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<title>Issue 3569: join_view fails to support ranges of ranges with non-default_initializable iterators</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="3569"><a href="lwg-defects.html#3569">3569</a>. <code>join_view</code> fails to support ranges of ranges with non-<code>default_initializable</code> iterators</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> 2021-06-16 <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.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>join_view::<i>iterator</i></code> has exposition-only members <code><i>outer_</i></code> &mdash; which holds an 
iterator into the adapted range &mdash; and <code><i>inner_</i></code> &mdash; which holds an iterator into the 
range denoted by <code><i>outer_</i></code>. After application of <a href="https://wg21.link/P2325R3" title=" Views should not be required to be default constructible">P2325R3</a> "Views should not be 
required to be default constructible" to the working draft, single-pass iterators can be 
<code>non-default_initializable</code>. P2325R3 constrains <code>join_view::<i>iterator</i></code>'s default constructor 
to require that the types of both <code><i>outer_</i></code> and <code><i>inner_</i></code> are <code>default_initializable</code>, 
indicating an intent to support such iterator types. However, the effect of the non-default constructor specified 
in 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a> paragraph 6 is to default-initialize <code><i>inner_</i></code>, which is 
ill-formed if its type is not <code>default_initializable</code>.
</p>

<p><i>[2021-06-23; Reflector poll]</i></p>

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

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
Wording relative to the <a href="https://github.com/cplusplus/draft/releases/download/n4892/n4892.pdf">post 2021-06 
virtual plenary working draft</a>. This PR is currently being implemented in MSVC.
</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>
<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;]
    <ins>optional&lt;</ins><i>InnerIter</i><ins>&gt;</ins> <i>inner_</i><del> = <i>InnerIter</i>()</del>;
    [&hellip;]
    constexpr decltype(auto) operator*() const { return <ins>*</ins>*<i>inner_</i>; }
    [&hellip;]
    friend constexpr decltype(auto) iter_move(const <i>iterator</i>&amp; i)
    noexcept(noexcept(ranges::iter_move(<ins>*</ins>i.<i>inner_</i>))) {
      return ranges::iter_move(<ins>*</ins>i.<i>inner_</i>);
    }
    
    friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
      noexcept(noexcept(ranges::iter_swap(<ins>*</ins>x.<i>inner_</i>, <ins>*</ins>y.<i>inner_</i>)))
      requires indirectly_swappable&lt;<i>InnerIter</i>&gt;;
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr void <i>satisfy();</i>       <i>// exposition only</i>
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
auto update_inner = [this](const iterator_t&lt;<i>Base</i>&gt;&amp; x) -> auto&amp;&amp; {
[&hellip;] 
};
for (; <i>outer_</i> != ranges::end(<i>parent_</i>-&gt;<i>base_</i>); ++<i>outer_</i>) {
  auto&amp;&amp; inner = update_inner(*<i>outer_</i>);
  <i>inner_</i> = ranges::begin(inner);
  if (<ins>*</ins><i>inner_</i> != ranges::end(inner))
    return;
}
if constexpr (<i>ref-is-glvalue</i>)
  <i>inner_</i><ins>.reset()</ins><del> = <i>InnerIter</i>()</del>;
</pre>
</blockquote>
</blockquote>
[&hellip;]
<pre>
constexpr <i>InnerIter</i> operator-&gt;() const
  requires <i>has-arrow</i>&lt;<i>InnerIter</i>&gt; &amp;&amp; copyable&lt;<i>InnerIter</i>&gt;;
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to: <code>return <ins>*</ins><i>inner_</i>;</code>
</p>
</blockquote>
<pre>
constexpr <i>iterator</i>&amp; operator++();
</pre>
<blockquote>
<p>
-9- Let <i>inner-range</i> be:
<p/>
[&hellip;]
<p/>
-10- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
auto&amp;&amp; inner_rng = <i>inner-range</i>;
if (++<ins>*</ins><i>inner_</i> == ranges::end(inner_rng)) {
  ++<i>outer_</i>;
  <i>satisfy</i>();
}
return *this;
</pre>
</blockquote>
</blockquote>
[&hellip;]
<pre>
constexpr <i>iterator</i>&amp; operator--()
  requires <i>ref-is-glvalue</i> &amp;&amp; bidirectional_range&lt;<i>Base</i>&gt; &amp;&amp;
           bidirectional_range&lt;range_reference_t&lt;Base&gt;&gt; &amp;&amp;
           common_range&lt;range_reference_t&lt;Base&gt;&gt;;
</pre>
<blockquote>
<p>
-13- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
if (<i>outer_</i> == ranges::end(<i>parent_</i>-&gt;<i>base_</i>))
  <i>inner_</i> = ranges::end(*--<i>outer_</i>);
while (<ins>*</ins><i>inner_</i> == ranges::begin(*<i>outer_</i>))
  <ins>*</ins><i>inner_</i> = ranges::end(*--<i>outer_</i>);
--<ins>*</ins><i>inner_</i>;
return *this;
</pre>
</blockquote>
</blockquote>
[&hellip;]
<pre>
friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  noexcept(noexcept(ranges::iter_swap(<ins>*</ins>x.<i>inner_</i>, <ins>*</ins>y.<i>inner_</i>)))
  requires indirectly_swappable&lt;<i>InnerIter</i>&gt;;
</pre>
<blockquote>
<p>
-16- <i>Effects:</i> Equivalent to: return <code>ranges::iter_swap(<ins>*</ins>x.<i>inner_</i>, <ins>*</ins>y.<i>inner_</i>);</code>
</p>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2021-08-23; Louis Dionne comments and provides improved wording]</i></p>

