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<title>Issue 3947: Unexpected constraints on adjacent_transform_view::base()</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#WP">WP</a> status.</em></p>
<h3 id="3947"><a href="lwg-defects.html#3947">3947</a>. Unexpected constraints on <code>adjacent_transform_view::base()</code></h3>
<p><b>Section:</b> 25.7.28.2 <a href="https://wg21.link/range.adjacent.transform.view">[range.adjacent.transform.view]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Bo Persson <b>Opened:</b> 2023-06-17 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In section 25.7.28.2 <a href="https://wg21.link/range.adjacent.transform.view">[range.adjacent.transform.view]</a> the class
<code>ranges::adjacent_transform_view</code> got two new <code>base()</code> members from
<a href="lwg-defects.html#3848" title="adjacent_view, adjacent_transform_view and slide_view missing base accessor (Status: C++23)">3848</a><sup><a href="https://cplusplus.github.io/LWG/issue3848" title="Latest snapshot">(i)</a></sup>.
<p/>
The first one looks like
</p>
<blockquote><pre>
constexpr V base() const &amp; requires copy_constructible&lt;<i>InnerView</i>&gt;
{ return <i>inner_</i>.base(); }
</pre></blockquote>
<p>
Here the requirement is that <code><i>InnerView</i></code> is copy constructible, when it in
fact returns an object of type <code>V</code>. That seems odd.
<p/>
I would expect the constraint to instead be <code>copy_constructible&lt;V&gt;</code>.
</p>

<p><i>[2023-10-27; Reflector poll]</i></p>

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

<p><i>[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3947"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</a>.
</p>

<ol>
<li><p>Modify 25.7.28.2 <a href="https://wg21.link/range.adjacent.transform.view">[range.adjacent.transform.view]</a>, class template 
<code>adjacent_transform_view</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;forward_range V, move_constructible F, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; (N &gt; 0) &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
      regular_invocable&lt;F&amp;, <i>REPEAT</i>(range_reference_t&lt;V&gt;, N)...&gt; &amp;&amp;
      <i>can-reference</i>&lt;invoke_result_t&lt;F&amp;, <i>REPEAT</i>(range_reference_t&lt;V&gt;, N)...&gt;&gt;
  class adjacent_transform_view : public view_interface&lt;adjacent_transform_view&lt;V, F, N&gt;&gt; {
    [&hellip;]
    adjacent_view&lt;V, N&gt; <i>inner_</i>; // <i>exposition only</i>
    
    using <i>InnerView</i> = adjacent_view&lt;V, N&gt;; // <i>exposition only</i>
    [&hellip;]
  public:
    [&hellip;]
    constexpr V base() const &amp; requires copy_constructible&lt;<ins>V</ins><del><i>InnerView</i></del>&gt; { return <i>inner_</i>.base(); }
    constexpr V base() &amp;&amp; { return std::move(<i>inner_</i>).base(); }
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

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