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<title>Issue 4272: For rank == 0, layout_stride is atypically convertible</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="4272"><a href="lwg-active.html#4272">4272</a>. For <code class='backtick'>rank == 0</code>, <code class='backtick'>layout_stride</code> is atypically convertible</h3>
<p><b>Section:</b> 23.7.3.4 <a href="https://wg21.link/mdspan.layout">[mdspan.layout]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Luc Grosheintz <b>Opened:</b> 2025-06-02 <b>Last modified:</b> 2025-08-29</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#mdspan.layout">active issues</a> in [mdspan.layout].</p>
<p><b>View all other</b> <a href="lwg-index.html#mdspan.layout">issues</a> in [mdspan.layout].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Commonly, two layouts are considered convertible, if the underlying
<code class='backtick'>extent_types</code> are convertible.
<p/>
However, for the two ctors <code class='backtick'>layout_left::mapping(layout_stride::mapping)</code> and
<code class='backtick'>layout_right::mapping(layout_stride::mapping)</code>, the condition is <code>rank &gt; 0</code>. 
Therefore,
</p>
<blockquote><pre>
using E1 = std::extents&lt;int&gt;;
using E2 = std::extents&lt;unsigned int&gt;;

static_assert(std::is_convertible_v&lt;
    std::layout_stride::mapping&lt;E2&gt;,
    std::layout_right::mapping&lt;E1&gt;
  &gt;);
</pre></blockquote>
<p>
even though:
</p>
<blockquote><pre>
static_assert(!std::is_convertible_v&lt;E2, E1&gt;);
</pre></blockquote>
<p>
Moreover, for rank 0 <code class='backtick'>layout_stride</code> can be converted to any
specialization of <code class='backtick'>layout_left</code> or <code class='backtick'>layout_right</code>; but not to every
specialization of <code class='backtick'>layout_stride</code>.
</p>

<p><i>[2025-06-12; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>

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Previous resolution [SUPERSEDED]: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

<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<blockquote class="note">
<p>
[<i>Drafting note</i>: As drive-by fixes the edits for <code>layout_left_padded&lt;&gt;::mapping</code> and
<code>layout_right_padded&lt;&gt;::mapping</code> also correct an editorial asymmetry between class
header synopsis declaration form and prototype specification form of the corresponding
constructors and adjust to the correct formatting of the exposition-only data member <code><i>rank_</i></code>.]
</p>
</blockquote>

<ol>
<li><p>Modify 23.7.3.4.5.1 <a href="https://wg21.link/mdspan.layout.left.overview">[mdspan.layout.left.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Extents&gt;
  class layout_left::mapping {
    [&hellip;]
    <i>// 23.7.3.4.5.2 <a href="https://wg21.link/mdspan.layout.left.cons">[mdspan.layout.left.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    
    constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.5.2 <a href="https://wg21.link/mdspan.layout.left.cons">[mdspan.layout.left.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
-14- <i>Preconditions</i>: [&hellip;]
<p/>
-15- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.6.1 <a href="https://wg21.link/mdspan.layout.right.overview">[mdspan.layout.right.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Extents&gt;
  class layout_right::mapping {
    [&hellip;]
    <i>// 23.7.3.4.6.2 <a href="https://wg21.link/mdspan.layout.right.cons">[mdspan.layout.right.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    
    constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.6.2 <a href="https://wg21.link/mdspan.layout.right.cons">[mdspan.layout.right.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
-14- <i>Preconditions</i>: [&hellip;]
<p/>
-15- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.8.1 <a href="https://wg21.link/mdspan.layout.leftpad.overview">[mdspan.layout.leftpad.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;size_t PaddingValue&gt;
  template&lt;class Extents&gt;
  class layout_left_padded&lt;PaddingValue&gt;::mapping {
    [&hellip;]
    <i>// 23.7.3.4.8.3 <a href="https://wg21.link/mdspan.layout.leftpad.cons">[mdspan.layout.leftpad.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.8.3 <a href="https://wg21.link/mdspan.layout.leftpad.cons">[mdspan.layout.leftpad.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-10- <i>Constraints</i>: [&hellip;]
<p/>
-11- <i>Preconditions</i>: [&hellip;]
<p/>
-12- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.9.1 <a href="https://wg21.link/mdspan.layout.rightpad.overview">[mdspan.layout.rightpad.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;size_t PaddingValue&gt;
  template&lt;class Extents&gt;
  class layout_right_padded&lt;PaddingValue&gt;::mapping {
    [&hellip;]
    <i>// 23.7.3.4.9.3 <a href="https://wg21.link/mdspan.layout.rightpad.cons">[mdspan.layout.rightpad.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.9.3 <a href="https://wg21.link/mdspan.layout.rightpad.cons">[mdspan.layout.rightpad.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-10- <i>Constraints</i>: [&hellip;]
<p/>
-11- <i>Preconditions</i>: [&hellip;]
<p/>
-12- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2025-06-20, Luc Grosheintz provides further wording improvements]</i></p>


