New issue from Hewill: "Ambiguities of simd::basic_vec constructor"

Author: Dani-Hub

xml/issue4391.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4391" status="New">
+<title>Ambiguities of `simd::basic_vec` constructor</title>
+<section><sref ref="[simd.ctor]"/></section>
+<submitter>Hewill Kang</submitter>
+<date>29 Sep 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+The broadcasting, generator-based, and range constructors of `simd::basic_vec` all take a single
+argument, and their constraints are not mutually exclusive.
+<p/>
+This means that when a type satisfies both characteristics, such as a range that can be converted to a
+`value_type`, this will lead to ambiguity:
+</p>
+<blockquote>
+<pre>
+#include &lt;simd&gt;
+
+struct S {
+  operator double() const;       // basic_vec(U&amp;&amp; value)
+  
+  double operator()(int) const;  // basic_vec(G&amp;&amp; gen)
+
+  double* begin() const;         // basic_vec(R&amp;&amp; r, flags&lt;Flags...&gt; = {});
+  double* end() const;
+  constexpr static int size() { return 2; }
+};
+
+int main() {
+  std::simd::vec&lt;double&gt; simd(S{}); // <span style="color:#C80000;font-weight:bold">error: call of overloaded 'basic_simd(S)' is ambiguous</span>
+}
+</pre>
+</blockquote>
+<p>
+Do we need more constraints, similar to the one in <code>string_view(R&amp;&amp; r)</code> that requires
+`R` not to be convertible to `const char*`, to make the above work, i.e., only invoke the
+broadcasting constructor?
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5014"/>.
+</p>
+
+<ol>
+
+
+<li><p>Modify <sref ref="[simd.ctor]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+template&lt;class G&gt; constexpr explicit basic_vec(G&amp;&amp; gen);
+</pre>
+<blockquote>
+<p>
+-8- Let <tt>From<sub><i>i</i></sub></tt> denote the type
+   <code>decltype(gen(integral_constant&lt;<i>simd-size-type</i>, <i>i</i>&gt;()))</code>.
+<p/>
+-9- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+   <li><p>
+   <ins>(9.?) &mdash; <code>constructible_from&lt;value_type, G&gt;</code> is <code>false</code>.</ins>
+   </p></li>
+   <li><p>
+    <ins>(9.?) &mdash;</ins>
+    <tt>From<sub><i>i</i></sub></tt> satisfies <code>convertible_to&lt;value_type&gt;</code> for all 
+    <tt><i>i</i></tt> in the range of [<code>0, size()</code>). In addition, for all <i>i</i> in the range of
+    [<code>0, size()</code>), if <tt>From<sub><i>i</i></sub></tt> is an arithmetic type, conversion from 
+    <tt>From<sub><i>i</i></sub></tt> to <code>value_type</code> is value-preserving.</p>
+   </li>
+</ol>
+</blockquote>
+[&hellip;]
+<pre>
+template&lt;class R, class... Flags&gt;
+  constexpr basic_vec(R&amp;&amp; r, flags&lt;Flags...&gt; = {});
+template&lt;class R, class... Flags&gt;
+  constexpr basic_vec(R&amp;&amp; r, const mask_type&amp; mask, flags&lt;Flags...&gt; = {});
+</pre>
+<blockquote>
+<p>
+-12- Let mask be `mask_type(true)` for the overload with no `mask` parameter.
+<p/>
+-13- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+   <li><p>
+   (13.1) &mdash; `R` models `ranges::contiguous_range` and `ranges::sized_range`,
+   </p></li>
+   <li><p>
+   (13.2) &mdash; `ranges::size(r)` is a constant expression, <del>and</del>
+   </p></li>
+   <li><p>
+   (13.3) &mdash; `ranges::size(r)` is equal to `size()`<del>.</del><ins>,</ins>
+   </p></li>
+   <li><p>
+   <ins>(13.?) &mdash; <code>constructible_from&lt;value_type, R&gt;</code> is `false`, and</ins>
+   </p></li>
+   <li><p>
+   <ins>(13.?) &mdash; <code>r(integral_constant&lt;<i>simd-size-type</i>, 0&gt;())</code> is not a valid
+            expression.</ins>
+   </p></li>
+</ol>
+</blockquote>
+</blockquote>
+</li>
+</ol>
+
+</resolution>
+
+</issue>