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New issue from Hewill: "mdspan constructor should disallow derived to base conversions"
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‎xml/issue4405.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="4405" status="New">
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<title>`mdspan` constructor should disallow derived to base conversions</title>
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<section>
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<sref ref="[mdspan.mdspan.cons]"/>
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</section>
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<submitter>Hewill Kang</submitter>
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<date>05 Oct 2025</date>
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<priority>99</priority>
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<discussion>
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<p>
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Unlike `ranges::subrange` or `span`, `mdspan` syntactically allows a multidimensional
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viewing base class via a derived class pointer (<a href="https://godbolt.org/z/c56h9fjs8">demo</a>):
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</p>
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<blockquote><pre>
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#include &lt;span&gt;
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#include &lt;ranges&gt;
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#include &lt;mdspan&gt;
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struct Base {};
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struct Derived : Base {};
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std::array&lt;Derived, 12&gt; arr;
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std::ranges::subrange&lt;Base*&gt; s(arr); // error, slicing
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std::span&lt;Base&gt; sp(arr.data(), arr.size()); // error, slicing
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std::mdspan&lt;Base, std::dims&lt;1&gt;&gt; msp(arr.data(), arr.size()); // <span style="color:#C80000;font-weight:bold">ok</span>
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</pre></blockquote>
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<p>
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Given that we intend to reject object slicing for both `default_accessor` and
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`aligned_accessor`, there seems no reason not to reject this invalid pointer
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arithmetic for `mdspan`.
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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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<blockquote class="note">
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<p>
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[<i>Drafting note:</i> The exposition-only concept <code><i>convertible-to-non-slicing</i></code> comes from
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<sref ref="[range.subrange.general]"/>.]
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</p>
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</blockquote>
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<ol>
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<li><p>Modify <sref ref="[mdspan.mdspan.overview]"/> 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 ElementType, class Extents, class LayoutPolicy = layout_right,
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class AccessorPolicy = default_accessor&lt;ElementType&gt;&gt;
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class mdspan {
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public:
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using extents_type = Extents;
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using layout_type = LayoutPolicy;
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using accessor_type = AccessorPolicy;
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using mapping_type = typename layout_type::template mapping&lt;extents_type&gt;;
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using element_type = ElementType;
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using value_type = remove_cv_t&lt;element_type&gt;;
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using index_type = typename extents_type::index_type;
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using size_type = typename extents_type::size_type;
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using rank_type = typename extents_type::rank_type;
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using data_handle_type = typename accessor_type::data_handle_type;
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using reference = typename accessor_type::reference;
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[&hellip;]
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template&lt;class... OtherIndexTypes&gt;
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constexpr explicit mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> ptr, OtherIndexTypes... exts);
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template&lt;class OtherIndexType, size_t N&gt;
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constexpr explicit(N != rank_dynamic())
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mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, span&lt;OtherIndexType, N&gt; exts);
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template&lt;class OtherIndexType, size_t N&gt;
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constexpr explicit(N != rank_dynamic())
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mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const array&lt;OtherIndexType, N&gt;&amp; exts);
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constexpr mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const extents_type&amp; ext);
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constexpr mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const mapping_type&amp; m);
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constexpr mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const mapping_type&amp; m, const accessor_type&amp; a);
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[&hellip;]
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};
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[&hellip;]
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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 <sref ref="[mdspan.mdspan.cons]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class... OtherIndexTypes&gt;
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constexpr explicit mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, OtherIndexTypes... exts);
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</pre>
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<blockquote>
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<p>
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-4- Let `N` be `sizeof...(OtherIndexTypes)`.
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<p/>
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-5- <i>Constraints</i>: [&hellip;]
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<p/>
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[&hellip;]
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</p>
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</blockquote>
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<pre>
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template&lt;class OtherIndexType, size_t N&gt;
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constexpr explicit(N != rank_dynamic())
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mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, span&lt;OtherIndexType, N&gt; exts);
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template&lt;class OtherIndexType, size_t N&gt;
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constexpr explicit(N != rank_dynamic())
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mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const array&lt;OtherIndexType, N&gt;&amp; exts);
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</pre>
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<blockquote>
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<p>
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-8- <i>Constraints</i>: [&hellip;]
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<p/>
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[&hellip;]
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</p>
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</blockquote>
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<pre>
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constexpr mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const extents_type&amp; ext);
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</pre>
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<blockquote>
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<p>
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-11- <i>Constraints</i>: [&hellip;]
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<p/>
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[&hellip;]
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</p>
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</blockquote>
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<pre>
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constexpr mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const mapping_type&amp; m);
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</pre>
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<blockquote>
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<p>
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-14- <i>Constraints</i>: [&hellip;]
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<p/>
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[&hellip;]
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</p>
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</blockquote>
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<pre>
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constexpr mdspan(<ins><i>convertible-to-non-slicing</i>&lt;</ins>data_handle_type<ins>&gt; auto</ins> p, const mapping_type&amp; m, const accessor_type&amp; a);
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</pre>
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<blockquote>
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<p>
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-17- <i>Preconditions</i>: [&hellip;]
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<p/>
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[&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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