<p>
I believe the currently proposed resolution is missing the removal of the <code>default_initializable&lt;<i>InnerIter</i>&gt;</code> 
constraint on <code>join_view::<i>iterator</i></code>'s default constructor in 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a>. Indeed, 
after the currently-proposed resolution, <code>join_view::<i>iterator</i></code> reads like:
</p>
<blockquote><pre>
template&lt;input_range V&gt;
  requires [&hellip;]
struct join_view&lt;V&gt;::<i>iterator</i> {
private:
  optional&lt;<i>InnerIter</i>&gt; <i>inner_</i>; <i>// exposition only</i>
  [&hellip;]
public:
  <i>iterator</i>() requires default_initializable&lt;<i>OuterIter</i>&gt; &amp;&amp;
                      default_initializable&lt;<i>InnerIter</i>&gt; = default;
    [&hellip;]
};
</pre></blockquote>
<p>
I believe we should drop the <code>default_initializable&lt;<i>InnerIter</i>&gt;</code> constraint from the default constructor 
(that seems like an oversight unless I missed something):
</p>
<blockquote><pre>
template&lt;input_range V&gt;
  requires [&hellip;]
struct join_view&lt;V&gt;::<i>iterator</i> {
private:
  optional&lt;<i>InnerIter</i>&gt; <i>inner_</i>; <i>// exposition only</i>
  [&hellip;]
public:
  <i>iterator</i>() requires default_initializable&lt;<i>OuterIter</i>&gt; = default;
  [&hellip;]
};
</pre></blockquote>

<p><i>[Kona 2022-11-08; Accepted at joint LWG/SG9 session. Move to Immediate]</i></p>


<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3569"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 25.7.14.3 <a href="https://wg21.link/range.join.iterator">[range.join.iterator]</a> as indicated:</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;]
    <ins>optional&lt;</ins><i>InnerIter</i><ins>&gt;</ins> <i>inner_</i><del> = <i>InnerIter</i>()</del>;
    [&hellip;]
    <i>iterator</i>() requires default_initializable&lt;<i>OuterIter</i>&gt; <del>&amp;&amp;
                        default_initializable&lt;<i>InnerIter</i>&gt;</del> = default;
    [&hellip;]
    constexpr decltype(auto) operator*() const { return <ins>*</ins>*<i>inner_</i>; }
    [&hellip;]
    friend constexpr decltype(auto) iter_move(const <i>iterator</i>&amp; i)
    noexcept(noexcept(ranges::iter_move(<ins>*</ins>i.<i>inner_</i>))) {
      return ranges::iter_move(<ins>*</ins>i.<i>inner_</i>);
    }
    
    friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
      noexcept(noexcept(ranges::iter_swap(<ins>*</ins>x.<i>inner_</i>, <ins>*</ins>y.<i>inner_</i>)))
      requires indirectly_swappable&lt;<i>InnerIter</i>&gt;;
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
constexpr void <i>satisfy();</i>       <i>// exposition only</i>
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
auto update_inner = [this](const iterator_t&lt;<i>Base</i>&gt;&amp; x) -> auto&amp;&amp; {
[&hellip;] 
};
for (; <i>outer_</i> != ranges::end(<i>parent_</i>-&gt;<i>base_</i>); ++<i>outer_</i>) {
  auto&amp;&amp; inner = update_inner(*<i>outer_</i>);
  <i>inner_</i> = ranges::begin(inner);
  if (<ins>*</ins><i>inner_</i> != ranges::end(inner))
    return;
}
if constexpr (<i>ref-is-glvalue</i>)
  <i>inner_</i><ins>.reset()</ins><del> = <i>InnerIter</i>()</del>;
</pre>
</blockquote>
</blockquote>
[&hellip;]
<pre>
constexpr <i>InnerIter</i> operator-&gt;() const
  requires <i>has-arrow</i>&lt;<i>InnerIter</i>&gt; &amp;&amp; copyable&lt;<i>InnerIter</i>&gt;;
</pre>
<blockquote>
<p>
-8- <i>Effects:</i> Equivalent to: <code>return <ins>*</ins><i>inner_</i>;</code>
</p>
</blockquote>
<pre>
constexpr <i>iterator</i>&amp; operator++();
</pre>
<blockquote>
<p>
-9- Let <i>inner-range</i> be:
<p/>
[&hellip;]
<p/>
-10- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
auto&amp;&amp; inner_rng = <i>inner-range</i>;
if (++<ins>*</ins><i>inner_</i> == ranges::end(inner_rng)) {
  ++<i>outer_</i>;
  <i>satisfy</i>();
}
return *this;
</pre>
</blockquote>
</blockquote>
[&hellip;]
<pre>
constexpr <i>iterator</i>&amp; operator--()
  requires <i>ref-is-glvalue</i> &amp;&amp; bidirectional_range&lt;<i>Base</i>&gt; &amp;&amp;
           bidirectional_range&lt;range_reference_t&lt;Base&gt;&gt; &amp;&amp;
           common_range&lt;range_reference_t&lt;Base&gt;&gt;;
</pre>
<blockquote>
<p>
-13- <i>Effects:</i> Equivalent to:
</p>
<blockquote>
<pre>
if (<i>outer_</i> == ranges::end(<i>parent_</i>-&gt;<i>base_</i>))
  <i>inner_</i> = ranges::end(*--<i>outer_</i>);
while (<ins>*</ins><i>inner_</i> == ranges::begin(*<i>outer_</i>))
  <ins>*</ins><i>inner_</i> = ranges::end(*--<i>outer_</i>);
--<ins>*</ins><i>inner_</i>;
return *this;
</pre>
</blockquote>
</blockquote>
[&hellip;]
<pre>
friend constexpr void iter_swap(const <i>iterator</i>&amp; x, const <i>iterator</i>&amp; y)
  noexcept(noexcept(ranges::iter_swap(<ins>*</ins>x.<i>inner_</i>, <ins>*</ins>y.<i>inner_</i>)))
  requires indirectly_swappable&lt;<i>InnerIter</i>&gt;;
</pre>
<blockquote>
<p>
-16- <i>Effects:</i> Equivalent to: return <code>ranges::iter_swap(<ins>*</ins>x.<i>inner_</i>, <ins>*</ins>y.<i>inner_</i>);</code>
</p>
</blockquote>
</blockquote>
</li>

</ol>




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