New issue from Jonathan B. Coe: "Constraints on incomplete types in indirect and polymorphic"

Author: Dani-Hub

xml/issue4322.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4322" status="New">
+<title>Constraints on incomplete types in indirect and polymorphic</title>
+<section>
+<sref ref="[mem.composite.types]"/>
+</section>
+<submitter>Jonathan B. Coe</submitter>
+<date>20 Aug 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+The class templates <tt>indirect&lt;T&gt;</tt> and <tt>polymorphic&lt;T&gt;</tt> allow the template argument `T` 
+to be an incomplete type.
+<p/>
+Both classes can be instantiated when the type `T` is incomplete: constraints are written so that requirements 
+on incomplete types are not evaluated at class instantiation time.
+<p/>
+For constructors with additional template parameters, there are currently constraints written on the potentially 
+incomplete type `T` and the additional template parameters. Such constraints will not be evaluated at class 
+instantiation time but could be explicitly evaluated in contexts where support for an incomplete `T` is required.
+</p>
+<blockquote><pre>
+template&lt;typename U&gt;
+class A {
+  U u;
+public:
+  A(const <i>SomeType</i>&amp;) requires std::is_constructible_v&lt;U, <i>SomeType</i>&gt; 
+  {
+    // [&hellip;]
+  }
+};
+</pre></blockquote>
+<p>
+when `U` is <tt>indirect&lt;T&gt;</tt> or <tt>polymorphic&lt;T&gt;</tt> for some type `T`, the existence 
+of the requires clause will require that `T` is a complete type for constraints on indirect or polymorphic 
+to be evaluated.
+<p/>
+<i>Constraints</i> on `T` should be converted to <i>Mandates</i> on `T` so that constraint evaluation 
+does not require `T` to be a complete type.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5014"/>.
+</p>
+
+<ol>
+
+<li><p>Modify <sref ref="[indirect.ctor]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+template&lt;class U = T&gt;
+  constexpr explicit indirect(U&amp;&amp; u);
+</pre>
+<blockquote>
+<p>
+-17- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p>(17.1) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, indirect&gt;</tt> is `false`,</p></li>
+<li><p>(17.2) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, in_place_t&gt;</tt> is `false`, <ins>and</ins></p></li>
+<li><p><del>(17.3) &mdash; <tt>is_constructible_v&lt;T, U&gt;</tt> is `true`, and</del></p></li>
+<li><p>(17.4) &mdash; <tt>is_default_constructible_v&lt;Allocator&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>is_constructible_v&lt;T, U&gt;</tt> is `true`.</ins>
+<p/>
+-18- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class U = T&gt;
+  constexpr explicit indirect(allocator_arg_t, const Allocator&amp; a, U&amp;&amp; u);
+</pre>
+<blockquote>
+<p>
+-19- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p>(19.1) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, indirect&gt;</tt> is `false`, <ins>and</ins></p></li>
+<li><p>(19.2) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, in_place_t&gt;</tt> is `false`<ins>.</ins><del>, and</del></p></li>
+<li><p><del>(19.3) &mdash; <tt>is_constructible_v&lt;T, U&gt;</tt> is `true`</del></p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>is_constructible_v&lt;T, U&gt;</tt> is `true`.</ins>
+<p/>
+-20- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class... Us&gt;
+  constexpr explicit indirect(in_place_t, Us&amp;&amp;... us);
+</pre>
+<blockquote>
+<p>
