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<title>Issue 3481: viewable_range mishandles lvalue move-only views</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="3481"><a href="lwg-defects.html#3481">3481</a>. <code>viewable_range</code> mishandles lvalue move-only views</h3>
<p><b>Section:</b> 25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-08-29 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.refinements">issues</a> in [range.refinements].</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>
The <code>viewable_range concept</code> (25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a>) and the <code>views:all</code> 
range adaptor (25.7.6 <a href="https://wg21.link/range.all">[range.all]</a>) are duals: <code>viewable_range</code> is intended to admit 
exactly types <code>T</code> for which <code>views::all(declval&lt;T&gt;())</code> is well-formed. (Recall 
that <code>views::all(<i>meow</i>)</code> is a prvalue whose type models <code>view</code> when it is 
well-formed.) Before the addition of move-only view types to the design, this relationship was in 
place (modulo an incredibly pathological case: a <code>volatile</code> value of a view type with 
<code>volatile</code>-qualified <code>begin</code> and <code>end</code> models <code>viewable_range</code> but is 
rejected by <code>views::all</code> unless it also has a <code>volatile</code>-qualified copy constructor 
and copy assignment operator). Adding move-only views to the design punches a bigger hole, however: 
<code>viewable_range</code> admits lvalues of move-only view types for which <code>views::all</code> is 
ill-formed because these lvalues cannot be decay-copied.
<p/>
It behooves us to restore the correspondence between <code>viewable_range</code> and <code>views::all</code> 
so that instantiations of components constrained with <code>viewable_range</code> (which often appears 
indirectly as <code>views::all_t&lt;R&gt;</code> in deduction guides) continue to be well-formed when 
the constraints are satisfied.
</p>

<p><i>[2020-09-06; Reflector prioritization]</i></p>

<p>
Set priority to 2 during reflector discussions.
</p>

<p><i>[2021-05-24; Reflector poll]</i></p>

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

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3481"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a> as indicated:</p>

<blockquote>
<p>
-5- The <code>viewable_range</code> concept specifies the requirements of a <code>range</code> 
type that can be converted to a <code>view</code> safely.
</p>
<pre>
template&lt;class T&gt;
  concept viewable_range =
    range&lt;T&gt; &amp;&amp; <del>(borrowed_range&lt;T&gt; || view&lt;remove_cvref_t&lt;T&gt;&gt;);</del>
    <ins>((view&lt;remove_cvref_t&lt;T&gt;&gt; &amp;&amp; constructible_from&lt;remove_cvref_t&lt;T&gt;, T&gt;) ||</ins>
    <ins>(!view&lt;remove_cvref_t&lt;T&gt;&gt; &amp;&amp; borrowed_range&lt;T&gt;));</ins>
</pre>
</blockquote>
</li>

</ol>




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