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<title>Issue 2892: Relax the prohibition on libraries adding constexpr</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="2892"><a href="lwg-closed.html#2892">2892</a>. Relax the prohibition on libraries adding <code>constexpr</code></h3>
<p><b>Section:</b> 16.4.6.7 <a href="https://wg21.link/constexpr.functions">[constexpr.functions]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Great Britain <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#constexpr.functions">issues</a> in [constexpr.functions].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<b>Addresses GB 38</b>

<p>Relax the prohibition on libraries adding <code>constexpr</code>; this was a constraint requested by library implementers when 
<code>constexpr</code> was new, and those same implementers now feel unduly constrained.</p>
<p>Proposed change: Rewrite the whole sub-clause to support libraries adding <code>constexpr</code> in a compatible manner, 
much like the freedom to add a <code>noexcept</code> specification.</p>

<p><i>[2017-02-20, Marshall adds wording]</i></p>

<p>The simplest change would be to strike the sentence "An implementation shall not [&hellip;] explicitly required". However, 
people seem to want a definite permission here, so I have provided one.</p>

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

<p>Priority 1</p>

<p><i>[2018-06 Rapperswil Thursday issues processing]</i></p>

<p>This is a feature request; not a defect. We've had two cross-group meetings over this, and there is no consensus for changing this. Closing as NAD.</p>



<p id="res-2892"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.</p>

<ol>
<li><p>Modify 16.4.6.7 <a href="https://wg21.link/constexpr.functions">[constexpr.functions]</a> as indicated:</p>
<blockquote>
<p>This International Standard explicitly requires that certain standard library functions are constexpr (7.1.5). <ins>An 
implementation may declare additional standard library function signatures as constexpr</ins><del>An implementation shall 
not declare any standard library function signature as constexpr except for those where it is explicitly required</del>. 
Within any header that provides any non-defining declarations of constexpr functions or constructors an implementation 
shall provide corresponding definitions.</p>
</blockquote>
</li>

</ol>





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