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<title>Issue 3280: View converting constructors can cause constraint recursion and are unneeded</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++20">C++20</a> status.</em></p>
<h3 id="3280"><a href="lwg-defects.html#3280">3280</a>. View converting constructors can cause constraint recursion and are unneeded</h3>
<p><b>Section:</b> 25.7.8.2 <a href="https://wg21.link/range.filter.view">[range.filter.view]</a>, 25.7.9.2 <a href="https://wg21.link/range.transform.view">[range.transform.view]</a>, 25.7.10.2 <a href="https://wg21.link/range.take.view">[range.take.view]</a>, 25.7.14.2 <a href="https://wg21.link/range.join.view">[range.join.view]</a>, 25.7.17.2 <a href="https://wg21.link/range.split.view">[range.split.view]</a>, 25.7.21.2 <a href="https://wg21.link/range.reverse.view">[range.reverse.view]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2019-09-09 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>1
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following program fails to compile:
</p>
<blockquote><pre>
#include &lt;ranges&gt;

int main() {
  namespace ranges = std::ranges;
  int a[] = {1, 7, 3, 6, 5, 2, 4, 8};
  auto r0 = ranges::view::reverse(a);
  auto is_even = [](int i) { return i % 2 == 0; };
  auto r1 = ranges::view::filter(r0, is_even);
  int sum = 0;
  for (auto i : r1) {
    sum += i;
  }
  return sum - 20;
}
</pre></blockquote>
<p>
The problem comes from constraint recursion, caused by the following constructor:
</p>
<blockquote><pre>
template&lt;viewable_range R&gt;
  requires bidirectional_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
constexpr explicit reverse_view(R&amp;&amp; r);
</pre></blockquote>
<p>
This constructor owes its existence to class template argument deduction; it is the constructor 
we intend to use to resolve <code>reverse_view{r}</code>, which (in accordance to the deduction guide) 
will construct an object of type <code>reverse_view&lt;all_view&lt;decltype(r)&gt;&gt;</code>.
<p/>
However, we note that <code>all_view&lt;R&gt;</code> is always one of:
</p>
<ul>
<li><p><code>decay_t&lt;R&gt;</code></p></li>
<li><p><code>ref_view&lt;remove_reference_t&lt;R&gt;&gt;</code></p></li>
<li><p><code>subrange&lt;iterator_t&lt;R&gt;, sentinel_t&lt;R&gt;, [sized?]&gt;</code></p></li>
</ul>
<p>
In all cases, there is a conversion from <code>r</code> to the destination type. As a result, the 
following non-template <code>reverse_view</code> constructor can fulfill the duty that the above 
constructor was meant to fulfill, and does <em>not</em> cause constraint recursion:
</p>
<blockquote><pre>
constexpr explicit reverse_view(V r);
</pre></blockquote>
<p>
In short, the problematic constructor can simply be removed with no negative impact on the design. 
And the similar constructors from the other range adaptors should similarly be stricken.
<p/>
Suggested priority P1. The view types are unusable without this change.
<p/>
This proposed resolution has been implemented in range-v3 and has been shipping for some time.
</p>

<p><i>[2019-10 Priority set to 1 after reflector discussion]</i></p>

<p><i>[2019-10 Status set to ready Wednesday night discussion in Belfast.]</i></p>



