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<title>Issue 4185: Ill-formed, no diagnostic required on runtime behavior</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="4185"><a href="lwg-active.html#4185">4185</a>. Ill-formed, no diagnostic required on runtime behavior</h3>
<p><b>Section:</b> 29.9.7 <a href="https://wg21.link/linalg.helpers">[linalg.helpers]</a>, 18.4.9 <a href="https://wg21.link/concept.swappable">[concept.swappable]</a>, 24.3.3.2 <a href="https://wg21.link/iterator.cust.swap">[iterator.cust.swap]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-12-18 <b>Last modified:</b> 2025-02-07</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Several exposition-only function objects in 29.9.7 <a href="https://wg21.link/linalg.helpers">[linalg.helpers]</a> are specified with strange 
wording &mdash; "If the function selected by overload resolution does not return [&hellip;], the program 
is ill-formed, no diagnostic required;". These conditions don't seem to be static properties because what 
the selected function returns can be completely dependent on the runtime environment and input.
<p/>
Likewise, 18.4.9 <a href="https://wg21.link/concept.swappable">[concept.swappable]</a> and 24.3.3.2 <a href="https://wg21.link/iterator.cust.swap">[iterator.cust.swap]</a> also contain such weird 
usage. Presumably we should say the behavior is undefined in these cases.
</p>

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

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-4185"><b>Proposed resolution:</b></p>





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