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<title>Issue 4161: Some free functions don't automatically work for program-defined std::complex&lt;NonFloatingPoint&gt;</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#New">New</a> status.</em></p>
<h3 id="4161"><a href="lwg-active.html#4161">4161</a>. Some free functions don't automatically work for program-defined <code>std::complex&lt;<i>NonFloatingPoint</i>&gt;</code></h3>
<p><b>Section:</b> 29.4 <a href="https://wg21.link/complex.numbers">[complex.numbers]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-09-29 <b>Last modified:</b> 2025-02-07</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#complex.numbers">issues</a> in [complex.numbers].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>std::complex&lt;<i>NonFloatingPoint</i>&gt;</code> is possibly a program-defined specialization and thus 
instantiation of such a specialization can have determined effect. However, it's improbable for implementations 
to ensure that some free functions, e.g. <code>abs</code>, <code>sin</code>, <code>exp</code>, work for such a specialization.
<p/>
Moreover, <a href="https://wg21.link/P2819R2" title=" Add tuple protocol to complex">P2819R2</a> made all <code>std::complex</code> specializations tuple-like types, despite that 
<code>std::get</code> overloads need to touch implementation details and thus don't work for program-defined specializations.
<p/>
It seems better to make only <code>complex&lt;floating-point&gt;</code> tuple-like types, and add <i>Constraints</i> 
to some free functions that can't be guaranteed to work to only accept cases where <code>T</code> is a <i>cv</i>-unqualified 
floating-point type. However, it's unclear to me how to handle cases where the implementation intentionally supports 
<code>complex&lt;X&gt;</code> (where <code>X</code> is not a program-defined type) as extension, and whether <i>Mandates</i> 
should be used.
</p>

<p><i>[2025-02-07; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
"Program-defined specializations of <code class='backtick'>std::complex</code>
(i.e. explicit specializations and program-defined partial specializations)
seem unsupportably broken. We should say so explicitly until somebody writes
a paper to fix them."
</p>



<p id="res-4161"><b>Proposed resolution:</b></p>





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