New issue from Matthias Kretz: "simd<complex>::real/imag is overconstrained"

Author: Dani-Hub

xml/issue4230.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4230" status="New">
+<title><tt>simd&lt;complex&gt;::real/imag</tt> is overconstrained</title>
+<section>
+<sref ref="[simd.complex.access]"/>
+</section>
+<submitter>Matthias Kretz</submitter>
+<date>18 Mar 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+<sref ref="[simd.complex.access]"/> overconstrains the arguments to `real` and `imag`.
+<tt>complex&lt;T&gt;::real/imag</tt> allows conversions to `T` whereas <tt>simd&lt;complex&lt;T&gt;&gt;</tt>
+requires basically an exact match (<tt>same_as&lt;simd&lt;T&gt;&gt;</tt> modulo ABI tag differences).
+</p>
+<blockquote><pre>
+complex&lt;double&gt; c = {};
+c.real(1.f); // OK
+
+simd&lt;complex&lt;double&gt;&gt; sc = {};
+sc.real(simd&lt;float&gt;(1.f)); // <span style="color:red;font-weight:bolder">ill-formed, should be allowed</span>
+</pre></blockquote>
+<p>
+The design intent was to match the <tt>std::complex&lt;T&gt;</tt> interface. In which case
+the current wording doesn't match that intent. `complex` doesn't say <tt>real(same_as&lt;T&gt; auto)</tt> 
+but 'real(T)', which allows conversions.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5008"/>.
+</p>
+
+<ol>
+
+<li><p>Modify <sref ref="[simd.overview]"/>, class template `basic_simd` synopsis, as indicated:</p>
+
+<blockquote>
+<pre>
+[&hellip;]
+<i>// <sref ref="[simd.complex.access]"/>, basic_simd complex-value accessors</i>
+constexpr auto real() const noexcept;
+constexpr auto imag() const noexcept;
+<del>template&lt;<i>simd-floating-point</i> V&gt;</del>
+  constexpr void real(<ins><i>see below</i></ins><del>const V&amp;</del> v) noexcept;
+<del>template&lt;<i>simd-floating-point</i> V&gt;</del>
+  constexpr void imag(<ins><i>see below</i></ins><del>const V&amp;</del> v) noexcept;
+[&hellip;]
+</pre>
+</blockquote>
+</li>
+
+<li><p>Modify <sref ref="[simd.complex.access]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+<del>template&lt;<i>simd-floating-point</i> V&gt;</del>
+  constexpr void real(<ins><i>see below</i></ins><del>const V&amp;</del> v) noexcept;
+<del>template&lt;<i>simd-floating-point</i> V&gt;</del>
+  constexpr void imag(<ins><i>see below</i></ins><del>const V&amp;</del> v) noexcept;
+</pre>
+<blockquote>
+<p>
+-3- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p><del>(3.1) &mdash;</del> <tt><i>simd-complex</i>&lt;basic_simd&gt;</tt> is modeled<ins>.</ins><del>,</del></p></li>
+<li><p><del>(3.2) &mdash; <tt>same_as&lt;typename V::value_type, typename T::value_type&gt;</tt> is modeled, and</del></p></li>
+<li><p><del>(3.3) &mdash; `V::size() == size()` is `true`.</del></p></li>
+</ol>
+<p>
+-4- <i>Effects</i>: Replaces each element of the `basic_simd` object such that the <tt><i>i</i></tt><sup>th</sup> 
+element is replaced with <tt>value_type(v[<i>i</i>], operator[](<i>i</i>).imag())</tt> or 
+<tt>value_type(operator[](<i>i</i>).real(), v[<i>i</i>])</tt> for `real` and `imag` respectively, 
+for all <tt><i>i</i></tt> in the range `[0, size())`.
+<p/>
+<ins>-?- <i>Remarks</i>: The type of argument `v` is 
+<tt>const rebind_t&lt;typename T::value_type, basic_simd&lt;T, Abi&gt;&gt;&amp;</tt>.</ins>
+</p>
+</blockquote>
+</blockquote>
+</li>
+
+</ol>
+</resolution>
+
+</issue>