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<title>Issue 3967: The specification for std::is_nothrow_* traits may be ambiguous in some cases involving noexcept(false)</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="3967"><a href="lwg-active.html#3967">3967</a>. The specification for <code>std::is_nothrow_*</code> traits may be ambiguous in some cases involving <code>noexcept(false)</code></h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-08-04 <b>Last modified:</b> 2023-08-05</p>
<p><b>Priority: </b>Not Prioritized
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<p><b>Discussion:</b></p>
<p>
<code>std::is_nothrow_*</code> traits are currently specified in the form "(some operation) is known not to throw any exceptions". 
Under the following circumstance, the current specification may be ambiguous:
</p>
<ul>
<li><p>a trait detects an explicitly defaulted special member function, and</p></li>
<li><p>the implicit except specification of that function would be <code>noexcept(true)</code>, but</p></li>
<li><p>the function is explicitly marked with <code>noexcept(false)</code>.</p></li>
</ul>
<p>
It seems that an implementation may decompose the defaulted special member function and then conclude that the trait 
should give a <code>true</code> result, or may just use the exception specification which gives a <code>false</code> result.
<p/>
Implementors seemingly hold different opinions, see <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106611">GCC Bug 106611</a> 
and <a href="https://quuxplusone.github.io/blog/2023/04/17/noexcept-false-equals-default/">this blog post</a>.
<p/> 
Perhaps we may need to clarify that the implementation divergence is permitted, or unambiguously specify these 
traits to give results consistent with exception specifications.
</p>


<p id="res-3967"><b>Proposed resolution:</b></p>





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