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<title>Issue 3731: zip_view and adjacent_view are underconstrained</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="3731"><a href="lwg-active.html#3731">3731</a>. <code>zip_view</code> and <code>adjacent_view</code> are underconstrained</h3>
<p><b>Section:</b> 25.7.25.2 <a href="https://wg21.link/range.zip.view">[range.zip.view]</a>, 25.7.27.2 <a href="https://wg21.link/range.adjacent.view">[range.adjacent.view]</a>, 25.7.33.2 <a href="https://wg21.link/range.cartesian.view">[range.cartesian.view]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-07-04 <b>Last modified:</b> 2023-08-12</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.zip.view">issues</a> in [range.zip.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>
Both <code>zip_view::<i>iterator</i></code>'s (25.7.25.3 <a href="https://wg21.link/range.zip.iterator">[range.zip.iterator]</a>) and 
<code>adjacent_view::<i>iterator</i></code>'s (25.7.27.3 <a href="https://wg21.link/range.adjacent.iterator">[range.adjacent.iterator]</a>) 
<code>operator*</code> have similar <i>Effects:</i> elements:
</p>
<blockquote><pre>
return <i>tuple-transform</i>([](auto&amp; i) -&gt; decltype(auto) { return *i; }, <i>current_</i>);
</pre></blockquote>
<p>
where <code><i>tuple-transform</i></code> is defined as:
</p>
<blockquote><pre>
template&lt;class F, class Tuple&gt;
constexpr auto <i>tuple-transform</i>(F&amp;&amp; f, Tuple&amp;&amp; tuple) { <i>// exposition only</i>
  return apply([&amp;]&lt;class... Ts&gt;(Ts&amp;&amp;... elements) {
    return <i>tuple-or-pair</i>&lt;invoke_result_t&lt;F&amp;, Ts&gt;...&gt;(
      invoke(f, std::forward&lt;Ts&gt;(elements))...
    );
  }, std::forward&lt;Tuple&gt;(tuple));
}
</pre></blockquote>
<p>
That is, <code>zip_view::<i>iterator</i></code> will invoke the <code>operator*</code> of each iterator of 
<code>Views</code> and return a <code>tuple</code> containing its <code>reference</code>.
<p/>
This is not a problem when the <code>reference</code> of iterators is actually the reference type. However, 
when the <code>operator*</code> returns a prvalue of non-movable type, <code><i>tuple-transform</i></code> will 
be ill-formed since there are no suitable constructors for <code>tuple</code>:
</p>
<blockquote><pre>
#include &lt;ranges&gt;

struct NonMovable {
  NonMovable() = default;
  NonMovable(NonMovable&amp;&amp;) = delete;
};

auto r = std::views::iota(0, 5)
       | std::views::transform([](int) { return NonMovable{}; });
auto z = std::views::zip(r);
auto f = *z.begin(); // <span style="color:red;font-weight:bolder">hard error</span>

</pre></blockquote>
<p>
We should constrain the <code>range_reference_t</code> of the underlying range to be <code>move_constructible</code> 
when it is not a reference type, which also solves similar issues in <code>zip_view::<i>iterator</i></code> and 
<code>adjacent_view::<i>iterator</i></code>'s <code>operator[]</code> and <code>iter_move</code>.
</p>

<p><i>[2022-08-23; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
"The constraint should just be <code>move_constructible</code>."
</p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a>, header <code>&lt;ranges&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  <i>// 25.7.25 <a href="https://wg21.link/range.zip">[range.zip]</a>, zip view</i>
  <ins>template&lt;class Ref&gt;
    concept <i>tuple-constructible-reference</i> = <i>see below</i>; <i>// exposition only</i></ins>

  template&lt;input_range... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) <ins>&amp;&amp;
             (<i>tuple-constructible-reference</i>&lt;range_reference_t&lt;Views&gt;&gt; &amp;&amp; ...)</ins>
  class zip_view;

  [&hellip;]

