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<title>Issue 3791: join_view::iterator::operator-- may be ill-formed</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#Resolved">Resolved</a> status.</em></p>
<h3 id="3791"><a href="lwg-defects.html#3791">3791</a>. <code>join_view::<i>iterator</i>::operator--</code> may be ill-formed</h3>
<p><b>Section:</b> 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-09-30 <b>Last modified:</b> 2023-03-23</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.join.iterator">issues</a> in [range.join.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, <code>join_view::<i>iterator</i>::operator--</code> has the following <i>Effects</i>:
</p>
<blockquote><pre>
if (<i>outer_</i> == ranges::end(<i>parent_</i>-><i>base_</i>))
  <i>inner_</i> = ranges::end(*--<i>outer_</i>);
while (<i>inner_</i> == ranges::begin(*<i>outer_</i>))
  <i>inner_</i> = ranges::end(*--<i>outer_</i>);
--<i>inner_</i>;
return *this;
</pre></blockquote>
<p>
which uses <code>ranges::end(*--<i>outer_</i>)</code> to get the sentinel of the inner range.
However, <code>*--<i>outer_</i></code> may return an rvalue reference to a non-borrowed range,
in which case calling <code>ranges::end</code> will be ill-formed,
<a href="https://godbolt.org/z/s4bxTWfqv">for example</a>:
</p>
<blockquote><pre>
#include &lt;ranges&gt;
#include &lt;vector&gt;

int main() {
  std::vector&lt;std::vector&lt;int&gt;&gt; v;
  auto r = v | std::views::transform([](auto&amp; x) -&gt; auto&amp;&amp; { return std::move(x); })
             | std::views::join;
  auto e = --r.end(); // <span style="color:red;font-weight:bolder">hard error</span>
}
</pre></blockquote>
<p>
The proposed resolution uses a temporary reference to bind <code>*--<i>outer_</i></code>, so that
<code>ranges::end</code> is always invoked on an lvalue range, which is consistent with the behavior of
<code>join_with_view::<i>iterator</i>::operator--</code>.
</p>

<p><i>[2022-10-12; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
"Could introduce an <code>as_lvalue</code> lambda (like
<code>auto as_lvalue = []&lt;class T&gt;(T&amp;&amp; x) -&gt; T&amp; { return (T&amp;)x; };</code>) and use it throughout."
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a> as indicated:</p>

<blockquote>
<pre>
constexpr <i>iterator</i>&amp; operator--()
  requires <i>ref-is-glvalue</i> &amp;&amp; bidirectional_range&lt;<i>Base</i>&gt; &amp;&amp;
            bidirectional_range&lt;range_reference_t&lt;<i>Base</i>&gt;&gt; &amp;&amp;
            common_range&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;;
</pre>
</blockquote>
<p>
-13- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
if (<i>outer_</i> == ranges::end(<i>parent_</i>-><i>base_</i>)) <ins>{
  auto&amp;&amp; inner = *--<i>outer_</i>;</ins>
  <i>inner_</i> = ranges::end(<ins>inner</ins><del>*--<i>outer_</i></del>);
<ins>}</ins>
while (<ins>true</ins><del><i>inner_</i> == ranges::begin(*<i>outer_</i>)</del>) <ins>{
  if (auto&amp;&amp; tmp = *<i>outer_</i>; <i>inner_</i> == ranges::begin(tmp)) {
    auto&amp;&amp; inner = *--<i>outer_</i>;</ins>
    <i>inner_</i> = ranges::end(<ins>inner</ins><del>*--<i>outer_</i></del>);
  <ins>} else {
    break;
  }
}</ins>
--<i>inner_</i>;
return *this;
</pre></blockquote>
</li>
</ol>
</blockquote>

<p><i>[2023-03-22 Resolved by the adoption of <a href="https://wg21.link/P2770R0" title=" Stashing stashing iterators for proper flattening">P2770R0</a> in Issaquah. Status changed: New &rarr; Resolved.]</i></p>




<p id="res-3791"><b>Proposed resolution:</b></p>





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