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<title>Issue 3982: is-derived-from-view-interface should require that T is derived from view_interface&lt;T&gt;</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#NAD">Tentatively NAD</a> status.</em></p>
<h3 id="3982"><a href="lwg-active.html#3982">3982</a>. <code><i>is-derived-from-view-interface</i></code> should require that <code>T</code> is derived from <code>view_interface&lt;T&gt;</code></h3>
<p><b>Section:</b> 25.4.5 <a href="https://wg21.link/range.view">[range.view]</a> <b>Status:</b> <a href="lwg-active.html#NAD">Tentatively NAD</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-08-22 <b>Last modified:</b> 2023-10-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.view">issues</a> in [range.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Tentatively NAD">Tentatively NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, the wording of <code><i>is-derived-from-view-interface</i></code> only detects whether type <code>T</code> is unambiguously 
derived from one base class <code>view_interface&lt;U&gt;</code> where <code>U</code> is not required to be <code>T</code>, which is not 
the intention of CRTP.
</p>

<p><i>[2023-10-30; Reflector poll]</i></p>

<p>Set status to Tentatively NAD.
The wording correctly handles the case where T derives from <code>Base</code>
which derives from <code>view_interface&lt;Base&gt;</code>.
We don't want it to only be satisfied for direct inheritance from
<code>view_interface&lt;T&gt;</code>, but from any specialization of
<code>view_interface</code>.
Previously the concept only checked for inheritance from <code>view_base</code>
but it was changed when <code>view_interface</code> stopped inheriting from
<code>view_base</code>.
</p>



<p id="res-3982"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4958" title=" Working Draft, Programming Languages — C++">N4958</a>.
</p>

<ol>

<li><p>Modify 25.4.5 <a href="https://wg21.link/range.view">[range.view]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
  constexpr bool <i>is-derived-from-view-interface</i> = <i>see below</i>;            // exposition only
template&lt;class T&gt;
  constexpr bool enable_view =
    derived_from&lt;T, view_base&gt; || <i>is-derived-from-view-interface</i>&lt;T&gt;;
</pre>
<blockquote>
<p>
-6- For a type <code>T</code>, <code><i>is-derived-from-view-interface</i>&lt;T&gt;</code>
is <code>true</code> if and only if <code>T</code> has exactly one public base class
<code>view_interface&lt;<ins>T</ins><del>U</del>&gt;</code> <del>for some type <code>U</code></del> and
<code>T</code> has no base classes of type <code>view_interface&lt;<ins>U</ins><del>V</del>&gt;</code> for any
other type <code><ins>U</ins><del>V</del></code>.
<p/>
</p>
</blockquote>
</blockquote>

</li>

</ol>





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