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jwakely merged 2 commits into from
Aug 29, 2025
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LWG4272: New proposed resolution fixing constructor from padded layouts #462

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LWG4272: New proposed resolution fixing constructor from padded layouts
tomaszkam Aug 27, 2025
b4a98c3
Update xml/issue4272.xml
jwakely Aug 29, 2025
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251 changes: 249 additions & 2 deletions xml/issue4272.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -250,9 +250,8 @@ template<class OtherExtents>
</superseded>

<note>2025-06-20, Luc Grosheintz provides further wording improvements</note>
</discussion>

<resolution>
<superseded>
<p>
This wording is relative to <paper num="N5008"/>.
</p>
Expand Down Expand Up @@ -493,7 +492,255 @@ LayoutRightPaddedMapping::padding_value == dynamic_extent)
</blockquote>
</li>

</ol>
</superseded>

<note>2025-09-27, Tomasz Kamiński fixes constraints in constructors from padded layouts</note>
</discussion>

<resolution>
<p>
This wording is relative to <paper num="N5008"/>.
</p>

<blockquote class="note">
<p>
[<i>Drafting note</i>: As drive-by fixes the edits for <tt>layout_left_padded&lt;&gt;::mapping</tt> and
<tt>layout_right_padded&lt;&gt;::mapping</tt> 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 <tt><i>rank_</i></tt>.]
</p>
</blockquote>

<ol>
<li><p>Modify <sref ref="[mdspan.layout.left.overview]"/> as indicated:</p>

<blockquote>
<pre>
namespace std {
template&lt;class Extents&gt;
class layout_left::mapping {
[&hellip;]
<i>// <sref ref="[mdspan.layout.left.cons]"/>, 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 <sref ref="[mdspan.layout.left.cons]"/> 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 `explicit` 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 <sref ref="[mdspan.layout.right.overview]"/> as indicated:</p>

<blockquote>
<pre>
namespace std {
template&lt;class Extents&gt;
class layout_right::mapping {
[&hellip;]
<i>// <sref ref="[mdspan.layout.right.cons]"/>, 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 <sref ref="[mdspan.layout.right.cons]"/> 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 `explicit` 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 <sref ref="[mdspan.layout.leftpad.overview]"/> 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>// <sref ref="[mdspan.layout.leftpad.cons]"/>, 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 <sref ref="[mdspan.layout.leftpad.cons]"/> 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 `explicit` 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 `explicit` 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 <sref ref="[mdspan.layout.rightpad.overview]"/> 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>// <sref ref="[mdspan.layout.rightpad.cons]"/>, 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 <sref ref="[mdspan.layout.rightpad.cons]"/> 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 `explicit` 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 `explicit` 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>
</resolution>


</issue>