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<title>Issue 2514: Type traits must not be final</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="2514"><a href="lwg-defects.html#2514">2514</a>. Type traits must not be <code>final</code></h3>
<p><b>Section:</b> 21.3.2 <a href="https://wg21.link/meta.rqmts">[meta.rqmts]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2015-07-03 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.rqmts">issues</a> in [meta.rqmts].</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>
We should make it clear that all standard <code>UnaryTypeTraits</code>,
<code>BinaryTypeTraits</code> and <code>TransformationTraits</code> are not <code>final</code>.
<p/>
Otherwise it is not safe to use them as arguments to a template like this:
</p>
<blockquote><pre>
template&lt;typename C1, typename C2&gt;
struct conjunction
  : conditional_t&lt;C1::value, C2, C1&gt;
{ };
</pre></blockquote>

<p><i>[2016-08-03 Chicago LWG]</i></p>

<p>
Walter, Nevin, and Jason provide initial Proposed Resolution.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This wording is relative to N4606.</p>

<ol>
<li><p>Change 21.3.2 <a href="https://wg21.link/meta.rqmts">[meta.rqmts]</a> as indicated:</p>

<blockquote>
<p>
-1- A <em>UnaryTypeTrait</em> describes a property of a type. It shall be a <ins>non-<code>final</code></ins> 
class template [&hellip;]
<p/>
-2- A <em>BinaryTypeTrait</em> describes a relationship between two types. It shall be a 
<ins>non-<code>final</code></ins> class template [&hellip;]
<p/>
-3- A <em>TransformationTrait</em> modifies a property of a type. It shall be a 
<ins>non-<code>final</code></ins> class template [&hellip;]
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2016-08-04 Chicago LWG]</i></p>

<p>
LWG discusses and expresses preference for a more general, Library-wide, resolution. Walter and Nevin 
provide a new Proposed Resolution consistent with such guidance.
</p>

<p><i>[2016-08 - Chicago]</i></p>

<p>Thurs PM: Moved to Tentatively Ready</p>


<p id="res-2514"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4606.</p>

<ol>
<li><p>Add a new paragraph add the end of 16.4.6.13 <a href="https://wg21.link/derivation">[derivation]</a> as indicated:</p>

<blockquote>
<p>
<ins>-?- All types specified in the C++ standard library shall be non-<code>final</code>
types unless otherwise specified.</ins>
</p>
</blockquote>
</li>
</ol>





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