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New issue to address US 102-209: Metafunctions should not be defined in terms of constant subexpressions
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‎xml/issue4428.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="4428" status="New">
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<title>Metafunctions should not be defined in terms of constant subexpressions</title>
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<section>
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<sref ref="[meta.reflection.access.queries]"/>
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<sref ref="[meta.reflection.annotation]"/>
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<sref ref="[meta.reflection.array]"/>
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</section>
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<submitter>Jonathan Wakely</submitter>
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<date>24 Oct 2025</date>
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<priority>99</priority>
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<discussion>
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<b>Addresses US 102-209</b>
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<p>
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"is a constant (sub)expression" is incorrect now that errors are reported
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via exceptions.
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</p>
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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="[meta.reflection.access.queries]"/> as indicated:</p>
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<blockquote>
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<pre>
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consteval bool has_inaccessible_nonstatic_data_members(info r, access_context ctx);
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</pre>
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<blockquote>
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<p>-5- <i>Returns</i>:
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`true` if `is_accessible(`<i>R</i>`, ctx)` is `false` for any <i>R</i>
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in `nonstatic_data_members_of(r, access_context::unchecked())`.
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Otherwise, `false`.
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</p>
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<p>-6- <i>Throws</i>:
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`meta::exception` unless
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<ul>
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<li>(6.1) &mdash;
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<ins>The evaluation of</ins>
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`nonstatic_data_members_of(r, access_context::unchecked())`
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<del>
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is a constant subexpression
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</del>
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<ins>
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would not exit via an exception
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</ins>
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and
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</li>
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<li>(6.2) &mdash;
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`r` does not represent a closure type.
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</li>
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</ul>
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</p>
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</blockquote>
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<pre>
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consteval bool has_inaccessible_bases(info r, access_context ctx);
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</pre>
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<blockquote>
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<p>-5- <i>Returns</i>:
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`true` if `is_accessible(`<i>R</i>`, ctx)` is `false` for any <i>R</i>
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in `bases_of(r, access_context::unchecked())`.
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Otherwise, `false`.
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</p>
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<p>-6- <i>Throws</i>:
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`meta::exception` unless
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<ins>The evaluation of</ins>
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`bases_of(r, access_context::unchecked())`
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<del>is a constant subexpression</del>
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<ins>would not exit via an exception</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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<li><p>Modify <sref ref="[meta.reflection.annotation]"/> as indicated:</p>
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<blockquote>
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<pre>
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consteval vector&lt;info&gt; annotations_of_with_type(info item, info type);
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</pre>
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<blockquote>
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<p>-4- <i>Returns</i>:
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A `vector` containing each element `e` of `annotations_of(item)`
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where `remove_const(type_of(e)) == remove_const(type)`
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is `true`, preserving their order.
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</p>
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<p>-5- <i>Throws</i>:
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`meta::exception` unless
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<ul>
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<li>(5.1) &mdash;
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<ins>The evaluation of</ins>
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`annotations_of(item)`
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<del>
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is a constant subexpression
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</del>
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<ins>
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would not exit via an exception
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</ins>
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and
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</li>
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<li>(5.2) &mdash;
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`dealias(type)` represents a type that is complete
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from some point in the evaluation context.
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</li>
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</ul>
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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="[meta.reflection.annotation]"/> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;ranges::input_range R&gt;
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consteval info reflect_constant_array(R&amp;&amp; r);
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</pre>
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<blockquote>
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<p>-8-
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Let `T` be <code>ranges::range_value_t&lt;R&gt;</code>.
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</p>
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<p>-9- <i>Mandates</i>:
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`T` is a structural type (<sref ref="[temp.param]"/>),
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<code>is_constructible_v&lt;T, ranges::range_reference_t&lt;R&gt;&gt;</code>
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is `true`, and
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<code>is_copy_constructible_v&lt;T&gt;</code>
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is `true`.
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</p>
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<p>-10-
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Let <i>V</i> be the pack of values of type `info` of the same size as `r`,
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where the <i>i</i><sup>th</sup> element is
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<code>reflect_constant(e<sub><i>i</i></sub>)</code>,
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where <code>e<sub><i>i</i></sub></code> is
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the <i>i</i><sup>th</sup> element of `r`.
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</p>
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<p>-11-
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Let <i>P</i> be
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<ul>
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<li>(11.1) &mdash;
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If `sizeof...(`<i>V</i>`) > 0` is `true`,
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then the template parameter object (<sref ref="[temp.param]"/>)
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of type `const T[sizeof...(`<i>V</i>`)]` initialized with `{[:V:]...}`.
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</li>
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<li>(11.2) &mdash;
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Otherwise, the template parameter object of type <code>array&lt;T, 0&gt;</code>
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initialized with `{}`.
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</li>
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</ul>
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</p>
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<p>-12- <i>Returns</i>: `^^`<i>P</i>.
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</p>
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<p>-13- <i>Throws</i>:
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`meta::exception` unless
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<ins>The evaluation of</ins>
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`reflect_constant(e)`
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<del>is a constant subexpression</del>
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<ins>would not exit via an exception</ins>
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for every element `e` of `r`.
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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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