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

<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<blockquote class="note">
<p>
[<i>Drafting note</i>: As drive-by fixes the edits for <code>layout_left_padded&lt;&gt;::mapping</code> and
<code>layout_right_padded&lt;&gt;::mapping</code> also correct an editorial asymmetry between class
header synopsis declaration form and prototype specification form of the corresponding
constructors and adjust to the correct formatting of the exposition-only data member <code><i>rank_</i></code>.]
</p>
</blockquote>

<ol>
<li><p>Modify 23.7.3.4.5.1 <a href="https://wg21.link/mdspan.layout.left.overview">[mdspan.layout.left.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Extents&gt;
  class layout_left::mapping {
    [&hellip;]
    <i>// 23.7.3.4.5.2 <a href="https://wg21.link/mdspan.layout.left.cons">[mdspan.layout.left.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    
    constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.5.2 <a href="https://wg21.link/mdspan.layout.left.cons">[mdspan.layout.left.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
-14- <i>Preconditions</i>: [&hellip;]
<p/>
-15- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.6.1 <a href="https://wg21.link/mdspan.layout.right.overview">[mdspan.layout.right.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Extents&gt;
  class layout_right::mapping {
    [&hellip;]
    <i>// 23.7.3.4.6.2 <a href="https://wg21.link/mdspan.layout.right.cons">[mdspan.layout.right.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    
    constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.6.2 <a href="https://wg21.link/mdspan.layout.right.cons">[mdspan.layout.right.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
-14- <i>Preconditions</i>: [&hellip;]
<p/>
-15- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.8.1 <a href="https://wg21.link/mdspan.layout.leftpad.overview">[mdspan.layout.leftpad.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;size_t PaddingValue&gt;
  template&lt;class Extents&gt;
  class layout_left_padded&lt;PaddingValue&gt;::mapping {
    [&hellip;]
    <i>// 23.7.3.4.8.3 <a href="https://wg21.link/mdspan.layout.leftpad.cons">[mdspan.layout.leftpad.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.8.3 <a href="https://wg21.link/mdspan.layout.leftpad.cons">[mdspan.layout.leftpad.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-10- <i>Constraints</i>: [&hellip;]
<p/>
-11- <i>Preconditions</i>: [&hellip;]
<p/>
-12- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
<pre>
template&lt;class LayoutLeftPaddedMapping&gt;
  constexpr explicit(<i>see below</i>)
    mapping(const LayoutLeftPaddedMapping&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-16- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:
</p>
<blockquote><pre>
<ins>!is_convertible_v&lt;typename LayoutLeftPaddedMapping::extents_type, extents_type&gt; &amp;&amp;</ins>
rank_&gt; 1 &amp;&amp;
(padding_value != dynamic_extent ||
 LayoutLeftPaddedMapping::padding_value == dynamic_extent)
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.9.1 <a href="https://wg21.link/mdspan.layout.rightpad.overview">[mdspan.layout.rightpad.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;size_t PaddingValue&gt;
  template&lt;class Extents&gt;
  class layout_right_padded&lt;PaddingValue&gt;::mapping {
    [&hellip;]
    <i>// 23.7.3.4.9.3 <a href="https://wg21.link/mdspan.layout.rightpad.cons">[mdspan.layout.rightpad.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.9.3 <a href="https://wg21.link/mdspan.layout.rightpad.cons">[mdspan.layout.rightpad.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-10- <i>Constraints</i>: [&hellip;]
<p/>
-11- <i>Preconditions</i>: [&hellip;]
<p/>
-12- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
<pre>
template&lt;class LayoutRightPaddedMapping&gt;
  constexpr explicit(<i>see below</i>)
    mapping(const LayoutRightPaddedMapping&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-17- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:
</p>
<blockquote><pre>
<ins>!is_convertible_v&lt;typename LayoutRightPaddedMapping::extents_type, extents_type&gt; &amp;&amp;</ins>
<i>rank_</i> &gt; 1 &amp;&amp;
(padding_value != dynamic_extent ||
LayoutRightPaddedMapping::padding_value == dynamic_extent)
</pre></blockquote>
</blockquote>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2025-09-27, Tomasz Kamiński fixes constraints in constructors from padded layouts]</i></p>



