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<title>Issue 3500: join_view::iterator::operator-&gt;() is bogus</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="3500"><a href="lwg-defects.html#3500">3500</a>. <code>join_view::iterator::operator-&gt;()</code> is bogus</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> Michael Schellenberger Costa <b>Opened:</b> 2020-11-15 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</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>
There seems to be a copy &amp; paste error in the <code>join_view::iterator::operator-&gt;()</code> 
specification. According to 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a> p8 it is specified as:
</p>
<blockquote>
<pre>
constexpr iterator_t&lt;<i>Base</i>&gt; operator-&gt;() const
  requires <i>has-arrow</i>&lt;iterator_t&lt;Base&gt;&gt; &amp;&amp; copyable&lt;iterator_t&lt;Base&gt;&gt;;
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to <code>return <i>inner_</i>;</code>
</p>
</blockquote>
</blockquote>
<p>
Now <code><i>inner_</i></code> is of type <code>iterator_t&lt;range_reference_t&lt;Base&gt;&gt;</code>. So it 
is unclear how that should be converted to <code>iterator_t&lt;<i>Base</i>&gt;</code>, or why the 
constraints concern the outer iterator type <code>iterator_t&lt;<i>Base</i>&gt;</code>. On the other hand 
returning <code><i>outer_</i></code> would not make any sense here.
<p/>
As far as I can tell we should replace <code>iterator_t&lt;<i>Base</i>&gt;</code> with 
<code>iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;</code> so that the new specification would 
read
</p>
<blockquote>
<pre>
constexpr iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt; operator-&gt;() const
 requires <i>has-arrow</i>&lt;iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;&gt; 
   &amp;&amp; copyable&lt;iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;&gt;;
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to <code>return <i>inner_</i>;</code>
</p>
</blockquote>
</blockquote>
<p>
Generally it would help readability of the specification a lot if we would introduce some exposition only aliases:
</p>
<blockquote>
<pre>
using OuterIter = iterator_t&lt;<i>Base</i>&gt;; <i>//exposition-only</i>
using InnerIter = iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt; <i>//exposition-only</i>
</pre>
</blockquote>
<p>
and use them throughout <code>join_view::<i>iterator</i></code>.
</p>

<p><i>[2020-11-21; Reflector prioritization]</i></p>

<p>
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
</p>
<p><i>[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3500"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4868">N4868</a>. 
</p>

<ol>
<li>
<p>
Modify 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a>, class template <code>join_view::<i>iterator</i></code> synopsis, 
as indicated:
</p>

<blockquote>
<blockquote>
<pre>
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;)
struct join_view&lt;V&gt;::<i>iterator</i> {
private:
  using <i>Parent</i> = <i>// exposition only</i>
    conditional_t&lt;Const, const join_view, join_view&gt;;
  using Base = conditional_t&lt;Const, const V, V&gt;; <i>// exposition only</i>
  <ins>using <i>OuterIter</i> = iterator_t&lt;<i>Base</i>&gt;; <i>//exposition-only</i>
  using <i>InnerIter</i> = iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt; <i>//exposition-only</i></ins>
  static constexpr bool <i>ref-is-glvalue</i> = <i>// exposition only</i>
    is_reference_v&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;;
  <ins><i>OuterIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del> <i>outer_</i> = <ins><i>OuterIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del>(); <i>// exposition only</i>
  <ins><i>InnerIter</i></ins><del>iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;</del> <i>inner_</i> = <i>// exposition only</i>
    <ins><i>InnerIter</i></ins><del>iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;</del>();
  <i>Parent</i>* <i>parent_</i> = nullptr; <i>// exposition only</i>
  constexpr void <i>satisfy</i>(); <i>// exposition only</i>
public:
  [&hellip;]
  <i>iterator</i>() = default;
  constexpr <i>iterator</i>(<i>Parent</i>&amp; parent, <ins><i>OuterIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del> outer);
  constexpr <i>iterator</i>(<i>iterator</i>&lt;!Const&gt; i)
    requires Const &amp;&amp;
             convertible_to&lt;iterator_t&lt;V&gt;, <ins><i>OuterIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del>&gt; &amp;&amp;
             convertible_to&lt;iterator_t&lt;<i>InnerRng</i>&gt;,
                            <ins><i>InnerIter</i></ins><del>iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;</del>&gt;;

  constexpr decltype(auto) operator*() const { return *<i>inner_</i>; }
  
  constexpr <ins><i>InnerIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del> operator-&gt;() const
    requires <i>has-arrow</i>&lt;<ins><i>InnerIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del>&gt; 
      &amp;&amp; copyable&lt;<ins><i>InnerIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del>&gt;;
  
  constexpr <i>iterator</i>&amp; operator++();
  [&hellip;]
</pre>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
constexpr void <i>satisfy</i>(); <i>// exposition only</i>
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
[&hellip;]
if constexpr (<i>ref-is-glvalue</i>)
  <i>inner_</i> = <ins><i>InnerIter</i></ins><del>iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;</del>();
</pre></blockquote>
</blockquote>
<pre>
constexpr <i>iterator</i>(<i>Parent</i>&amp; parent, <ins><i>OuterIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del> outer);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
constexpr <i>iterator</i>(<i>iterator</i>&lt;!Const&gt; i)
  requires Const &amp;&amp;
           convertible_to&lt;iterator_t&lt;V&gt;, <ins><i>OuterIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del>&gt; &amp;&amp;
           convertible_to&lt;iterator_t&lt;<i>InnerRng</i>&gt;,
                          <ins><i>InnerIter</i></ins><del>iterator_t&lt;range_reference_t&lt;<i>Base</i>&gt;&gt;</del>&gt;;
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
constexpr <ins><i>InnerIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del> operator-&gt;() const
  requires <i>has-arrow</i>&lt;<ins><i>InnerIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del>&gt; 
    &amp;&amp; copyable&lt;<ins><i>InnerIter</i></ins><del>iterator_t&lt;<i>Base</i>&gt;</del>&gt;;
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to <code>return <i>inner_</i>;</code>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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