  <i>// 25.7.27 <a href="https://wg21.link/range.adjacent">[range.adjacent]</a>, adjacent view</i>
  template&lt;forward_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; (N &gt; 0) <ins>&amp;&amp; 
             <i>tuple-constructible-reference</i>&lt;range_reference_t&lt;V&gt;&gt;</ins>
  class adjacent_view;
}
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.25.2 <a href="https://wg21.link/range.zip.view">[range.zip.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  <ins>template&lt;class Ref&gt;
    concept <i>tuple-constructible-reference</i> =            <i>// exposition only</i>
      is_reference_v&lt;Ref&gt; || move_constructible&lt;Ref&gt;;</ins>
  [&hellip;]
  template&lt;input_range... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) <ins>&amp;&amp;
             (<i>tuple-constructible-reference</i>&lt;range_reference_t&lt;Views&gt;&gt; &amp;&amp; ...)</ins>
  class zip_view : public view_interface&lt;zip_view&lt;Views...&gt;&gt; {
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.27.2 <a href="https://wg21.link/range.adjacent.view">[range.adjacent.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;forward_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; (N &gt; 0) <ins>&amp;&amp; 
             <i>tuple-constructible-reference</i>&lt;range_reference_t&lt;V&gt;&gt;</ins>
  class adjacent_view : public view_interface&lt;adjacent_view&lt;V, N&gt;&gt; {
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2022-09-25; Hewill provides improved wording]</i></p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a>, header <code>&lt;ranges&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]

  <ins>template&lt;class R&gt;
    concept <i>has-tuplable-ref</i> =            // <i>exposition only</i>
      move_constructible&lt;range_reference_t&lt;R&gt;&gt;;</ins>

  // <i>25.7.25 <a href="https://wg21.link/range.zip">[range.zip]</a>, zip view</i>
  template&lt;input_range... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) <ins>&amp;&amp;
             (<i>has-tuplable-ref</i>&lt;Views&gt; &amp;&amp; ...)</ins>
  class zip_view;

  [&hellip;]

  // <i>25.7.27 <a href="https://wg21.link/range.adjacent">[range.adjacent]</a>, adjacent view</i>
  template&lt;forward_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; (N &gt; 0) <ins>&amp;&amp; <i>has-tuplable-ref</i>&lt;V&gt;</ins>
  class adjacent_view;

  [&hellip;]

  // <i>25.7.33 <a href="https://wg21.link/range.cartesian">[range.cartesian]</a>, cartesian product view</i>
  template&lt;input_range First, forward_range... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; view&lt;Vs&gt;) <ins>&amp;&amp;
             (<i>has-tuplable-ref</i>&lt;First&gt; &amp;&amp; ... &amp;&amp; <i>has-tuplable-ref</i>&lt;Vs&gt;)</ins>
  class cartesian_product_view;

