecon]</i></p>

<p>Each variable template that grants permission to specialize already
states requirements more precisely than proposed here anyway.
For example, <code>disable_sized_range</code> only allows it for
cv-unqualified program-defined types.
Adding less precise wording here wouldn't be an improvement.
Add a note to make it clear we didn't just forget to say something here,
and to remind us to state requirements for each variable template in future.
</p>
<p><i>[2022-08-25; Jonathan Wakely provides improved wording]</i></p>



<p><i>[2022-09-28; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

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



<p id="res-3177"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.</p>

<ul>
<li><p>Change 16.4.5.2.1 <a href="https://wg21.link/namespace.std">[namespace.std]</a> as indicated:</p>
<blockquote>
<p>
-2- Unless explicitly prohibited, a program may add a template specialization for any standard library class
template to namespace <code>std</code> provided that (a) the added declaration depends on at least one program-defined
type and (b) the specialization meets the standard library requirements for the original template.(footnote 163)
</p>
<p>
-3- The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard
library variable template, except where explicitly permitted by the specification of that variable 
template.
</p>

<p>
<ins>
[<i>Note 1</i>:
The requirements on an explicit or partial specialization are stated
by each variable template that grants such permission.
&mdash; <i>end note</i>]
</ins>
</p>

</blockquote>
</li>
</ul>






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