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<title>Issue 3766: view_interface::cbegin is underconstrained</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++23">C++23</a> status.</em></p>
<h3 id="3766"><a href="lwg-defects.html#3766">3766</a>. <code>view_interface::cbegin</code> is underconstrained</h3>
<p><b>Section:</b> 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-09-04 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#view.interface.general">issues</a> in [view.interface.general].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, <code>view_interface::cbegin</code> simply returns <code>ranges::cbegin(<i>derived</i>())</code>, 
which returns the type alias <code>const_iterator</code> for its iterator, which requires that the template 
parameter <code>I</code> must model the <code>input_iterator</code>.
<p/>
Given that <code>view_interface::cbegin</code> does not have any constraints, when <code>D</code> models only 
<code>output_range</code>, calling its <code>cbegin()</code> will result in a hard error inside the function body:
</p>
<blockquote><pre>
#include &lt;ranges&gt;
#include &lt;vector&gt;

int main() {
  std::vector&lt;int&gt; v;
  auto r = std::views::counted(std::back_inserter(v), 3);
  auto b = r.cbegin(); // <span style="color:red;font-weight:bolder">hard error</span>
}
</pre></blockquote>
<p>
We should add a constraint for <code>view_interface::cbegin</code> that <code>D</code> must model <code>input_range</code>.
</p>

<p><i>[2022-09-23; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>
<p>
This should be done for <code>cend</code> too.
</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Previous resolution [SUPERSEDED]: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 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;class D&gt;
    requires is_class_v&lt;D&gt; &amp;&amp; same_as&lt;D, remove_cv_t&lt;D&gt;&gt;
  class view_interface {
    [&hellip;]
  public:
    [&hellip;]
    constexpr auto cbegin() <ins>requires input_range&lt;D&gt;</ins> {
      return ranges::cbegin(<i>derived</i>());
    }
    constexpr auto cbegin() const requires <ins>input_</ins>range&lt;const D&gt; {
      return ranges::cbegin(<i>derived</i>());
    }
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2022-09-25; Hewill provides improved wording]</i></p>


<p><i>[Kona 2022-11-08; Accepted at joint LWG/SG9 session. Move to Immediate]</i></p>


<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3766"><b>Proposed resolution:</b></p>
<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 25.5.3.1 <a href="https://wg21.link/view.interface.general">[view.interface.general]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;class D&gt;
    requires is_class_v&lt;D&gt; &amp;&amp; same_as&lt;D, remove_cv_t&lt;D&gt;&gt;
  class view_interface {
    [&hellip;]
  public:
    [&hellip;]
    constexpr auto cbegin() <ins>requires input_range&lt;D&gt;</ins> {
      return ranges::cbegin(<i>derived</i>());
    }
    constexpr auto cbegin() const requires <ins>input_</ins>range&lt;const D&gt; {
      return ranges::cbegin(<i>derived</i>());
    }
    constexpr auto cend() <ins>requires input_range&lt;D&gt;</ins> {
      return ranges::cend(<i>derived</i>());
    }
    constexpr auto cend() const requires <ins>input_</ins>range&lt;const D&gt; {
      return ranges::cend(<i>derived</i>());
    }
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

</ol>





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