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<title>Issue 4116: enumerate_view::iterator and cartesian_product_view::iterator should not
    always provide 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#New">New</a> status.</em></p>
<h3 id="4116"><a href="lwg-active.html#4116">4116</a>. <code>enumerate_view::<i>iterator</i></code> and <code>cartesian_product_view::<i>iterator</i></code> should not
    always provide <code>iterator_category</code></h3>
<p><b>Section:</b> 25.7.24.3 <a href="https://wg21.link/range.enumerate.iterator">[range.enumerate.iterator]</a>, 25.7.33.3 <a href="https://wg21.link/range.cartesian.iterator">[range.cartesian.iterator]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-07 <b>Last modified:</b> 2024-07-07</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.enumerate.iterator">active issues</a> in [range.enumerate.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.enumerate.iterator">issues</a> in [range.enumerate.iterator].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
These two iterators do not support <code>*r++</code> for non-forward iterators,
so we should not provide <code>iterator_category</code> as they are not C++17 iterators.
</p>


<p id="res-4116"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>
<li><p>Modify 25.7.24.3 <a href="https://wg21.link/range.enumerate.iterator">[range.enumerate.iterator]</a> as indicated:</p>

<blockquote><pre>
namespace std::ranges {
  template&lt;view V&gt;
    requires <i>range-with-movable-references</i>&lt;V&gt;
  template&lt;bool Const&gt;
  class enumerate_view&lt;V&gt;::<i>iterator</i> {
    using <i>Base</i> = <i>maybe-const</i>&lt;Const, V&gt;;                         // <i>exposition only</i>

  public:
    using iterator_category = input_iterator_tag;      <ins>// present only if <i>Base</i>
                                                       // models forward_range</ins>
    using iterator_concept = <i>see below</i>;
    [&hellip;]
  };
}
</pre></blockquote>

</li>

<li><p>Modify 25.7.33.3 <a href="https://wg21.link/range.cartesian.iterator">[range.cartesian.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range First, forward_range... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; view&lt;Vs&gt;)
  template&lt;bool Const&gt;
  class cartesian_product_view&lt;First, Vs...&gt;::<i>iterator</i> {
  public:
    using iterator_category = input_iterator_tag;      <ins>// present only if <i><i>maybe-const</i>&lt;Const, First&gt;</i>
                                                       // models forward_range</ins>
    using iterator_concept  = see below;
    [&hellip;]
  };
}
</pre>
</blockquote>

</li>
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