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<!DOCTYPE html>
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<html lang="en">
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<meta charset="utf-8">
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<title>Issue 4256: Incorrect constrains for function_ref constructors from nontype_t</title>
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<meta property="og:title" content="Issue 4256: Incorrect constrains for function_ref constructors from nontype_t">
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<meta property="og:description" content="C++ library issue. Status: New">
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
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<h3 id="4256"><a href="lwg-active.html#4256">4256</a>. Incorrect constrains for <code>function_ref</code> constructors from <code>nontype_t</code></h3>
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<p><b>Section:</b> 22.10.17.6.3 <a href="https://wg21.link/func.wrap.ref.ctor">[func.wrap.ref.ctor]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Tomasz Kami&nacute;ski <b>Opened:</b> 2025-05-14 <b>Last modified:</b> 2025-05-14</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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For the following class:
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</p>
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<pre>
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struct M
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{
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void operator();
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};
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</pre>
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<p>
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The constructor of <code>function_ref&lt;void()&gt;</code> from <code>nontype_t</code>
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is considered to be valid candidate
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(is_constructible_v&lt;function_ref&lt;void()&gt;, nontype_t&lt;M{}&gt;&gt; is <code>true</code>),
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despite the fact that the corresponding invocation of template argument object, that is const lvalue,
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is ill-formed. As consequence we produce hard error from inside of this constructor.
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</p>
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<p>
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This is caused by the fact that for constructors with non-type <code>auto f</code> parameter,
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we are checking if <code><i>is-invocable-using</i>&lt;F&gt;</code> is <code>true</code>,
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where <code>F</code> is <code>decltype(f)</code> i.e. <code>M</code> for the example.
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We should use <code>const F&amp;</code> or <code>decltype((f))</code>.
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</p>
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<p id="res-4256"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 22.10.17.6.3 <a href="https://wg21.link/func.wrap.ref.ctor">[func.wrap.ref.ctor]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;auto f&gt; constexpr function_ref(nontype_t&lt;f&gt;) noexcept;
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</pre>
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<blockquote>
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<p>-8- Let <code>F</code> be <code>decltype(f)</code>.</p>
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<p>-9- <i>Constraints</i>: <code><i>is-invocable-using</i>&lt;<ins>const </ins>F<ins>&amp;</ins>&gt;</code> is <code>true</code>.</p>
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[&hellip;]
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</blockquote>
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<pre>
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template&lt;auto f, class U&gt; constexpr function_ref(nontype_t&lt;f&gt;, U&amp;&amp; obj) noexcept;
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</pre>
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<blockquote>
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<p>-12- Let <code>T</code> be <code>remove_reference_t&lt;U&gt;</code> and <code>F</code> be <code>decltype(f)</code>.</p>
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<p>-13- <i>Constraints:</i>:</p>
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<ul>
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<li><p>(13.1) <code>is_rvalue_reference_v&lt;U&amp;&amp;&gt;</code> is false, and</p></li>
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<li><p>(13.2) <code><i>is-invocable-using</i>&lt;<ins>const </ins>F<ins>&amp;</ins>, T <i>cv</i>&amp;&gt;</code> is <code>true</code>.</p></li>
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</ul>
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[&hellip;]
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</blockquote>
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<pre>
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template&lt;auto f, class T&gt; constexpr function_ref(nontype_t&lt;f&gt;, T <i>cv</i>* obj) noexcept;
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</pre>
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<blockquote>
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<p>-17- Let <code>F</code> be <code>decltype(f)</code>.</p>
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<p>-16- <i>Constraints</i>: <code><i>is-invocable-using</i>&lt;<ins>const </ins>F<ins>&amp;</ins>, T <i>cv</i>*&gt;</code> is <code>true</code>.</p>
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[&hellip;]
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</blockquote>
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</blockquote>
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</li>
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</ol>
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</body>
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</html>

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