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‎issue4272.html‎

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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>
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<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>
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<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>
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<b>Submitter:</b> Luc Grosheintz <b>Opened:</b> 2025-06-02 <b>Last modified:</b> 2025-06-14</p>
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<b>Submitter:</b> Luc Grosheintz <b>Opened:</b> 2025-06-02 <b>Last modified:</b> 2025-06-28</p>
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<p><b>Priority: </b>2
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</p>
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<p><b>View all other</b> <a href="lwg-index.html#mdspan.layout">issues</a> in [mdspan.layout].</p>
@@ -105,6 +105,214 @@ <h3 id="4272"><a href="lwg-active.html#4272">4272</a>. For <code class='backtick
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Set priority to 2 after reflector poll.
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</p>
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<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
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<blockquote class="note">
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<blockquote class="note">
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<p>
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[<i>Drafting note</i>: As drive-by fixes the edits for <code>layout_left_padded&lt;&gt;::mapping</code> and
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<code>layout_right_padded&lt;&gt;::mapping</code> also correct an editorial asymmetry between class
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header synopsis declaration form and prototype specification form of the corresponding
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constructors and adjust to the correct formatting of the exposition-only data member <code><i>rank_</i></code>.]
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</p>
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</blockquote>
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<ol>
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<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>
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<blockquote>
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<pre>
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namespace std {
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template&lt;class Extents&gt;
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class layout_left::mapping {
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[&hellip;]
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<i>// 23.7.3.4.5.2 <a href="https://wg21.link/mdspan.layout.left.cons">[mdspan.layout.left.cons]</a>, constructors</i>
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[&hellip;]
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
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constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
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[&hellip;]
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};
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}
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</pre>
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</blockquote>
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</li>
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<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>
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<blockquote>
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<pre>
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
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</pre>
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<blockquote>
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<p>
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-13- <i>Constraints</i>: [&hellip;]
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<p/>
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-14- <i>Preconditions</i>: [&hellip;]
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<p/>
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-15- <i>Effects</i>: [&hellip;]
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<p/>
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<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
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</p>
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<blockquote><pre>
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<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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<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>
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<blockquote>
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<pre>
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namespace std {
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template&lt;class Extents&gt;
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class layout_right::mapping {
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[&hellip;]
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<i>// 23.7.3.4.6.2 <a href="https://wg21.link/mdspan.layout.right.cons">[mdspan.layout.right.cons]</a>, constructors</i>
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[&hellip;]
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
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constexpr mapping&amp; operator=(const mapping&amp;) noexcept = default;
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[&hellip;]
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};
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}
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</pre>
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</blockquote>
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</li>
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<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>
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<blockquote>
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<pre>
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
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</pre>
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<blockquote>
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<p>
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-13- <i>Constraints</i>: [&hellip;]
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<p/>
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-14- <i>Preconditions</i>: [&hellip;]
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<p/>
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-15- <i>Effects</i>: [&hellip;]
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<p/>
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<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
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</p>
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<blockquote><pre>
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<ins>!(extents_type::rank() == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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<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>
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<blockquote>
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<pre>
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namespace std {
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template&lt;size_t PaddingValue&gt;
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template&lt;class Extents&gt;
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class layout_left_padded&lt;PaddingValue&gt;::mapping {
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[&hellip;]
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<i>// 23.7.3.4.8.3 <a href="https://wg21.link/mdspan.layout.leftpad.cons">[mdspan.layout.leftpad.cons]</a>, constructors</i>
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[&hellip;]
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>extents_type::rank() &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
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[&hellip;]
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};
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}
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</pre>
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</blockquote>
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</li>
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<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>
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<blockquote>
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<pre>
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
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</pre>
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<blockquote>
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<p>
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-10- <i>Constraints</i>: [&hellip;]
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<p/>
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-11- <i>Preconditions</i>: [&hellip;]
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<p/>
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-12- <i>Effects</i>: [&hellip;]
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<p/>
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<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
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</p>
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<blockquote><pre>
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<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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<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>
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<blockquote>
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<pre>
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namespace std {
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template&lt;size_t PaddingValue&gt;
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template&lt;class Extents&gt;
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class layout_right_padded&lt;PaddingValue&gt;::mapping {
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[&hellip;]
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<i>// 23.7.3.4.9.3 <a href="https://wg21.link/mdspan.layout.rightpad.cons">[mdspan.layout.rightpad.cons]</a>, constructors</i>
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[&hellip;]
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp;);
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[&hellip;]
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};
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}
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</pre>
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</blockquote>
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</li>
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<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>
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<blockquote>
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<pre>
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template&lt;class OtherExtents&gt;
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constexpr explicit(<del>rank_ &gt; 0</del><ins><i>see below</i></ins>)
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mapping(const layout_stride::mapping&lt;OtherExtents&gt;&amp; other);
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</pre>
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<blockquote>
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<p>
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-10- <i>Constraints</i>: [&hellip;]
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<p/>
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-11- <i>Preconditions</i>: [&hellip;]
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<p/>
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-12- <i>Effects</i>: [&hellip;]
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<p/>
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<ins>-?- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:</ins>
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</p>
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<blockquote><pre>
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<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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</ol>
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</blockquote>
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<p><i>[2025-06-20, Luc Grosheintz provides further wording improvements]</i></p>
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<p id="res-4272"><b>Proposed resolution:</b></p>
@@ -259,6 +467,26 @@ <h3 id="4272"><a href="lwg-active.html#4272">4272</a>. For <code class='backtick
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<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
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</pre></blockquote>
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</blockquote>
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<pre>
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template&lt;class LayoutLeftPaddedMapping&gt;
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constexpr explicit(<i>see below</i>)
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mapping(const LayoutLeftPaddedMapping&amp; other);
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</pre>
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<blockquote>
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<p>
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-13- <i>Constraints</i>: [&hellip;]
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<p/>
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[&hellip;]
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<p/>
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-16- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:
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</p>
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<blockquote><pre>
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<ins>!is_convertible_v&lt;typename LayoutLeftPaddedMapping::extents_type, extents_type&gt; &amp;&amp;</ins>
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rank_&gt; 1 &amp;&amp;
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(padding_value != dynamic_extent ||
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LayoutLeftPaddedMapping::padding_value == dynamic_extent)
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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@@ -305,6 +533,26 @@ <h3 id="4272"><a href="lwg-active.html#4272">4272</a>. For <code class='backtick
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<ins>!(<i>rank_</i> == 0 &amp;&amp; is_convertible_v&lt;OtherExtents, extents_type&gt;)</ins>
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</pre></blockquote>
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</blockquote>
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<pre>
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template&lt;class LayoutRightPaddedMapping&gt;
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constexpr explicit(<i>see below</i>)
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mapping(const LayoutRightPaddedMapping&amp; other);
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</pre>
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<blockquote>
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<p>
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-13- <i>Constraints</i>: [&hellip;]
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<p/>
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[&hellip;]
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<p/>
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-17- <i>Remarks</i>: The expression inside <code class='backtick'>explicit</code> is equivalent to:
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</p>
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<blockquote><pre>
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<ins>!is_convertible_v&lt;typename LayoutRightPaddedMapping::extents_type, extents_type&gt; &amp;&amp;</ins>
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<i>rank_</i> &gt; 1 &amp;&amp;
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(padding_value != dynamic_extent ||
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LayoutRightPaddedMapping::padding_value == dynamic_extent)
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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