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New issue from Lénárd Szolnoki: "Constant expression ranges::size(r) Constraints and Mandates in [simd]"
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‎xml/issue4409.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="4409" status="New">
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<title>Constant expression `ranges::size(r)` <i>Constraints</i> and <i>Mandates</i> in [simd]</title>
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<section>
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<sref ref="[simd]"/>
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</section>
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<submitter>Lénárd Szolnoki</submitter>
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<date>10 Oct 2025</date>
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<priority>99</priority>
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<discussion>
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<p>
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Various <i>Constraints</i> and <i>Mandates</i> in <sref ref="[simd]"/> are conditioned on `ranges::size(r)`,
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assuming that subsequent operations on the result are also constant expression. This is not a given,
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since `ranges::size(r)` is allowed to return an integer-class type, and it is not mandated that operations
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on integer-class types are usable in constant expressions.
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<p/>
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In particular:
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</p>
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<ol>
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<li><p>
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<sref ref="[simd.ctor]"/> p13:
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</p>
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<blockquote>
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<p>
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<i>Constraints</i>:
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</p>
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<ul>
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<li><p>`R` models `ranges::contiguous_range` and `ranges::sized_range`,</p></li>
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<li><p>`ranges::size(r)` is a constant expression, and</p></li>
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<li><p>`ranges::size(r)` is equal to `size()`.</p></li>
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</ul>
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</blockquote>
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<p>
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Here `ranges::size(r) == size()` might not be a constant expression, even when `ranges::size(r)`
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is. Operations that are not accounted for:
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</p>
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<ul>
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<li><p>Possible conversion from `range::size(r)` to <tt><i>simd-size-type</i></tt>.</p></li>
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<li><p>Possible operator overloading on ==.</p></li>
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</ul>
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</li>
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<li><p>
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<sref ref="[simd.ctor]"/> p17+18:
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</p>
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<blockquote>
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<p>
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<i>Constraints</i>:
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</p>
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<ul>
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<li><p>`R` models `ranges::contiguous_range` and `ranges::sized_range`, and</p></li>
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<li><p>`ranges::size(r)` is a constant expression</p></li>
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</ul>
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<p>
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<i>Remarks:</i> The deduced type is equivalent to <tt>vec&lt;ranges::range_value_t&lt;R&gt;, ranges::size(r)&gt;</tt>.
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</p>
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</blockquote>
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<p>
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Here the second constraint is simply redundant, if failure to form the type is in the immediate context.
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If we want to type it out in the constraints then we should account for conversion to <tt><i>simd-size-type</i></tt>
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and narrowing.
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</p>
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</li>
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<li><p>
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<sref ref="[simd.loadstore]"/> p2:
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</p>
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<blockquote>
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<p>
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<i>Mandates</i>: If `ranges::size(r)` is a constant expression then `ranges::size(r)` ≥ `V::size()`.
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</p>
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</blockquote>
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<p>
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Here <tt>ranges::size(r) &gt;= V::size()</tt> might not be a constant expression, even when
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`ranges::size(r)` is. It is unclear why a mathematical operator symbol is used here (and further
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down in the preconditions).
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</p>
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</li>
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</ol>
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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="[simd.ctor]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class R, class... Flags&gt;
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constexpr basic_vec(R&amp;&amp; r, flags&lt;Flags...&gt; = {});
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template&lt;class R, class... Flags&gt;
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constexpr basic_vec(R&amp;&amp; r, const mask_type&amp; mask, flags&lt;Flags...&gt; = {});
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</pre>
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<blockquote>
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<p>
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-12- Let `mask` be `mask_type(true)` for the overload with no `mask` parameter.
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<p/>
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-13- <i>Constraints</i>:
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</p>
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<ol style="list-style-type: none">
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<li><p>
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(13.1) &mdash; `R` models `ranges::contiguous_range` and `ranges::sized_range`, <ins>and</ins>
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</p></li>
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<li><p>
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(13.2) &mdash; <tt>ranges::size(r) <ins>== size()</ins></tt> is a constant expression, and <ins>evaluates to `true`.</ins>
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</p></li>
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<li><p>
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<del>(13.3) &mdash; `ranges::size(r)` is equal to `size()`.</del>
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</p></li>
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</ol>
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[&hellip;]
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</blockquote>
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<pre>
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template&lt;class R, class... Ts&gt;
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basic_vec(R&amp;&amp; r, Ts...) -&gt; <i>see below</i>;
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</pre>
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<blockquote>
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<p>
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-17- <i>Constraints</i>:
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</p>
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<ol style="list-style-type: none">
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<li><p>
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<del>(17.1) &mdash;</del> `R` models `ranges::contiguous_range` and `ranges::sized_range`<del>, and</del><ins>.</ins>
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</p></li>
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<li><p>
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<del>(17.2) &mdash; `ranges::size(r)` is a constant expression.</del>
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</p></li>
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</ol>
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<p>
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-18- <i>Remarks</i>: The deduced type is equivalent to <tt>vec&lt;ranges::range_value_t&lt;R&gt;, ranges::size(r)&gt;</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="[simd.loadstore]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class V = <i>see below</i>, ranges::contiguous_range R, class... Flags&gt;
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requires ranges::sized_range&lt;R&gt;
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constexpr V unchecked_load(R&amp;&amp; r, flags&lt;Flags...&gt; f = {});
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[&hellip;]
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template&lt;class V = <i>see below</i>, contiguous_iterator I, sized_sentinel_for&lt;I&gt; S, class... Flags&gt;
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constexpr V unchecked_load(I first, S last, const typename V::mask_type&amp; mask,
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flags&lt;Flags...&gt; f = {});
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</pre>
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<blockquote>
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<p>
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-1- Let [&hellip;]
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<p/>
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-2- <i>Mandates</i>: If <tt>ranges::size(r) <ins>&gt;= V::size()</ins></tt> is a constant expression
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then <del>`ranges::size(r)` ≥ `V::size()`</del><ins>it evaluates to `true`</ins>.
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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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