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<title>Issue 3599: The const overload of lazy_split_view::begin should be constrained by const Pattern</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="3599"><a href="lwg-active.html#3599">3599</a>. The <code>const</code> overload of <code>lazy_split_view::begin</code> should be constrained by <code>const Pattern</code></h3>
<p><b>Section:</b> 25.7.16.2 <a href="https://wg21.link/range.lazy.split.view">[range.lazy.split.view]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2021-09-23 <b>Last modified:</b> 2021-10-14</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.lazy.split.view">active issues</a> in [range.lazy.split.view].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.lazy.split.view">issues</a> in [range.lazy.split.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>
 Consider the following code snippet:
</p>
<blockquote>
<pre>
#include &lt;ranges&gt;

int main() {
  auto p = std::views::iota(0)
         | std::views::take(1)
         | std::views::reverse;
  auto r = std::views::single(42)
         | std::views::lazy_split(p);
  auto f = r.front();
}
</pre>
</blockquote>
<p>
<code>r.front()</code> is ill-formed even if <code>r</code> is a <code>forward_range</code>.
</p>
<p>
This is because the <code>const</code> overload of <code>lazy_split_view::begin</code> is not constrained by the
<code>const Pattern</code>, which makes it still well-formed in such cases. When the <code>const</code> overload 
of <code>view_interface&lt;lazy_split_view&lt;V, Pattern&gt;&gt;::front</code> is instantiated, the 
<code>subrange{parent_-&gt;pattern_}</code> inside <code>lazy_split_view::<i>outer-iterator</i>&lt;true&gt;::operator++()</code> 
will cause a hard error since <code>const Pattern</code> is not a <code>range</code>.
</p>

<p><i>[2021-09-24; Daniel comments]</i></p>

<p>
This issue is related to LWG <a href="lwg-defects.html#3592" title="lazy_split_view needs to check the simpleness of Pattern (Status: C++23)">3592</a><sup><a href="https://cplusplus.github.io/LWG/issue3592" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2021-10-14; Reflector poll]</i></p>

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



<p id="res-3599"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 25.7.16.2 <a href="https://wg21.link/range.lazy.split.view">[range.lazy.split.view]</a>, class template <code>lazy_split_view</code> synopsis, as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {

  [&hellip;]

  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;)
  class lazy_split_view : public view_interface&lt;lazy_split_view&lt;V, Pattern&gt;&gt; {
  private:
    [&hellip;]
  public:
    [&hellip;]

    constexpr auto begin() {
      [&hellip;]
    }
    
    constexpr auto begin() const requires forward_range&lt;V&gt; &amp;&amp; forward_range&lt;const V&gt; <ins>&amp;&amp;
                                          forward_range&lt;const Pattern&gt;</ins>{
      return <i>outer-iterator</i>&lt;true&gt;{*this, ranges::begin(<i>base_</i>)};
    }
    
    constexpr auto end() requires forward_range&lt;V&gt; &amp;&amp; common_range&lt;V&gt; {
      [&hellip;]
    }

    constexpr auto end() const {
      if constexpr (forward_range&lt;V&gt; &amp;&amp; forward_range&lt;const V&gt; &amp;&amp; common_range&lt;const V&gt; <ins>&amp;&amp;
                    forward_range&lt;const Pattern&gt;</ins>)
        return <i>outer-iterator</i>&lt;true&gt;{*this, ranges::end(<i>base_</i>)};
      else
        return default_sentinel;
    }
  };
  
  [&hellip;]
  
}
</pre>
</blockquote>
</blockquote>
</li>

</ol>





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