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<title>Issue 3447: Deduction guides for take_view and drop_view have different constraints</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="3447"><a href="lwg-defects.html#3447">3447</a>. Deduction guides for <code>take_view</code> and <code>drop_view</code> have different constraints</h3>
<p><b>Section:</b> 25.7.10.2 <a href="https://wg21.link/range.take.view">[range.take.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jens Maurer <b>Opened:</b> 2020-05-15 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.take.view">issues</a> in [range.take.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>
From this <a href="https://github.com/cplusplus/draft/issues/3997">editorial issue request</a>:
<p/>
(Note "<code>range R</code>" vs "<code>class R</code>".)
<p/>
In 25.7.10.2 <a href="https://wg21.link/range.take.view">[range.take.view]</a>, the deduction guide for <code>take_view</code> is declared as:
</p>
<blockquote><pre>
template&lt;range R&gt;
  take_view(R&amp;&amp;, range_difference_t&lt;R&gt;)
    -&gt; take_view&lt;views::all_t&lt;R&gt;&gt;;
</pre></blockquote>
<p>
In 25.7.12.2 <a href="https://wg21.link/range.drop.view">[range.drop.view]</a>, the deduction guide for <code>drop_view</code> is declared as:
</p>
<blockquote><pre>
template&lt;class R&gt;
  drop_view(R&amp;&amp;, range_difference_t&lt;R&gt;) -&gt; drop_view&lt;views::all_t&lt;R&gt;&gt;;
</pre></blockquote>
<p>
Note the difference between their template parameter lists.
<p/>
Suggested resolution:
<p/>
Change the deduction guide of <code>take_view</code> from
</p>
<blockquote><pre>
template&lt;range R&gt;
</pre></blockquote>
<p>
to
</p>
<blockquote><pre>
template&lt;class R&gt;
</pre></blockquote>
<p>
</p>

<p><i>[2020-07-17; Moved to Ready in telecon]</i></p>

<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3447"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.</p>

<ol>
<li><p>Modify 25.7.10.2 <a href="https://wg21.link/range.take.view">[range.take.view]</a>, class template <code>take_view</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;<del>range</del><ins>class</ins> R&gt;
  take_view(R&amp;&amp;, range_difference_t&lt;R&gt;)
    -&gt; take_view&lt;views::all_t&lt;R&gt;&gt;;
[&hellip;]
</pre>
</blockquote>
</li>

</ol>




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