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<title>Issue 4019: Reversing an infinite range leads to an infinite loop</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#SG9">SG9</a> status.</em></p>
<h3 id="4019"><a href="lwg-active.html#4019">4019</a>. Reversing an infinite range leads to an infinite loop</h3>
<p><b>Section:</b> 25.7.21 <a href="https://wg21.link/range.reverse">[range.reverse]</a> <b>Status:</b> <a href="lwg-active.html#SG9">SG9</a>
 <b>Submitter:</b> Barry Revzin <b>Opened:</b> 2023-11-25 <b>Last modified:</b> 2024-11-14</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.reverse">issues</a> in [range.reverse].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#SG9">SG9</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Consider the following:
</p>
<blockquote><pre>
auto a = views::iota(0) | views::reverse;
auto b = views::repeat(42) | views::reverse;
</pre></blockquote>
<p>
Here, <code>views::iota(0)</code> and <code>views::repeat(42)</code> are both non-common bidirectional (even random-access) ranges. 
They are also infinite ranges, even if the standard doesn't really recognize that. 
<p/>
<code>views::reverse</code> on a non-common range will actually compute the end iterator for you. So while both declarations 
of <code>a</code> and <code>b</code> above compile, attempting to use either in any way will lead to an infinite loop when you 
<code>try a.begin()</code> or <code>b.begin()</code>.
<p/>
A reddit post suggested disallowing reversing a non-common range but that likely breaks reasonable use-cases. We could at
the very least recognize ranges whose sentinel is <code>unreachable_t</code> and reject those from consideration. For instance, 
we could change 24.4.4.4 <a href="https://wg21.link/range.iter.op.next">[range.iter.op.next]</a>/3 to <i>Mandate</i> that <code>S</code> is not <code>unreachable_t</code>.
</p>

<p><i>[2024-03-11; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll. Ask SG9 to look. Probably needs a paper.
</p>
<p>
Infinite ranges are invalid and giving an invalid range to the library is
undefined. But this is not a particularly satisfactory answer given that
we provide such ranges ourselves...
</p>

<p><i>[St. Louis 2024-06-28; LWG and SG9 joint session]</i></p>

<p>
Poll: SG9 and LWG believe this is not a defect?
<pre>
|SF| F| N| A|SA|
| 3| 3| 0| 2| 0|
</pre>
Weak consensus => needs to go to LEWG
</p>
<p>
Poll: SG9 and LWG agree that standard could do something to create
fewer sharp edges here, and we encourage a paper exploring options.
<pre>
|SF| F| N| A|SA|
| 4| 2| 2| 0| 0|
</pre>
</p>

<p><i>[2024-11-14; Related to LWG <a href="lwg-active.html#4097" title="views::reverse should be specialized for some view types (Status: LEWG)">4097</a><sup><a href="https://cplusplus.github.io/LWG/issue4097" title="Latest snapshot">(i)</a></sup>.]</i></p>



<p id="res-4019"><b>Proposed resolution:</b></p>





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