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<title>Issue 3730: std::ranges::drop_view may have different size type from its underlying view</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="3730"><a href="lwg-active.html#3730">3730</a>. <code>std::ranges::drop_view</code> may have different size type from its underlying view</h3>
<p><b>Section:</b> 25.7.12.2 <a href="https://wg21.link/range.drop.view">[range.drop.view]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2022-07-03 <b>Last modified:</b> 2022-07-17</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.drop.view">active issues</a> in [range.drop.view].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.drop.view">issues</a> in [range.drop.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The bodies of both overloads of <code>drop_view&lt;V&gt;::size</code> are specified as:
</p>
<blockquote><pre>
const auto s = ranges::size(<i>base_</i>);
const auto c = static_cast&lt;decltype(s)&gt;(<i>count_</i>);
return s &lt; c ? 0 : s - c;
</pre></blockquote>
<p>
Given the return type is specified with <code>auto</code>, the actual return type 
is the <em>promoted type</em> of the size type of the underlying view, which may be 
different from the underlying size type (e.g. if the underlying size is <code>unsigned short</code>).
<p/>
Note that <code>take_view</code> always has the same size type as its underlying view. 
So I think the difference on the size types is an oversight. On the other hand, the 
<code>const</code> used here seems redundant and inconsistent with other parts of the 
standard wording, although implementations may tend to use it.
</p>

<p><i>[2022-07-08; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
"The PR is incorrect - integer-class types do not support mixed-signedess operations, so you 
have to cast one of the two first."
</p>

<p><i>[2022-07-17; Daniel comments]</i></p>

<p>
This issue should be resolved by keeping LWG <a href="lwg-closed.html#3739" title="chunk_view::size should preserve the signedness of the size of the underlying range (Status: NAD)">3739</a><sup><a href="https://cplusplus.github.io/LWG/issue3739" title="Latest snapshot">(i)</a></sup> and <a href="lwg-closed.html#3740" title="slide_view::size should preserve the signedness of underlying range's size (Status: NAD)">3740</a><sup><a href="https://cplusplus.github.io/LWG/issue3740" title="Latest snapshot">(i)</a></sup> in mind.
</p>


<p id="res-3730"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 25.7.12.2 <a href="https://wg21.link/range.drop.view">[range.drop.view]</a>, class template <code>drop_view</code> synopsis, as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> <code>s</code> and <code><i>count_</i></code> usually have different types, but I think 
it's safe to perform comparison and subtraction, as <code><i>count_</i></code> is non-negative as long as 
the behavior is well-defined.]
</p>
</blockquote>

<blockquote>
<pre>
[&hellip;]
constexpr auto size() requires sized_range&lt;V&gt; {
  <del>const</del> auto s = ranges::size(<i>base_</i>);
  <del>const auto c = static_cast&lt;decltype(s)&gt;(<i>count_</i>);</del>
  return <ins>static_cast&lt;decltype(s)&gt;(</ins>s &lt; <del>c</del><ins><i>count_</i></ins> ? 0 : s - <del>c</del><ins><i>count_</i>)</ins>;
}

constexpr auto size() const requires sized_range&lt;const V&gt; {
  <del>const</del> auto s = ranges::size(<i>base_</i>);
  <del>const auto c = static_cast&lt;decltype(s)&gt;(<i>count_</i>);</del>
  return <ins>static_cast&lt;decltype(s)&gt;(</ins>s &lt; <del>c</del><ins><i>count_</i></ins> ? 0 : s - <del>c</del><ins><i>count_</i>)</ins>;
}
[&hellip;]
</pre>
</blockquote>
</li>

</ol>





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