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<title>Issue 3971: Join ranges of rvalue references with ranges of prvalues</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">NAD</a> status.</em></p>
<h3 id="3971"><a href="lwg-closed.html#3971">3971</a>. Join ranges of rvalue references with ranges of prvalues</h3>
<p><b>Section:</b> 25.7.15.2 <a href="https://wg21.link/range.join.with.view">[range.join.with.view]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-08-21 <b>Last modified:</b> 2024-06-24</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.join.with.view">issues</a> in [range.join.with.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The issue that <code>concat_view</code> implicitly breaks equality-preserving by concatenating range of references with range 
of prvalues seems to be reflected in <code>join_view</code> as well.
<p/>
When the reference of the inner range is <code>string&amp;&amp;</code> and the reference of the pattern range is prvalue 
<code>string</code>, dereferencing its iterator will move the elements of the inner range to the returned <code>string</code>, 
which makes the second dereference get an empty <code>string</code> (<a href="https://godbolt.org/z/5YYzK5qsr">demo</a>):
</p>
<blockquote>
<pre>
vector v1{"hello"s};
vector v2{"world"s};
vector v{v1 | views::as_rvalue, v2 | views::as_rvalue};
auto pattern = views::iota(0, 1) | views::transform([](int) { return ", "s; });
ranges::forward_range auto joined = v | views::join_with(pattern);
fmt::print("{}\n", joined); // ["hello", ", ", "world"]
fmt::print("{}\n", joined); // <span style="color:red;font-weight:bolder">["", ", ", ""]</span>
</pre>
</blockquote>

<p>Not sure if we should ban such less common case.</p>

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

<p>
Set priority to 3 after reflector poll. Send to SG9.
</p>

<p><i>[2024-03-19; Tokyo: feedback from SG9]</i></p>

<p>
SG9 believe that LWG3971 is a use-after-move and is working as intended.
We further believe that matching the behavior of <code class='backtick'>concat_view</code>
(which is the current behavior) is important. This is not a defect.
</p>

<p><i>[St. Louis 2024-06-24 Status changed: Open &rarr; NAD.]</i></p>



<p id="res-3971"><b>Proposed resolution:</b></p>






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