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<title>Issue 4098: views::adjacent&lt;0&gt; should reject non-forward ranges</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="4098"><a href="lwg-defects.html#4098">4098</a>. <code>views::adjacent&lt;0&gt;</code> should reject non-forward ranges</h3>
<p><b>Section:</b> 25.7.27.1 <a href="https://wg21.link/range.adjacent.overview">[range.adjacent.overview]</a>, 25.7.28.1 <a href="https://wg21.link/range.adjacent.transform.overview">[range.adjacent.transform.overview]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-05-10 <b>Last modified:</b> 2024-07-08</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.adjacent.overview">issues</a> in [range.adjacent.overview].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Following-up LWG <a href="lwg-defects.html#4082" title="views::concat(r) is well-formed when r is an output_range (Status: WP)">4082</a><sup><a href="https://cplusplus.github.io/LWG/issue4082" title="Latest snapshot">(i)</a></sup> and LWG <a href="lwg-defects.html#4083" title="views::as_rvalue should reject non-input ranges (Status: WP)">4083</a><sup><a href="https://cplusplus.github.io/LWG/issue4083" title="Latest snapshot">(i)</a></sup>, the current wording makes
<code>views::adjacent&lt;0&gt;(r)</code> and <code>views::adjacent_transform&lt;0&gt;(r, [] { return 0; })</code>
well-formed even when <code>r</code> is just an input range or an output range, which seems to be an oversight.
</p>

<p><i>[2024-06-24; Reflector poll]</i></p>

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

<p><i>[St. Louis 2024-06-29; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4098"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>

<li><p>Modify 25.7.27.1 <a href="https://wg21.link/range.adjacent.overview">[range.adjacent.overview]</a> as indicated:</p>

<blockquote>
<p>
-2- The name <code>views::adjacent&lt;N&gt;</code> denotes a range adaptor object
(25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>). Given a subexpression <code>E</code> and a constant expression
<code>N</code>, the expression <code>views::adjacent&lt;N&gt;(E)</code> is expression-equivalent to
</p>
<ol style="list-style-type:none">
<li>
<p>(2.1) &mdash; <code>((void)E, auto(views::empty&lt;tuple&lt;&gt;&gt;))</code> if
  <code>N</code> is equal to <code>0</code> <ins>and <code>decltype((E))</code> models <code>forward_range</code></ins>,
</p>
</li>
<li>
<p>(2.2) &mdash; otherwise, <code>adjacent_view&lt;views::all_t&lt;decltype((E))&gt;, N&gt;(E)</code>.</p>
</li>
</ol>
</blockquote>

</li>

<li><p>Modify 25.7.28.1 <a href="https://wg21.link/range.adjacent.transform.overview">[range.adjacent.transform.overview]</a> as indicated:</p>

<blockquote>
<p>
-2- The name <code>views::adjacent_transform&lt;N&gt;</code> denotes a range adaptor object
(25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>). Given subexpressions <code>E</code> and <code>F</code> 
and a constant expression <code>N</code>:
</p>
<ol style="list-style-type:none">
<li>
<p>(2.1) &mdash; If <code>N</code> is equal to <code>0</code> <ins>and <code>decltype((E))</code> 
models <code>forward_range</code></ins>, <code>views::adjacent_transform&lt;N&gt;(E, F)</code>
is expression-equivalent to <code>((void)E, views::zip_transform(F))</code>, except that
the evaluations of <code>E</code> and <code>F</code> are indeterminately sequenced.
</p>
</li>
<li>
<p>(2.2) &mdash; Otherwise, the expression <code>views::adjacent_transform&lt;N&gt;(E, F)</code> is
expression-equivalent to <code>adjacent_transform_view&lt;views::all_t&lt;decltype((E))&gt;, 
decay_t&lt;decltype((F))&gt;, N&gt;(E, F)</code>.
</p>
</li>
</ol>
</blockquote>

</li>
</ol>






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