+-21- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p><del>(21.1) &mdash; <tt>is_constructible_v&lt;T, Us...&gt;</tt> is `true`, and</del></p></li>
+<li><p><del>(21.2) &mdash;</del> <tt>is_default_constructible_v&lt;Allocator&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>is_constructible_v&lt;T, Us...&gt;</tt> is `true`.</ins>
+<p/>
+-22- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class... Us&gt;
+  constexpr explicit indirect(allocator_arg_t, const Allocator&amp; a,
+                              in_place_t, Us&amp;&amp; ...us);
+</pre>
+<blockquote>
+<p>
+-23- <i><del>Constraints</del><ins>Mandates</ins></i>: <tt>is_constructible_v&lt;T, Us...&gt;</tt> is `true`
+<p/>
+-24- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class I, class... Us&gt;
+  constexpr explicit indirect(in_place_t, initializer_list&lt;I&gt; ilist, Us&amp;&amp;... us);
+</pre>
+<blockquote>
+<p>
+-25- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p><del>(25.1) &mdash; <tt>is_constructible_v&lt;T, initializer_list&lt;I&gt;&amp;, Us...&gt;</tt> is `true`, and</del></p></li>
+<li><p><del>(25.2) &mdash;</del> <tt>is_default_constructible_v&lt;Allocator&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>is_constructible_v&lt;T, initializer_list&lt;I&gt;&amp;, Us...&gt;</tt> is `true`.</ins>
+<p/>
+<p/>
+-26- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class I, class... Us&gt;
+  constexpr explicit indirect(allocator_arg_t, const Allocator&amp; a,
+                              in_place_t, initializer_list&lt;I&gt; ilist, Us&amp;&amp;... us);
+</pre>
+<blockquote>
+<p>
+-27- <i><del>Constraints</del><ins>Mandates</ins></i>: <tt>is_constructible_v&lt;T, initializer_list&lt;I&gt;&amp;, Us...&gt;</tt> 
+is `true`
+<p/>
+-28- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+
+</blockquote>
+
+</li>
+
+<li><p>Modify <sref ref="[polymorphic.ctor]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+template&lt;class U = T&gt;
+  constexpr explicit polymorphic(U&amp;&amp; u);
+</pre>
+<blockquote>
+<p>
+-12- <i>Constraints</i>: Where `UU` is <tt>remove_cvref_t&lt;U&gt;</tt>,
+</p>
+<ol style="list-style-type: none">
+<li><p>(12.1) &mdash; <tt>is_same_v&lt;UU, polymorphic&gt;</tt> is `false`,</p></li>
+<li><p><del>(12.2) &mdash; <tt>derived_from&lt;UU, T&gt;</tt> is `true`,</del></p></li>
+<li><p>(12.3) &mdash; <tt>is_constructible_v&lt;UU, U&gt;</tt> is `true`,</p></li>
+<li><p>(12.4) &mdash; <tt>is_copy_constructible_v&lt;UU&gt;</tt> is `true`,</p></li>
+<li><p>(12.5) &mdash; `UU` is not a specialization of `in_place_type_t`, and</p></li>
+<li><p>(12.6) &mdash; <tt>is_default_constructible_v&lt;Allocator&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>derived_from&lt;UU, T&gt;</tt> is `true`.</ins>
+<p/>
+-13- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class U = T&gt;
+  constexpr explicit polymorphic(allocator_arg_t, const Allocator&amp; a, U&amp;&amp; u);
+</pre>
+<blockquote>
+<p>
+-14- <i>Constraints</i>: Where `UU` is <tt>remove_cvref_t&lt;U&gt;</tt>,
+</p>
+<ol style="list-style-type: none">
+<li><p>(14.1) &mdash; <tt>is_same_v&lt;UU, polymorphic&gt;</tt> is `false`,</p></li>
+<li><p><del>(14.2) &mdash; <tt>derived_from&lt;UU, T&gt;</tt> is `true`,</del></p></li>
+<li><p>(14.3) &mdash; <tt>is_constructible_v&lt;UU, U&gt;</tt> is `true`,</p></li>
+<li><p>(14.4) &mdash; <tt>is_copy_constructible_v&lt;UU&gt;</tt> is `true`, and</p></li>