  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.25.2 <a href="https://wg21.link/range.zip.view">[range.zip.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  
  template&lt;input_range... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) <ins>&amp;&amp;
             (<i>has-tuplable-ref</i>&lt;Views&gt; &amp;&amp; ...)</ins>
  class zip_view : public view_interface&lt;zip_view&lt;Views...&gt;&gt; {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.25.3 <a href="https://wg21.link/range.zip.iterator">[range.zip.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;input_range... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) <ins>&amp;&amp;
             (<i>has-tuplable-ref</i>&lt;Views&gt; &amp;&amp; ...)</ins>
  template&lt;bool Const&gt;
  class zip_view&lt;Views...&gt;::<i>iterator</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.25.4 <a href="https://wg21.link/range.zip.sentinel">[range.zip.sentinel]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0) <ins>&amp;&amp;
             (<i>has-tuplable-ref</i>&lt;Views&gt; &amp;&amp; ...)</ins>
  template&lt;bool Const&gt;
  class zip_view&lt;Views...&gt;::<i>sentinel</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.27.2 <a href="https://wg21.link/range.adjacent.view">[range.adjacent.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;forward_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; (N &gt; 0) <ins>&amp;&amp; <i>has-tuplable-ref</i>&lt;V&gt;</ins>
  class adjacent_view : public view_interface&lt;adjacent_view&lt;V, N&gt;&gt; {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.27.3 <a href="https://wg21.link/range.adjacent.iterator">[range.adjacent.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;forward_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; (N &gt; 0) <ins>&amp;&amp; <i>has-tuplable-ref</i>&lt;V&gt;</ins>
  template&lt;bool Const&gt;
  class adjacent_view&lt;V, N&gt;::<i>iterator</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.27.4 <a href="https://wg21.link/range.adjacent.sentinel">[range.adjacent.sentinel]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;forward_range V, size_t N&gt;
    requires view&lt;V&gt; &amp;&amp; (N &gt; 0) <ins>&amp;&amp; <i>has-tuplable-ref</i>&lt;V&gt;</ins>
  template&lt;bool Const&gt;
  class adjacent_view&lt;V, N&gt;::<i>sentinel</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.33.2 <a href="https://wg21.link/range.cartesian.view">[range.cartesian.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;input_range First, forward_range... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; view&lt;Vs&gt;) <ins>&amp;&amp;
             (<i>has-tuplable-ref</i>&lt;First&gt; &amp;&amp; ... &amp;&amp; <i>has-tuplable-ref</i>&lt;Vs&gt;)</ins>
  class cartesian_product_view : public view_interface&lt;cartesian_product_view&lt;First, Vs...&gt;&gt; {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.33.3 <a href="https://wg21.link/range.cartesian.iterator">[range.cartesian.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;input_range First, forward_range... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; view&lt;Vs&gt;) <ins>&amp;&amp;
             (<i>has-tuplable-ref</i>&lt;First&gt; &amp;&amp; ... &amp;&amp; <i>has-tuplable-ref</i>&lt;Vs&gt;)</ins>
  template&lt;bool Const&gt;
  class cartesian_product_view&lt;First, Vs...&gt;::<i>iterator</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2023-08-08; Hewill provides improved wording]</i></p>




<p id="res-3731"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</a>.
</p>

<ol>
<li><p>Modify 26.2 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a>, header <code>&lt;ranges&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]

  <ins>// <i>25.5.2 <a href="https://wg21.link/range.utility.helpers">[range.utility.helpers]</a>, helper concepts</i>
  template&lt;class R&gt;
    concept <i>range-with-movable-references</i> = <i>see below</i>; // <i>exposition only</i></ins>

  // <i>25.5.3 <a href="https://wg21.link/view.interface">[view.interface]</a>, class template view_interface</i>
  template&lt;class D&gt;
    requires is_class_v&lt;D&gt; &amp;&amp; same_as&lt;D, remove_cv_t&lt;D&gt;&gt;
  class view_interface;                                                             // <i>freestanding</i>

  [&hellip;]

  // <i>25.7.24 <a href="https://wg21.link/range.enumerate">[range.enumerate]</a>, enumerate view</i>
  template&lt;<del>input_range</del><ins>view</ins> V&gt;
    requires <ins><i>range-with-movable-references</i>&lt;V&gt;</ins><del>view&lt;View&gt;</del>
  class enumerate_view;                                                             // <i>freestanding</i>

  [&hellip;]

  // <i>25.7.25 <a href="https://wg21.link/range.zip">[range.zip]</a>, zip view</i>
  template&lt;<ins><i>range-with-movable-references</i></ins><del>input_range</del>... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0)
  class zip_view;                                                                   // <i>freestanding</i>

  [&hellip;]

  // <i>25.7.27 <a href="https://wg21.link/range.adjacent">[range.adjacent]</a>, adjacent view</i>
  template&lt;<ins><i>range-with-movable-references</i></ins><del>forward_range</del> V, size_t N&gt;
    requires <ins>forward_range&lt;V&gt; &amp;&amp;</ins> view&lt;V&gt; &amp;&amp; (N &gt; 0)
  class adjacent_view;                                                              // <i>freestanding</i>

