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<title>Issue 4091: concat_view rejects non-movable references</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#New">New</a> status.</em></p>
<h3 id="4091"><a href="lwg-active.html#4091">4091</a>. <code>concat_view</code> rejects non-movable references</h3>
<p><b>Section:</b> 25.7.18.2 <a href="https://wg21.link/range.concat.view">[range.concat.view]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-05-01 <b>Last modified:</b> 2024-06-24</p>
<p><b>Priority: </b>4
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.concat.view">active issues</a> in [range.concat.view].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.concat.view">issues</a> in [range.concat.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In order to prevent non-equality-preserving behavior of <code>operator*</code> and <code>iter_move</code>, 
<code>concat_view</code> introduces the <code><i>concat-indirectly-readable</i></code> concept, part of which is:
</p>
<blockquote><pre>
template&lt;class Ref, class RRef, class It&gt;
  concept <i>concat-indirectly-readable-impl</i> =                         // <i>exposition only</i>
    requires (const It it) {
      { *it } -&gt; convertible_to&lt;Ref&gt;;
      { ranges::iter_move(it) } -&gt; convertible_to&lt;RRef&gt;;
    };
</pre></blockquote>
<p>
This isn't quite right because <code>convertible_to</code> checks <code>is_convertible_v</code> which doesn't 
understand copy elision. This makes the current <code>concat_view</code> unable to work with ranges whose 
reference is non-movable prvalue:
</p>
<blockquote><pre>
auto r = std::views::iota(0, 5)
       | std::views::transform([](int) { return NonMovable{}; });
auto c1 = std::ranges::concat_view(r);    // <span style="color:red;font-weight:bolder">ill-formed, concat_indirectly_readable not satisfied</span>
auto c2 = std::ranges::concat_view(r, r); // <span style="color:red;font-weight:bolder">ditto</span>
</pre></blockquote>
<p>
Since <code>std::visit&lt;R&gt;</code> is used in the implementation to perform reference conversion for the 
underlying iterator, the more accurate one should be <code>is_invocable_r</code> which does understand guaranteed 
elision.
<p/>
Note that <code>join_with_view</code> has the same issue because <code><i>compatible-joinable-ranges</i></code> 
requires that the <code>value_type</code> of the inner range and pattern range must satisfy <code>common_with</code>, 
which always fails for non-movable types. However, this can be automatically resolved by LWG 
<a href="lwg-defects.html#4074" title="compatible-joinable-ranges is underconstrained (Status: WP)">4074</a><sup><a href="https://cplusplus.github.io/LWG/issue4074" title="Latest snapshot">(i)</a></sup>'s resolution.
</p>

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

<p>
Set priority to 4 after reflector poll.
"Proposed resolution loses the existing requirement that the conversion
is equality-preserving."
"Don't care about rejecting non-movable reference types."
</p>



<p id="res-4091"><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 25.7.18.2 <a href="https://wg21.link/range.concat.view">[range.concat.view]</a> as indicated:</p>

<blockquote>
<p>
-1- The exposition-only <code><i>concat-indirectly-readable</i></code> concept is equivalent to:
</p>
<blockquote>
<pre>
<ins>template&lt;class Ref, class CRef&gt;
concept <i>concat-ref-compatible-with</i> = is_invocable_r_v&lt;CRef, Ref()&gt;;  // <i>exposition only</i></ins>
  
template&lt;class Ref, class RRef, class It&gt;
concept <i>concat-indirectly-readable-impl</i> =                         // <i>exposition only</i>
  requires (const It it) {
    { *it } -&gt; <del>convertible_to</del><ins><i>concat-ref-compatible-with</i></ins>&lt;Ref&gt;;
    { ranges::iter_move(it) } -&gt; <del>convertible_to</del><ins><i>concat-ref-compatible-with</i></ins>&lt;RRef&gt;;
  };

template&lt;class... Rs&gt;
  concept <i>concat-indirectly-readable</i> =                              // <i>exposition only</i>
    common_reference_with&lt;<i>concat-reference-t</i>&lt;Rs...&gt;&amp;&amp;,
                          <i>concat-value-t</i>&lt;Rs...&gt;&amp;&gt; &amp;&amp;
    common_reference_with&lt;<i>concat-reference-t</i>&lt;Rs...&gt;&amp;&amp;,
                          <i>concat-rvalue-reference-t</i>&lt;Rs...&gt;&amp;&amp;&gt; &amp;&amp;
    common_reference_with&lt;<i>concat-rvalue-reference-t</i>&lt;Rs...&gt;&amp;&amp;,
                          <i>concat-value-t</i>&lt;Rs...&gt; const&amp;&gt; &amp;&amp;
    (<i>concat-indirectly-readable-impl</i>&lt;<i>concat-reference-t</i>&lt;Rs...&gt;,
                                     <i>concat-rvalue-reference-t</i>&lt;Rs...&gt;,
                                     iterator_t&lt;Rs&gt;&gt; &amp;&amp; ...);
</pre>
</blockquote>
</blockquote>

</li>
</ol>






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