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<title>Issue 3553: Useless constraint in split_view::outer-iterator::value_type::begin()</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="3553"><a href="lwg-defects.html#3553">3553</a>. Useless constraint in <code>split_view::<i>outer-iterator</i>::value_type::begin()</code></h3>
<p><b>Section:</b> 25.7.16.4 <a href="https://wg21.link/range.lazy.split.outer.value">[range.lazy.split.outer.value]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2021-05-18 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.lazy.split.outer.value">issues</a> in [range.lazy.split.outer.value].</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>copyable</code> constraint in <code>split_view::<i>outer-iterator</i>::value_type::begin()</code> 
is useless because <code><i>outer-iterator</i></code> always satisfies <code>copyable</code>.
<p/>
When <code>V</code> does not model <code>forward_range</code>, the <code><i>outer-iterator</i></code> only contains a pointer, so it 
is obviously copyable; When <code>V</code> is a <code>forward_range</code>, the <code>iterator_t&lt;Base&gt;</code> is a 
<code>forward_iterator</code>, so it is copyable, which makes <code><i>outer-iterator</i></code> also copyable.
<p/>
Suggested resolution: Remove the no-<code>const begin()</code> and useless <code>copyable</code> constraint in
 [range.split.outer.value].
</p>

<p><i>[2021-05-26; Reflector poll]</i></p>

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

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3553"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify  [range.split.outer.value], class <code>split_view::<i>outer-iterator</i>::value_type</code> synopsis, 
as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, forward_range Pattern&gt;
    requires view&lt;V&gt; &amp;&amp; view&lt;Pattern&gt; &amp;&amp;
             indirectly_comparable&lt;iterator_t&lt;V&gt;, iterator_t&lt;Pattern&gt;, ranges::equal_to&gt; &amp;&amp;
             (forward_range&lt;V&gt; || <i>tiny-range</i>&lt;Pattern&gt;)
  template&lt;bool Const&gt;
  struct split_view&lt;V, Pattern&gt;::<i>outer-iterator</i>&lt;Const&gt;::value_type
    : view_interface&lt;value_type&gt; {
  private:
    <i>outer-iterator i_</i> = <i>outer-iterator</i>(); <i>// exposition only</i>
  public:
    value_type() = default;
    constexpr explicit value_type(<i>outer-iterator</i> i);
    constexpr <i>inner-iterator</i>&lt;Const&gt; begin() const <del>requires copyable&lt;<i>outer-iterator</i>&gt;</del>;
    <del>constexpr <i>inner-iterator</i>&lt;Const&gt; begin() requires (!copyable&lt;<i>outer-iterator</i>&gt;);</del>
    constexpr default_sentinel_t end() const;
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr <i>inner-iterator</i>&lt;Const&gt; begin() const <del>requires copyable&lt;<i>outer-iterator</i>&gt;</del>;
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> Equivalent to: <code>return <i>inner-iterator</i>&lt;Const&gt;{<i>i_</i>};</code>
</p>
</blockquote>
<pre>
<del>constexpr <i>inner-iterator</i>&lt;Const&gt; begin() requires (!copyable&lt;<i>outer-iterator</i>&gt;)</del>;
</pre>
<blockquote>
<p>
<del>-3- <i>Effects:</i> Equivalent to: <code>return <i>inner-iterator</i>&lt;Const&gt;{std::move(<i>i_</i>)};</code></del>
</p>
</blockquote>
[&hellip;]
</blockquote>
</li>
</ol>





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