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<title>Issue 3589: The const lvalue reference overload of get for subrange 
does not constrain I to be copyable when N == 0</title>
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does not constrain I to be copyable when N == 0">
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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="3589"><a href="lwg-defects.html#3589">3589</a>. The <code>const</code> lvalue reference overload of <code>get</code> for <code>subrange</code> 
does not constrain <code>I</code> to be <code>copyable</code> when <code>N == 0</code></h3>
<p><b>Section:</b> 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2021-09-08 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.subrange">issues</a> in [range.subrange].</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>
The <code>const</code> lvalue reference overload of <code>get</code> used for <code>subrange</code> 
only constraint that <code>N &lt; 2</code>, this will cause the "discards qualifiers" error 
inside the function body when applying <code>get</code> to the lvalue <code>subrange</code> that 
stores a non-<code>copyable</code> iterator since its <code>begin()</code> is non-<code>const</code> 
qualified, we probably need to add a constraint for it.
</p>

<p><i>[2021-09-20; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>


<p><i>[2021-09-20; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six votes in favour during reflector poll.
</p>

<p><i>[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3589"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;size_t N, class I, class S, subrange_kind K&gt;
    requires <ins>(</ins>(N <del>&lt; 2</del><ins>== 0 &amp;&amp; copyable&lt;I&gt;) || N == 1</ins>)
    constexpr auto get(const subrange&lt;I, S, K&gt;&amp; r);

  template&lt;size_t N, class I, class S, subrange_kind K&gt;
    requires (N &lt; 2)
    constexpr auto get(subrange&lt;I, S, K&gt;&amp;&amp; r);
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.5.4.3 <a href="https://wg21.link/range.subrange.access">[range.subrange.access]</a> as indicated:</p>

<blockquote>
<pre>
  [&hellip;]
  template&lt;size_t N, class I, class S, subrange_kind K&gt;
    requires <ins>(</ins>(N <del>&lt; 2</del><ins>== 0 &amp;&amp; copyable&lt;I&gt;) || N == 1</ins>)
    constexpr auto get(const subrange&lt;I, S, K&gt;&amp; r);
  template&lt;size_t N, class I, class S, subrange_kind K&gt;
    requires (N &lt; 2)
    constexpr auto get(subrange&lt;I, S, K&gt;&amp;&amp; r);
</pre>
<blockquote>
<p>
-10- <i>Effects</i>: Equivalent to:
</p>
<pre>
if constexpr (N == 0)
  return r.begin();
else
  return r.end();
</pre>
</blockquote>
</blockquote>
</li>

</ol>





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