  [&hellip;]

  // <i>25.7.33 <a href="https://wg21.link/range.cartesian">[range.cartesian]</a>, cartesian product view</i>
  template&lt;<ins><i>range-with-movable-references</i></ins><del>input_range</del> First, 
           <ins><i>range-with-movable-references</i></ins><del>forward_range</del>... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; <ins>(forward_range&lt;Vs&gt; &amp;&amp;</ins> view&lt;Vs&gt;<ins>)</ins>)
  class cartesian_product_view;                                                     // <i>freestanding</i>

  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.25.2 <a href="https://wg21.link/range.zip.view">[range.zip.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  
  template&lt;<ins><i>range-with-movable-references</i></ins><del>input_range</del>... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0)
  class zip_view : public view_interface&lt;zip_view&lt;Views...&gt;&gt; {
    [&hellip;]
  public:
    [&hellip;]
    constexpr auto begin() const requires (<ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const Views&gt; &amp;&amp; ...) {
      return <i>iterator</i>&lt;true&gt;(<i>tuple-transform</i>(ranges::begin, <i>views_</i>));
    }
    [&hellip;]
    constexpr auto end() const requires (<ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const Views&gt; &amp;&amp; ...) {
      if constexpr (!<i>zip-is-common</i>&lt;const Views...&gt;) {
        return <i>sentinel</i>&lt;true&gt;(<i>tuple-transform</i>(ranges::end, <i>views_</i>));
      } else if constexpr ((random_access_range&lt;const Views&gt; &amp;&amp; ...)) {
        return begin() + iter_difference_t&lt;<i>iterator</i>&lt;true&gt;&gt;(size());
      } else {
        return <i>iterator</i>&lt;true&gt;(<i>tuple-transform</i>(ranges::end, <i>views_</i>));
      }
    }
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.25.3 <a href="https://wg21.link/range.zip.iterator">[range.zip.iterator]</a> as indicated:</p>
<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;<ins><i>range-with-movable-references</i></ins><del>input_range</del>... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0)
  template&lt;bool Const&gt;
  class zip_view&lt;Views...&gt;::<i>iterator</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.25.4 <a href="https://wg21.link/range.zip.sentinel">[range.zip.sentinel]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;<ins><i>range-with-movable-references</i></ins><del>input_range</del>... Views&gt;
    requires (view&lt;Views&gt; &amp;&amp; ...) &amp;&amp; (sizeof...(Views) &gt; 0)
  template&lt;bool Const&gt;
  class zip_view&lt;Views...&gt;::<i>sentinel</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.27.2 <a href="https://wg21.link/range.adjacent.view">[range.adjacent.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;<ins><i>range-with-movable-references</i></ins><del>forward_range</del> V, size_t N&gt;
    requires <ins>forward_range&lt;V&gt; &amp;&amp;</ins> view&lt;V&gt; &amp;&amp; (N &gt; 0)
  class adjacent_view : public view_interface&lt;adjacent_view&lt;V, N&gt;&gt; {
    [&hellip;]
  public:
    [&hellip;]
    constexpr auto begin() const requires <ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const V&gt; {
      return <i>iterator</i>&lt;true&gt;(ranges::begin(<i>base_</i>), ranges::end(<i>base_</i>));
    }
    [&hellip;]
    constexpr auto end() const requires <ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const V&gt; {
      if constexpr (common_range&lt;const V&gt;) {
        return <i>iterator</i>&lt;true&gt;(<i>as-sentinel</i>{}, ranges::begin(<i>base_</i>), ranges::end(<i>base_</i>));
      } else {
        return <i>sentinel</i>&lt;true&gt;(ranges::end(<i>base_</i>));
      }
    }
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.27.3 <a href="https://wg21.link/range.adjacent.iterator">[range.adjacent.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;<ins><i>range-with-movable-references</i></ins><del>forward_range</del> V, size_t N&gt;
    requires <ins>forward_range&lt;V&gt; &amp;&amp;</ins> view&lt;V&gt; &amp;&amp; (N &gt; 0)
  template&lt;bool Const&gt;
  class adjacent_view&lt;V, N&gt;::<i>iterator</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.27.4 <a href="https://wg21.link/range.adjacent.sentinel">[range.adjacent.sentinel]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;<ins><i>range-with-movable-references</i></ins><del>forward_range</del> V, size_t N&gt;
    requires <ins>forward_range&lt;V&gt; &amp;&amp;</ins> view&lt;V&gt; &amp;&amp; (N &gt; 0)
  template&lt;bool Const&gt;
  class adjacent_view&lt;V, N&gt;::<i>sentinel</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 25.7.33.2 <a href="https://wg21.link/range.cartesian.view">[range.cartesian.view]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;<ins><i>range-with-movable-references</i></ins><del>input_range</del> First, 
           <ins><i>range-with-movable-references</i></ins><del>forward_range</del>... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; <ins>(forward_range&lt;Vs&gt; &amp;&amp;</ins> view&lt;Vs&gt;<ins>)</ins>)
  class cartesian_product_view : public view_interface&lt;cartesian_product_view&lt;First, Vs...&gt;&gt; {
    [&hellip;]
  public:
    [&hellip;]
    constexpr <i>iterator</i>&lt;true&gt; begin() const
      requires (<ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const First&gt; &amp;&amp; ... &amp;&amp; 
                <ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const Vs&gt;);
    [&hellip;]
    constexpr <i>iterator</i>&lt;true&gt; end() const
      requires <ins>(<i>range-with-movable-references</i>&lt;const First&gt; &amp;&amp; ... &amp;&amp; 
                <i>range-with-movable-references</i>&lt;const Vs&gt;) &amp;&amp;</ins>
               <i>cartesian-product-is-common</i>&lt;const First, const Vs...&gt;;
    [&hellip;]
  };
}
</pre>
</blockquote>

