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New issue from Luc Grosheintz: "Missing move in mdspan layout mapping::operator()"
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‎xml/issue4314.xml

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<?xml version='1.0' encoding='utf-8' standalone='no'?>
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<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
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<issue num="4314" status="New">
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<title>Missing move in `mdspan` layout `mapping::operator()`</title>
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<section>
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<sref ref="[mdspan.layout]"/>
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</section>
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<submitter>Luc Grosheintz</submitter>
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<date>13 Aug 2025</date>
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<priority>99</priority>
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<discussion>
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<p>
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Numerous template classes in <tt>&lt;mdspan&gt;</tt> have template parameter `IndexType`.
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While this template parameter is restricted to be a signed or unsigned integer,
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these classes often accept user-defined classes that convert to `IndexType`.
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<p/>
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They're either passed as an array/span of `OtherIndexType`; or as a template parameter pack.
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When passed as a template parameter pack, the common pattern is
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</p>
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<blockquote><pre>
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template&lt;class... OtherIndexTypes&gt;
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requires std::is_convertible_v&lt;OtherIndexTypes, IndexType> &amp;&amp; ...
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void <i>dummy</i>(OtherIndexTypes... indices)
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{
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<i>something</i>(static_cast&lt;IndexType&gt;(std::move(indices))...);
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}
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</pre></blockquote>
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<p>
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This pattern allows passing in objects that convert to IndexType only as
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an rvalue reference, e.g.
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</p>
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<blockquote><pre>
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class RValueInt
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{
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constexpr
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operator int() &amp;&amp; noexcept
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{ return m_int; }
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private:
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int m_int;
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};
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</pre></blockquote>
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<p>
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This pattern can be found in:
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</p>
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<ul>
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<li><p>a ctor of `extents`,</p></li>
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<li><p>a ctor of `mdspan`,</p></li>
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<li><p>`mdspan::operator[]`.</p></li>
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</ul>
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<p>
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The five standardized layout mappings use a different pattern in their
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operator(), namely,
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</p>
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<blockquote><pre>
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static_cast&lt;IndexType&gt;(indices)...
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</pre></blockquote>
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<p>
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This prevents the passing in objects of type `RValueInt`, because the
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conversion isn't happening from an rvalue reference. This is addressed by
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Items 1 - 5 in the Proposed Resolution.
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<p/>
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A different pattern can be found a ctor for `layout_{left,right}_padded`.
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Namely, directly passing an object of type `OtherIndexType` to
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<tt><i>LEAST-MULTIPLE-AT-LEAST</i></tt>. This is addressed in Items 6 and 7
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in the Proposed Resolution.
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<p/>
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This inconsistency was noticed while fixing
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<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121061">PR121061</a>
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and these changes have been applied to all layout mappings implemented in libstdc++.
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</p>
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</discussion>
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<resolution>
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<p>
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This wording is relative to <paper num="N5014"/>.
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</p>
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<ol>
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<li><p>Modify <sref ref="[mdspan.layout.left.obs]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class... Indices&gt;
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constexpr index_type operator()(Indices... i) const noexcept;
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</pre>
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<blockquote>
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<p>
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-2- <i>Constraints</i>: [&hellip;]
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<p/>
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-3- <i>Preconditions</i>: [&hellip;]
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<p/>
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-4- <i>Effects</i>: Let `P` be a parameter pack such that
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</p>
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<blockquote><pre>
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is_same_v&lt;index_sequence_for&lt;Indices...&gt;, index_sequence&lt;P...&gt;&gt;
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</pre></blockquote>
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<p>
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is `true`. Equivalent to:
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</p>
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<blockquote><pre>
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return ((static_cast&lt;index_type&gt;(<ins>std::move(</ins>i<ins>)</ins>) * stride(P)) + ... + 0);
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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 <sref ref="[mdspan.layout.right.obs]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class... Indices&gt;
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constexpr index_type operator()(Indices... i) const noexcept;
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</pre>
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<blockquote>
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<p>
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-2- <i>Constraints</i>: [&hellip;]
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<p/>
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-3- <i>Preconditions</i>: [&hellip;]
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<p/>
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-4- <i>Effects</i>: Let `P` be a parameter pack such that
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</p>
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<blockquote><pre>
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is_same_v&lt;index_sequence_for&lt;Indices...&gt;, index_sequence&lt;P...&gt;&gt;
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</pre></blockquote>
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<p>
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is `true`. Equivalent to:
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</p>
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<blockquote><pre>
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return ((static_cast&lt;index_type&gt;(<ins>std::move(</ins>i<ins>)</ins>) * stride(P)) + ... + 0);
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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 <sref ref="[mdspan.layout.stride.obs]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class... Indices&gt;
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constexpr index_type operator()(Indices... i) const noexcept;
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</pre>
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<blockquote>
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<p>
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-2- <i>Constraints</i>: [&hellip;]
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<p/>
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-3- <i>Preconditions</i>: [&hellip;]
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<p/>
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-4- <i>Effects</i>: Let `P` be a parameter pack such that
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</p>
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<blockquote><pre>
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is_same_v&lt;index_sequence_for&lt;Indices...&gt;, index_sequence&lt;P...&gt;&gt;
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</pre></blockquote>
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<p>
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is `true`. Equivalent to:
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</p>
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<blockquote><pre>
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return ((static_cast&lt;index_type&gt;(<ins>std::move(</ins>i<ins>)</ins>) * stride(P)) + ... + 0);
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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 <sref ref="[mdspan.layout.leftpad.obs]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class... Indices&gt;
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constexpr index_type operator()(Indices... idxs) const noexcept;
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</pre>
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<blockquote>
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<p>
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-3- <i>Constraints</i>: [&hellip;]
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<p/>
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-4- <i>Preconditions</i>: [&hellip;]
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<p/>
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-5- <i>Returns</i>: <tt>((static_cast&lt;index_type&gt;(<ins>std::move(</ins>idxs<ins>)</ins>) * stride(P_rank)) + ... + 0)</tt>.
