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<title>Issue 3773: views::zip_transform still requires F to be copy_constructible when empty pack</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="3773"><a href="lwg-defects.html#3773">3773</a>. <code>views::zip_transform</code> still requires <code>F</code> to be <code>copy_constructible</code> when empty pack</h3>
<p><b>Section:</b> 25.7.26.1 <a href="https://wg21.link/range.zip.transform.overview">[range.zip.transform.overview]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-09-12 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>Not Prioritized
</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>
After <a href="https://wg21.link/P2494R2" title=" Relaxing range adaptors to allow for move only types">P2494R2</a>, range adaptors only require callable to be
<code>move_constructible</code>, however, for <code>views::zip_transform</code>,
when empty pack, it still requires callable to be <code>copy_constructible</code>.
We should relax this requirement, too.
</p>

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

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

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



<p id="res-3773"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li>
<p>Modify 25.7.26.1 <a href="https://wg21.link/range.zip.transform.overview">[range.zip.transform.overview]</a> as indicated:</p>
<blockquote>
<p>
-2- The name <code>views::zip_transform</code> denotes a customization point object
(16.3.3.3.5 <a href="https://wg21.link/customization.point.object">[customization.point.object]</a>). 
Let <code>F</code> be a subexpression,
and let <code>Es...</code> be a pack of subexpressions.
</p>
<ol style="list-style-type:none">
<li><p>(2.1) — If <code>Es</code> is an empty pack, let <code>FD</code> be <code>decay_t&lt;decltype((F))&gt;</code>.</p>
<ol style="list-style-type:none">
<li><p>(2.1.1) — If <code><ins>move_constructible</ins><del>copy_constructible</del>&lt;FD&gt; &amp;&amp; regular_invocable&lt;FD&amp;&gt;</code> is <code>false</code>, or if <code>decay_t&lt;invoke_result_t&lt;FD&amp;&gt;&gt;</code> is not an object type, <code>views::zip_transform(F, Es...)</code> is ill-formed.</p></li>
<li><p>(2.1.2) — Otherwise, the expression <code>views::zip_transform(F, Es...)</code> is expression-equivalent to</p>
<pre>
      ((void)F, auto(views::empty&lt;decay_t&lt;invoke_result_t&lt;FD&amp;&gt;&gt;&gt;))</pre>
</li>
</ol>
</li>
<li><p>(2.2) — Otherwise, the expression <code>views::zip_transform(F, Es...)</code> is expression-equivalent to <code>zip_transform_view(F, Es...)</code>.</p></li>
</ol>
</blockquote>
</li>
</ol>





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