<blockquote>
[&hellip;]
<pre>
constexpr <i>iterator</i>&lt;true&gt; begin() const
  requires (<ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const First&gt; &amp;&amp; ... &amp;&amp; 
            <ins><i>range-with-movable-references</i></ins><del>range</del>&lt;const Vs&gt;);
</pre>
<p>
-3- <i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
return <i>iterator</i>&lt;true&gt;(*this, <i>tuple-transform</i>(ranges::begin, <i>bases_</i>));
</pre></blockquote>
<pre>
constexpr <i>iterator</i>&lt;false&gt; end()
  requires ((!<i>simple-view</i>&lt;First&gt; || ... || !<i>simple-view</i>&lt;Vs&gt;)
    &amp;&amp; <i>cartesian-product-is-common</i>&lt;First, Vs...&gt;);
constexpr <i>iterator</i>&lt;true&gt; end() const
  requires <ins>(<i>range-with-movable-references</i>&lt;const First&gt; &amp;&amp; ... &amp;&amp; <i>range-with-movable-references</i>&lt;const Vs&gt;)
    &amp;&amp;</ins> <i>cartesian-product-is-common</i>&lt;const First, const Vs...&gt;;
</pre>
<p>
-4- Let:
</p>
<blockquote>
[&hellip;]
</blockquote>
</blockquote>
</li>

<li><p>Modify 25.7.33.3 <a href="https://wg21.link/range.cartesian.iterator">[range.cartesian.iterator]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  [&hellip;]
  template&lt;<ins><i>range-with-movable-references</i></ins><del>input_range</del> First, <ins><i>range-with-movable-references</i></ins><del>forward_range</del>... Vs&gt;
    requires (view&lt;First&gt; &amp;&amp; ... &amp;&amp; <ins>(forward_range&lt;Vs&gt; &amp;&amp;</ins> view&lt;Vs&gt;<ins>)</ins>)
  class cartesian_product_view&lt;First, Vs...&gt;::<i>iterator</i> {
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>
</ol>





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