issue4256.xml

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<issue num="4256" status="New">
<title>Incorrect constrains for <code>function_ref</code> constructors from <code>nontype_t</code></title>
<section><sref ref="[func.wrap.ref.ctor]"/></section>
<submitter>Tomasz Kami&nacute;ski</submitter>
<date>14 May 2025</date>
<priority>99</priority>

<discussion>
<p>
For the following class:
</p>
<pre>
struct M
{
   void operator();
};
</pre>
<p>
The constructor of <code>function_ref&lt;void()&gt;</code> from <code>nontype_t</code>
is considered to be valid candidate
(is_constructible_v&lt;function_ref&lt;void()&gt;, nontype_t&lt;M{}&gt;&gt; is <code>true</code>),
despite the fact that the corresponding invocation of template argument object, that is const lvalue,
is ill-formed. As consequence we produce hard error from inside of this constructor.
</p>

<p>
This is caused by the fact that for constructors with non-type <code>auto f</code> parameter,
we are checking if <code><i>is-invocable-using</i>&lt;F&gt;</code> is <code>true</code>,
where <code>F</code> is <code>decltype(f)</code> i.e. <code>M</code> for the example.
We should use <code>const F&amp;</code> or <code>decltype((f))</code>.
</p>

</discussion>

<resolution>
<p>
This wording is relative to <paper num="N5008"/>.
</p>
<ol>

<li><p>Modify <sref ref="[func.wrap.ref.ctor]"/> as indicated:</p>

<blockquote>
<pre>
template&lt;auto f&gt; constexpr function_ref(nontype_t&lt;f&gt;) noexcept;
</pre>
<blockquote>
<p>-8- Let <tt>F</tt> be <tt>decltype(f)</tt>.</p>
<p>-9- <i>Constraints</i>: <tt><i>is-invocable-using</i>&lt;<ins>const </ins>F<ins>&amp;</ins>&gt;</tt> is <tt>true</tt>.</p>
[&hellip;]
</blockquote>

<pre>
template&lt;auto f, class U&gt; constexpr function_ref(nontype_t&lt;f&gt;, U&amp;&amp; obj) noexcept;
</pre>
<blockquote>
<p>-12- Let <tt>T</tt> be <tt>remove_reference_t&lt;U&gt;</tt> and <tt>F</tt> be <tt>decltype(f)</tt>.</p>
<p>-13- <i>Constraints:</i>:</p>
<ul>
  <li><p>(13.1) <tt>is_rvalue_reference_v&lt;U&amp;&amp;&gt;</tt> is false, and</p></li>
  <li><p>(13.2) <tt><i>is-invocable-using</i>&lt;<ins>const </ins>F<ins>&amp;</ins>, T <i>cv</i>&amp;&gt;</tt> is <tt>true</tt>.</p></li>
</ul>
[&hellip;]
</blockquote>

<pre>
template&lt;auto f, class T&gt; constexpr function_ref(nontype_t&lt;f&gt;, T <i>cv</i>* obj) noexcept;
</pre>
<blockquote>
<p>-17- Let <tt>F</tt> be <tt>decltype(f)</tt>.</p>
<p>-16- <i>Constraints</i>: <tt><i>is-invocable-using</i>&lt;<ins>const </ins>F<ins>&amp;</ins>, T <i>cv</i>*&gt;</tt> is <tt>true</tt>.</p>
[&hellip;]
</blockquote>
</blockquote>

</li>
</ol>

</resolution>

</issue>