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<title>Issue 3256: Feature testing macro for constexpr algorithms</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++20">C++20</a> status.</em></p>
<h3 id="3256"><a href="lwg-defects.html#3256">3256</a>. Feature testing macro for <code>constexpr</code> algorithms</h3>
<p><b>Section:</b> 17.3.1 <a href="https://wg21.link/support.limits.general">[support.limits.general]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Antony Polukhin <b>Opened:</b> 2019-08-14 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#support.limits.general">issues</a> in [support.limits.general].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Feature testing macro from <a href="https://wg21.link/p0202">P0202</a> "Add Constexpr Modifiers to Functions 
in <code>&lt;algorithm&gt;</code> and <code>&lt;utility&gt;</code> Headers" is missing in the WD.
<p/>
For user convenience and to reduce feature testing macro count it would be better to stick to initial version 
of P0202 that was providing only the <code>__cpp_lib_constexpr_algorithms</code>.
<p/>
So remove <code>__cpp_lib_constexpr_swap_algorithms</code>, define <code>__cpp_lib_constexpr_algorithms</code> to 
<code>201703L</code> if <a href="https://wg21.link/p0202">P0202</a> is implemented, to <code>201806L</code> if 
<a href="https://wg21.link/p0202">P0202</a>+<a href="https://wg21.link/p0879">P0879</a> are implemented.
</p>

<p><i>[2019-09-02 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.</p>


<p id="res-3256"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 
17.3.1 <a href="https://wg21.link/support.limits.general">[support.limits.general]</a> as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 36: Standard library feature-test macros [tab:support.ft]</caption>
<tr align="center">
<th>Macro name</th>
<th>Value</th>
<th>Header(s)</th>
</tr> 

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

<tr>
<td>
<code>__cpp_lib_constexpr_<del>swap_</del>algorithms</code>
</td>
<td>
<code>201806L</code>
</td>
<td>
<code>&lt;algorithm&gt;</code>
</td>
</tr>

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

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

</ol>




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