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<title>Issue 3907: Can iterator types of range adaptors and range factories be SCARY?</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="3907"><a href="lwg-active.html#3907">3907</a>. Can iterator types of range adaptors and range factories be SCARY?</h3>
<p><b>Section:</b> 25.6 <a href="https://wg21.link/range.factories">[range.factories]</a>, 25.7 <a href="https://wg21.link/range.adaptors">[range.adaptors]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-03-18 <b>Last modified:</b> 2023-06-01</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
[For the term SCARY see <a href="https://wg21.link/N2911" title=" Minimizing Dependencies within Generic Classes for Faster and Smaller Programs">N2911</a> and <a href="https://wg21.link/N2913" title=" SCARY Iterator Assignment and Initialization">N2913</a>.]
<p/>
In 25.6 <a href="https://wg21.link/range.factories">[range.factories]</a> and 25.7 <a href="https://wg21.link/range.adaptors">[range.adaptors]</a>, many iterator types are specified 
as exposition-only nested classes or nested class templates.
<p/>
This has some observable differences from specifying them as class templates declared in the namespace scope:
</p>
<ol>
<li><p>value types are generally not associated entities of iterators or sentinels during ADL, which prevents 
unintended overloads to be found;</p></li>
<li><p>two different range adaptor/factory types, e.g. <code>iota_view&lt;I, S1&gt;</code> and <code>iota_view&lt;I, S2&gt;</code>, 
have different iterator types.</p></li>
</ol>
<p>
The ADL reduction seems preferable. However, is it intended to required to any two different range 
adaptor/factory types to have two different iterator types, even when the same iterator type can work as expected?
<p/>
Sentinel types don't seem able to be SCARY as they are dependent on iterator types.
</p>

<p><i>[2023-06-01; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
"The ADL hiding stuff is a red herring - SCARY-ness doesn't prevent ADL hiding.
Shouldn't require them to be different types (and similarly should not require
const/non-const iterators to be implemented as class templates with a single
bool template parameter)."
</p>



<p id="res-3907"><b>Proposed resolution:</b></p>





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