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<title>Issue 3755: tuple-for-each can call user-defined operator,</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++23">C++23</a> status.</em></p>
<h3 id="3755"><a href="lwg-defects.html#3755">3755</a>. <code><i>tuple-for-each</i></code> can call user-defined <code>operator,</code></h3>
<p><b>Section:</b> 25.7.25.2 <a href="https://wg21.link/range.zip.view">[range.zip.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Nicole Mazzuca  <b>Opened:</b> 2022-08-26 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.zip.view">issues</a> in [range.zip.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The specification for <code><i>tuple-for-each</i></code> is:
</p>

<blockquote><pre>
template&lt;class F, class Tuple&gt;
constexpr auto <i>tuple-for-each</i>(F&amp;&amp; f, Tuple&amp;&amp; t) { <i>// exposition only</i>
  apply([&amp;]&lt;class... Ts&gt;(Ts&amp;&amp;... elements) {
    (invoke(f, std::forward&lt;Ts&gt;(elements)), ...);
  }, std::forward&lt;Tuple&gt;(t));
}
</pre></blockquote>

<p>Given</p>

<blockquote><pre>
struct Evil {
  void operator,(Evil) {
        abort();
    }
};
</pre></blockquote>

<p>
and <code>tuple&lt;int, int&gt; t</code>, then
<code><i>tuple-for-each</i>([](int) { return Evil{}; }, t)</code>,
the program will (unintentionally) abort.
</p>

<p>
It seems likely that our <code>Evil</code>'s <code>operator,</code>
should not be called.
</p>


<p><i>[2022-09-23; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after nine votes in favour during reflector poll.
</p>
<p>
Feedback from reviewers:
</p>
<blockquote>
<p>
"NAD. This exposition-only facility is only used with things that return <code>void</code>. 
As far as I know, users can't define <code>operator,</code> for <code>void</code>."
"If I see the <code>void</code> cast, I don't need to audit the uses or be concerned that 
we'll add a broken use in the future."
</p>
</blockquote>

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3755"><b>Proposed resolution:</b></p>

<p> This wording is relative to the forthcoming C++23 CD. </p>

<ul>
<li>
<p>
Modify  [range.adaptor.tuple] as indicated:
</p>

<blockquote><pre>
template&lt;class F, class Tuple&gt;
constexpr auto <i>tuple-for-each</i>(F&amp;&amp; f, Tuple&amp;&amp; t) { <i>// exposition only</i>
  apply([&amp;]&lt;class... Ts&gt;(Ts&amp;&amp;... elements) {
    (<ins>static_cast&lt;void&gt;(</ins>invoke(f, std::forward&lt;Ts&gt;(elements))<ins>)</ins>, ...);
  }, std::forward&lt;Tuple&gt;(t));
}
</pre></blockquote>

</li>
</ul>






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