<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3517: join_view::iterator's iter_swap is underconstrained</title>
<meta property="og:title" content="Issue 3517: join_view::iterator's iter_swap is underconstrained">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3517.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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="3517"><a href="lwg-defects.html#3517">3517</a>. <code>join_view::<i>iterator</i></code>'s <code>iter_swap</code> is underconstrained</h3>
<p><b>Section:</b> 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2021-01-28 <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.join.iterator">issues</a> in [range.join.iterator].</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>
<code>std::ranges::join_view::<i>iterator</i></code>'s hidden friend <code>iter_swap</code> is specified in 
25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a>/16 as:
</p>
<blockquote>
<pre>
friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  noexcept(noexcept(ranges::iter_swap(x.<i>inner_</i>, y.<i>inner_</i>)));
</pre>
<blockquote>
<p>
-16- <i>Effects:</i> Equivalent to: <code>return ranges::iter_swap(x.<i>inner_</i>, y.<i>inner_</i>);</code>
</p>
</blockquote>
</blockquote>
<p>
Notably, the expression <code>ranges::iter_swap(<i>meow</i>, <i>woof</i>)</code> is not valid for all 
iterators <code><i>meow</i></code> and <code><i>woof</i></code>, or even all input iterators of the same 
type as is the case here. This <code>iter_swap</code> overload should be constrained to require the 
type of <code>iterator::<i>inner_</i></code> (<code>iterator_t&lt;range_reference_t&lt;<i>maybe-const</i>&lt;Const, V&gt;&gt;&gt;)</code> 
to satisfy <code>indirectly_swappable</code>. Notably this is already the case for <code>iter_swap</code> 
friends of every other iterator adaptor in the Standard Library (<code>reverse_iterator</code>, <code>move_iterator</code>, 
<code>common_iterator</code>, <code>counted_iterator</code>, <code>filter_view::iterator</code>, <code>transform_view::iterator</code>, and <code>split_view::<i>inner-iterator</i></code>). The omission for <code>join_view::<i>iterator</i></code> seems to 
have simply been an oversight.
</p>

<p><i>[2021-03-12; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six 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-3517"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4878">N4878</a>. 
</p>

<ol>
<li><p>Modify 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> If <a href="lwg-defects.html#3500" title="join_view::iterator::operator-&gt;() is bogus (Status: C++23)">3500</a><sup><a href="https://cplusplus.github.io/LWG/issue3500" title="Latest snapshot">(i)</a></sup> is accepted before this issue, it is 
kindly suggested to the Project Editor to apply the equivalent replacement of
"<code>iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;</code>" by "<code><i>InnerIter</i></code>" to
the newly inserted <code>requires</code>.
</p>
</blockquote>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V&gt;
    requires view&lt;V&gt; &amp;&amp; input_range&lt;range_reference_t&lt;V&gt;&gt; &amp;&amp;
             (is_reference_v&lt;range_reference_t&lt;V&gt;&gt; ||
             view&lt;range_value_t&lt;V&gt;&gt;)
  template&lt;bool Const&gt;
  struct join_view&lt;V&gt;::<i>iterator</i> {
    [&hellip;]
    
    friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
      noexcept(noexcept(ranges::iter_swap(x.<i>inner_</i>, y.<i>inner_</i>)))
      <ins>requires indirectly_swappable&lt;iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;</ins>;
  };
}
</pre>
</blockquote>
[&hellip;]
<blockquote>
<pre>
friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  noexcept(noexcept(ranges::iter_swap(x.<i>inner_</i>, y.<i>inner_</i>)))
  <ins>requires indirectly_swappable&lt;iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;</ins>;
</pre>
<blockquote>
<p>
-16- <i>Effects:</i> Equivalent to: <code>return ranges::iter_swap(x.<i>inner_</i>, y.<i>inner_</i>);</code>
</p>
</blockquote>
</blockquote>
</li>
</ol>





</body>
</html>