+<li><p>(14.5) &mdash; `UU` is not a specialization of `in_place_type_t`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>derived_from&lt;UU, T&gt;</tt> is `true`.</ins>
+<p/>
+-15- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class U, class... Ts&gt;
+  constexpr explicit polymorphic(in_place_type_t&lt;U&gt;, Ts&amp;&amp;... ts);
+</pre>
+<blockquote>
+<p>
+-16- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p>(16.1) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, U&gt;</tt> is `true`,</p></li>
+<li><p><del>(16.2) &mdash; <tt>derived_from&lt;U, T&gt;</tt> is `true`,</del></p></li>
+<li><p>(16.3) &mdash; <tt>is_constructible_v&lt;U, Ts&gt;</tt> is `true`,</p></li>
+<li><p>(16.4) &mdash; <tt>is_copy_constructible_v&lt;U&gt;</tt> is `true`, and</p></li>
+<li><p>(16.5) &mdash; <tt>is_default_constructible_v&lt;Allocator&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>derived_from&lt;U, T&gt;</tt> is `true`.</ins>
+<p/>
+-17- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class U, class... Ts&gt;
+  constexpr explicit polymorphic(allocator_arg_t, const Allocator&amp; a,
+                                 in_place_type_t&lt;U&gt;, Ts&amp;&amp;... ts);
+</pre>
+<blockquote>
+<p>
+-18- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p>(18.1) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, U&gt;</tt> is `true`,</p></li>
+<li><p><del>(18.2) &mdash; <tt>derived_from&lt;U, T&gt;</tt> is `true`,</del></p></li>
+<li><p>(18.3) &mdash; <tt>is_constructible_v&lt;U, Ts&gt;</tt> is `true`, and</p></li>
+<li><p>(18.4) &mdash; <tt>is_copy_constructible_v&lt;U&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>derived_from&lt;U, T&gt;</tt> is `true`.</ins>
+<p/>
+-19- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class U, class I, class... Us&gt;
+  constexpr explicit polymorphic(in_place_type_t&lt;U&gt;, initializer_list&lt;I&gt; ilist, Us&amp;&amp;... us);
+</pre>
+<blockquote>
+<p>
+-20- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p>(20.1) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, U&gt;</tt> is `true`,</p></li>
+<li><p><del>(20.2) &mdash; <tt>derived_from&lt;U, T&gt;</tt> is `true`,</del></p></li>
+<li><p>(20.3) &mdash; <tt>is_constructible_v&lt;U, initializer_list&lt;I&gt;&amp;, Us...&gt;</tt> is `true`,</p></li>
+<li><p>(20.4) &mdash; <tt>is_copy_constructible_v&lt;U&gt;</tt> is `true`, and</p></li>
+<li><p>(20.5) &mdash; <tt>is_default_constructible_v&lt;Allocator&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>derived_from&lt;U, T&gt;</tt> is `true`.</ins>
+<p/>
+-21- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+<pre>
+template&lt;class U, class I, class... Us&gt;
+  constexpr explicit polymorphic(in_place_type_t&lt;U&gt;, initializer_list&lt;I&gt; ilist, Us&amp;&amp;... us);
+</pre>
+<blockquote>
+<p>
+-22- <i>Constraints</i>:
+</p>
+<ol style="list-style-type: none">
+<li><p>(22.1) &mdash; <tt>is_same_v&lt;remove_cvref_t&lt;U&gt;, U&gt;</tt> is `true`,</p></li>
+<li><p><del>(22.2) &mdash; <tt>derived_from&lt;U, T&gt;</tt> is `true`,</del></p></li>
+<li><p>(22.3) &mdash; <tt>is_constructible_v&lt;U, initializer_list&lt;I&gt;&amp;, Us...&gt;</tt> is `true`, and</p></li>
+<li><p>(22.4) &mdash; <tt>is_copy_constructible_v&lt;U&gt;</tt> is `true`.</p></li>
+</ol>
+<p>
+<ins>-?- <i>Mandates:</i> <tt>derived_from&lt;U, T&gt;</tt> is `true`.</ins>
+<p/>
+-23- <i>Effects</i>: [&hellip;]
+</p>
+</blockquote>
+</blockquote>
+
+</li>
+
+</ol>
+</resolution>
+
+</issue>