<p id="res-4272"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<blockquote class="note">
<p>
[<i>Drafting note</i>: As drive-by fixes the edits for <code>layout_left_padded&lt;&gt;::mapping</code> and
<code>layout_right_padded&lt;&gt;::mapping</code> also correct an editorial asymmetry between class
header synopsis declaration form and prototype specification form of the corresponding
constructors and adjust to the correct formatting of the exposition-only data member <code><i>rank_</i></code>.]
</p>
</blockquote>

<ol>
<li><p>Modify 23.7.3.4.5.1 <a href="https://wg21.link/mdspan.layout.left.overview">[mdspan.layout.left.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Extents&gt;
  class layout_left::mapping {
    [&hellip;]
    <i>// 23.7.3.4.5.2 <a href="https://wg21.link/mdspan.layout.left.cons">[mdspan.layout.left.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    
    constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.5.2 <a href="https://wg21.link/mdspan.layout.left.cons">[mdspan.layout.left.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
-14- <i>Preconditions</i>: [&hellip;]
<p/>
-15- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.6.1 <a href="https://wg21.link/mdspan.layout.right.overview">[mdspan.layout.right.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class Extents&gt;
  class layout_right::mapping {
    [&hellip;]
    <i>// 23.7.3.4.6.2 <a href="https://wg21.link/mdspan.layout.right.cons">[mdspan.layout.right.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    
    constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.6.2 <a href="https://wg21.link/mdspan.layout.right.cons">[mdspan.layout.right.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
-14- <i>Preconditions</i>: [&hellip;]
<p/>
-15- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.8.1 <a href="https://wg21.link/mdspan.layout.leftpad.overview">[mdspan.layout.leftpad.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;size_t PaddingValue&gt;
  template&lt;class Extents&gt;
  class layout_left_padded&lt;PaddingValue&gt;::mapping {
    [&hellip;]
    <i>// 23.7.3.4.8.3 <a href="https://wg21.link/mdspan.layout.leftpad.cons">[mdspan.layout.leftpad.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.8.3 <a href="https://wg21.link/mdspan.layout.leftpad.cons">[mdspan.layout.leftpad.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-10- <i>Constraints</i>: [&hellip;]
<p/>
-11- <i>Preconditions</i>: [&hellip;]
<p/>
-12- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
<pre>
template&lt;class LayoutLeftPaddedMapping&gt;
  constexpr explicit(<i>see below</i>)
    mapping(const LayoutLeftPaddedMapping&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-16- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:
</p>
<blockquote><pre>
<ins>!is_convertible_v&lt;typename LayoutLeftPaddedMapping::extents_type, extents_type&gt; ||</ins>
rank_&gt; 1 &amp;&amp;
(padding_value != dynamic_extent ||
 LayoutLeftPaddedMapping::padding_value == dynamic_extent)
</pre></blockquote>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.9.1 <a href="https://wg21.link/mdspan.layout.rightpad.overview">[mdspan.layout.rightpad.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;size_t PaddingValue&gt;
  template&lt;class Extents&gt;
  class layout_right_padded&lt;PaddingValue&gt;::mapping {
    [&hellip;]
    <i>// 23.7.3.4.9.3 <a href="https://wg21.link/mdspan.layout.rightpad.cons">[mdspan.layout.rightpad.cons]</a>, constructors</i>
    [&hellip;]
    template&lt;class OtherExtents&gt;
      constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
        mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 23.7.3.4.9.3 <a href="https://wg21.link/mdspan.layout.rightpad.cons">[mdspan.layout.rightpad.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class OtherExtents&gt;
  constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
    mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
</pre>
<blockquote>
<p>
-10- <i>Constraints</i>: [&hellip;]
<p/>
-11- <i>Preconditions</i>: [&hellip;]
<p/>
-12- <i>Effects</i>: [&hellip;]
<p/>
<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
</p>
<blockquote><pre>
<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
</pre></blockquote>
</blockquote>
<pre>
template&lt;class LayoutRightPaddedMapping&gt;
  constexpr explicit(<i>see below</i>)
    mapping(const LayoutRightPaddedMapping&amp; other);
</pre>
<blockquote>
<p>
-13- <i>Constraints</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-17- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:
</p>
<blockquote><pre>
<ins>!is_convertible_v&lt;typename LayoutRightPaddedMapping::extents_type, extents_type&gt; ||</ins>
<i>rank_</i> &gt; 1 &amp;&amp;
(padding_value != dynamic_extent ||
LayoutRightPaddedMapping::padding_value == dynamic_extent)
</pre></blockquote>
</blockquote>
</blockquote>
</li>

</ol>






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