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<title>Issue 3854: &sect;[res.on.exception.handling]/3 should not be applied to all standard library types</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="3854"><a href="lwg-active.html#3854">3854</a>. &sect;[res.on.exception.handling]/3 should not be applied to all standard library types</h3>
<p><b>Section:</b> 16.4.6.14 <a href="https://wg21.link/res.on.exception.handling">[res.on.exception.handling]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-01-07 <b>Last modified:</b> 2023-02-01</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#res.on.exception.handling">active issues</a> in [res.on.exception.handling].</p>
<p><b>View all other</b> <a href="lwg-index.html#res.on.exception.handling">issues</a> in [res.on.exception.handling].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Some standard library types, such as <code>std::pair</code>, <code>std::tuple</code>, and <code>std::array</code>, 
are currently allowed to have a member or element type with a destructor that is <code>noexcept(false)</code>. 
In order to conform to 16.4.6.14 <a href="https://wg21.link/res.on.exception.handling">[res.on.exception.handling]</a>/3, these types can't always have 
implicitly declared destructors because the implicit exception specification may be <code>noexcept(false)</code>.
<p/>
However, it may be intentional to implement these types with implicitly declared destructors, and 
mainstream implementations (libc++, libstdc++, and MSVC STL) are already doing so.
<p/>
Furtherly, explicitly declared destructors are not allowed for algorithm result types 
(26.5 <a href="https://wg21.link/algorithms.results">[algorithms.results]</a>), while currently these class templates are allowed to be instantiated 
with classes with potentially throwing destructors, which is contradictory to 16.4.6.14 <a href="https://wg21.link/res.on.exception.handling">[res.on.exception.handling]</a>/3.
</p>

<p><i>[2023-01-29; Daniel comments]</i></p>

<p>
This issue has very much overlap with LWG <a href="lwg-active.html#3229" title="&sect;[res.on.exception.handling]#3 cannot apply to types with implicitly declared destructors (Status: New)">3229</a><sup><a href="https://cplusplus.github.io/LWG/issue3229" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2023-02-01; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
Unclear who the "shall" imposes requirements on. Maybe split into constraint
on the library to not put <code>noexcept(false)</code> on destructors, and
constraint on users that the library can assume destructors don't throw.
Should also make it clear which parts of the subclause are normative and
which are not.
</p>



<p id="res-3854"><b>Proposed resolution:</b></p>





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