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<title>Issue 3301: transform_view::iterator has incorrect iterator_category</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#C++20">C++20</a> status.</em></p>
<h3 id="3301"><a href="lwg-defects.html#3301">3301</a>. <code>transform_view::iterator</code> has incorrect <code>iterator_category</code></h3>
<p><b>Section:</b> 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Michel Morin <b>Opened:</b> 2019-10-03 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>1
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.transform.iterator">issues</a> in [range.transform.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When the transformation function returns an rvalue, <code>transform_view::iterator</code> 
cannot model <code><i>cpp17-forward-iterator</i></code>. However, similar to LWG <a href="lwg-defects.html#3291" title="iota_view::iterator has the wrong iterator_category (Status: C++20)">3291</a><sup><a href="https://cplusplus.github.io/LWG/issue3291" title="Latest snapshot">(i)</a></sup>,
the current wording on <code>transform_view::iterator::iterator_category</code> does not 
consider this.
<p/>
As Casey Carter pointed out <a href="https://github.com/ericniebler/stl2/issues/637#issuecomment-535594100">here</a>,
the proposed wording below does not consider <code>input_iterator</code> that is not 
<code><i>cpp17-input-iterator</i></code> (this problem is not specific to the PR; it's pervasive 
in adapted iterators) and concepts-based determination would be a better fix for issues around 
<code>iterator_category</code>. But anyway, I consider this PR as a minimal fix at the moment.
</p>

<p><i>[2019-10-31 Issue Prioritization]</i></p>

<p>Priority to 1 after reflector discussion.</p>

<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> as indicated:</p>

<blockquote>
<p>
-2- <ins><code>iterator::iterator_category</code> is defined as follows:</ins> 
Let <code>C</code> denote the type <code>iterator_traits&lt;iterator_t&lt;Base&gt;&gt;::iterator_category</code>.
</p>
<ol style="list-style-type: none">
<li><p><ins>(2.?) &mdash; If <code>is_lvalue_reference_v&lt;iter_reference_t&lt;iterator_t&lt;Base&gt;&gt;&gt;</code> 
is <code>true</code>,</ins></p>
<ol style="list-style-type: none">
<li><p><ins>(2.?.?) &mdash;</ins> If <code>C</code> models <code>derived_from&lt;contiguous_iterator_tag&gt;</code>, 
then <code>iterator_category</code> denotes <code>random_access_iterator_tag</code>;</p></li>
<li><p><ins>(2.?.?) &mdash;</ins> <ins>O</ins><del>o</del>therwise, <code>iterator_category</code> denotes 
<code>C</code>.</p></li>
</ol>
</li>
<li><p><ins>(2.?) &mdash; Otherwise, <code>iterator_category</code> denotes 
<code>input_iterator_tag</code>.</ins></p></li>
</ol>
</blockquote>
</li>

</ol>
</blockquote>
<p><i>[2019-11-06, Tim updates P/R based on Belfast LWG evening session discussion]</i></p>

<p>
The check in the original P/R is incorrect; we want to check the transformation's result, not the base iterator.
</p>

<p><i>[2020-02-10 Move to Immediate Monday afternoon in Prague]</i></p>



<p id="res-3301"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 25.7.9.3 <a href="https://wg21.link/range.transform.iterator">[range.transform.iterator]</a> as indicated:</p>

<blockquote>
<p>
-2- <ins><code>iterator::iterator_category</code> is defined as follows:</ins> 
Let <code>C</code> denote the type <code>iterator_traits&lt;iterator_t&lt;Base&gt;&gt;::iterator_category</code>.
</p>
<ol style="list-style-type: none">
<li><p><ins>(2.?) &mdash; If <code>is_lvalue_reference_v&lt;invoke_result_t&lt;F&amp;, range_reference_t&lt;Base&gt;&gt;&gt;</code> 
is <code>true</code>,</ins></p>
<ol style="list-style-type: none">
<li><p><ins>(2.?.?) &mdash;</ins> If <code>C</code> models <code>derived_from&lt;contiguous_iterator_tag&gt;</code>, 
then <code>iterator_category</code> denotes <code>random_access_iterator_tag</code>;</p></li>
<li><p><ins>(2.?.?) &mdash;</ins> <ins>O</ins><del>o</del>therwise, <code>iterator_category</code> denotes 
<code>C</code>.</p></li>
</ol>
</li>
<li><p><ins>(2.?) &mdash; Otherwise, <code>iterator_category</code> denotes 
<code>input_iterator_tag</code>.</ins></p></li>
</ol>
</blockquote>
</li>

</ol>





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