<p id="res-3280"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 25.7.8.2 <a href="https://wg21.link/range.filter.view">[range.filter.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, indirect_unary_predicate&lt;iterator_t&lt;V&gt;&gt; Pred&gt;
    requires view&lt;V&gt; &amp;&amp; is_object_v&lt;Pred&gt;
  class filter_view : public view_interface&lt;filter_view&lt;V, Pred&gt;&gt; {
  [&hellip;]
  public:
    filter_view() = default;
    constexpr filter_view(V base, Pred pred);
    <del>template&lt;input_range R&gt;
      requires viewable_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
    constexpr filter_view(R&amp;&amp; r, Pred pred);</del>
    [&hellip;]
  };
  [&hellip;]
}
</pre>
[&hellip;]
<pre>
<del>template&lt;input_range R&gt;
  requires viewable_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
constexpr filter_view(R&amp;&amp; r, Pred pred);</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects:</i> Initializes <code>base_</code> with <code>views::all(std::forward&lt;R&gt;(r))</code> 
and initializes <code>pred_</code> with <code>std::move(pred)</code>.</del>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.9.2 <a href="https://wg21.link/range.transform.view">[range.transform.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range V, copy_constructible F&gt;
    requires view&lt;V&gt; &amp;&amp; is_object_v&lt;F&gt; &amp;&amp;
    regular_invocable&lt;F&amp;, range_reference_t&lt;V&gt;&gt;
  class transform_view : public view_interface&lt;transform_view&lt;V, F&gt;&gt; {
  private:
    [&hellip;]
  public:
    transform_view() = default;
    constexpr transform_view(V base, F fun);
    <del>template&lt;input_range R&gt;
      requires viewable_range&lt;R> &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
    constexpr transform_view(R&amp;&amp; r, F fun);</del>
    [&hellip;]
  };
  [&hellip;]
}
</pre>
[&hellip;]
<pre>
<del>template&lt;input_range R&gt;
  requires viewable_range&lt;R> &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
constexpr transform_view(R&amp;&amp; r, F fun);</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects:</i> Initializes <code>base_</code> with <code>views::all(std::forward&lt;R&gt;(r))</code> 
and <code>fun_</code> with <code>std::move(fun)</code>.</del>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.10.2 <a href="https://wg21.link/range.take.view">[range.take.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;view V&gt;
  class take_view : public view_interface&lt;take_view&lt;V&gt;&gt; {
  private:
    [&hellip;]
  public:
    take_view() = default;
    constexpr take_view(V base, range_difference_t&lt;V&gt; count);
    <del>template&lt;viewable_range R&gt;
      requires constructible_from&lt;V, all_view&lt;R&gt;&gt;
    constexpr take_view(R&amp;&amp; r, range_difference_t&lt;V&gt; count);</del>
    [&hellip;]
  };
[&hellip;]
}
</pre>
[&hellip;]
<pre>
<del>template&lt;viewable_range R&gt;
  requires constructible_from&lt;V, all_view&lt;R&gt;&gt;
constexpr take_view(R&amp;&amp; r, range_difference_t&lt;V&gt; count);</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects:</i> Initializes <code>base_</code> with <code>views::all(std::forward&lt;R&gt;(r))</code> 
and <code>count_</code> with <code>count</code>.</del>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.14.2 <a href="https://wg21.link/range.join.view">[range.join.view]</a> as indicated:</p>

<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;)
  class join_view : public view_interface&lt;join_view&lt;V&gt;&gt; {
  private:
    [&hellip;]
  public:
    join_view() = default;
    constexpr explicit join_view(V base);
    <del>template&lt;input_range R&gt;
      requires viewable_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
    constexpr explicit join_view(R&amp;&amp; r);</del>
    [&hellip;]
  };
[&hellip;]
}
</pre>
[&hellip;]
<pre>
<del>template&lt;input_range R&gt;
  requires viewable_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
constexpr explicit join_view(R&amp;&amp; r);</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects:</i> Initializes <code>base_</code> with <code>views::all(std::forward&lt;R&gt;(r))</code>.</del>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.17.2 <a href="https://wg21.link/range.split.view">[range.split.view]</a> as indicated:</p>

<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 split_view : public view_interface&lt;split_view&lt;V, Pattern&gt;&gt; {
  private:
    [&hellip;]
  public:
    split_view() = default;
    constexpr split_view(V base, Pattern pattern);
    <del>template&lt;input_range R, forward_range P&gt;
      requires constructible_from&lt;V, all_view&lt;R&gt;&gt; &amp;&amp;
               constructible_from&lt;Pattern, all_view&lt;P&gt;&gt;
    constexpr split_view(R&amp;&amp; r, P&amp;&amp; p);</del>
    [&hellip;]
  };
[&hellip;]
}
</pre>
[&hellip;]
<pre>
<del>template&lt;input_range R, forward_range P&gt;
  requires constructible_from&lt;V, all_view&lt;R&gt;&gt; &amp;&amp;
           constructible_from&lt;Pattern, all_view&lt;P&gt;&gt;
constexpr split_view(R&amp;&amp; r, P&amp;&amp; p);</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects:</i> Initializes <code>base_</code> with <code>views::all(std::forward&lt;R&gt;(r))</code>, 
and <code>pattern_</code> with <code>views::all(std::forward&lt;P&gt;(p))</code>.</del>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.21.2 <a href="https://wg21.link/range.reverse.view">[range.reverse.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;view V&gt;
    requires bidirectional_range&lt;V&gt;
  class reverse_view : public view_interface&lt;reverse_view&lt;V&gt;&gt; {
  private:
    [&hellip;]
  public:
    reverse_view() = default;
    constexpr explicit reverse_view(V r);
    <del>template&lt;viewable_range R&gt;
      requires bidirectional_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
    constexpr explicit reverse_view(R&amp;&amp; r);</del>
    [&hellip;]
  };
[&hellip;]
}
</pre>
[&hellip;]
<pre>
<del>template&lt;viewable_range R&gt;
  requires bidirectional_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;
constexpr explicit reverse_view(R&amp;&amp; r);</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects:</i> Initializes <code>base_</code> with <code>views::all(std::forward&lt;R&gt;(r))</code>.</del>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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