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</p>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify <sref ref="[mdspan.layout.rightpad.obs]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class... Indices&gt;
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constexpr index_type operator()(Indices... idxs) const noexcept;
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</pre>
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<blockquote>
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<p>
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-3- <i>Constraints</i>: [&hellip;]
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<p/>
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-4- <i>Preconditions</i>: [&hellip;]
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<p/>
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-5- <i>Returns</i>: <tt>((static_cast&lt;index_type&gt;(<ins>std::move(</ins>idxs<ins>)</ins>) * stride(P_rank)) + ... + 0)</tt>.
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</p>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify <sref ref="[mdspan.layout.leftpad.cons]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class OtherIndexType&gt;
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constexpr mapping(const extents_type&amp; ext, OtherIndexType pad<ins>ding</ins>);
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</pre>
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<blockquote>
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<p>
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<ins><ins>Let <tt>pad = static_cast&lt;index_type&gt;(std::move(padding))</tt>.</ins></ins>
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<p/>
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-3- <i>Constraints</i>: [&hellip;]
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<p/>
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-4- <i>Preconditions</i>:
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</p>
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<ol style="list-style-type: none">
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<li><p>(4.1) &mdash; <tt>pad<ins>ding</ins></tt> is representable as a value of type `index_type`.</p></li>
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<li><p>(4.2) &mdash; <del><tt>extents_type::<i>index-cast</i>(pad)</tt></del><ins>pad</ins> is greater than zero.</p></li>
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<li><p>(4.3) &mdash; If <tt><i>rank_</i></tt> is greater than one, then
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<tt><i>LEAST-MULTIPLE-AT-LEAST</i>(pad, ext.extent(0))</tt> is representable as a value of type `index_type`.</p></li>
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<li><p>(4.4) &mdash; If <tt><i>rank_</i></tt> is greater than one, then the product of
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<tt><i>LEAST-MULTIPLE-AT-LEAST</i>(pad, ext.extent(0))</tt> and all values <tt>ext.extent(<i>k</i>)</tt> with
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<tt><i>k</i></tt> in the range of <tt>[1, <i>rank_</i>)</tt> is representable as a value of type `index_type`.</p></li>
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<li><p>(4.5) &mdash; If `padding_value` is not equal to `dynamic_extent`, `padding_value` equals
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<tt><del>extents_type::<i>index-cast</i>(pad)</del><ins>pad</ins></tt>.</p></li>
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</ol>
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<p>
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-5- <i>Effects</i>: [&hellip;]
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</p>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify <sref ref="[mdspan.layout.rightpad.cons]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class OtherIndexType&gt;
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constexpr mapping(const extents_type&amp; ext, OtherIndexType pad<ins>ding</ins>);
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</pre>
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<blockquote>
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<p>
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<ins><ins>Let <tt>pad = static_cast&lt;index_type&gt;(std::move(padding))</tt>.</ins></ins>
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<p/>
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-3- <i>Constraints</i>: [&hellip;]
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<p/>
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-4- <i>Preconditions</i>:
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</p>
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<ol style="list-style-type: none">
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<li><p>(4.1) &mdash; <tt>pad<ins>ding</ins></tt> is representable as a value of type `index_type`.</p></li>
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<li><p>(4.2) &mdash; <del><tt>extents_type::<i>index-cast</i>(pad)</tt></del><ins>pad</ins> is greater than zero.</p></li>
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<li><p>(4.3) &mdash; If <tt><i>rank_</i></tt> is greater than one, then
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<tt><i>LEAST-MULTIPLE-AT-LEAST</i>(pad, ext.extent(<i>rank_</i> - 1))</tt> is representable as a value of type `index_type`.</p></li>
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<li><p>(4.4) &mdash; If <tt><i>rank_</i></tt> is greater than one, then the product of
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<tt><i>LEAST-MULTIPLE-AT-LEAST</i>(pad, ext.extent(<i>rank_</i> - 1))</tt> and all values <tt>ext.extent(<i>k</i>)</tt> with
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<tt><i>k</i></tt> in the range of <tt>[1, <i>rank_</i> - 1)</tt> is representable as a value of type `index_type`.</p></li>
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<li><p>(4.5) &mdash; If `padding_value` is not equal to `dynamic_extent`, `padding_value` equals
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<tt><del>extents_type::<i>index-cast</i>(pad)</del><ins>pad</ins></tt>.</p></li>
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</ol>
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<p>
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-5- <i>Effects</i>: [&hellip;]
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</p>
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</blockquote>
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</blockquote>
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</li>
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</ol>
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</resolution>
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</issue>

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