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<!DOCTYPE issue SYSTEM "lwg-issue.dtd">

<issue num="4324" status="New">

<title><code>unique_ptr&lt;void&gt;::operator*</code> is not SFINAE-friendly</title>

<section>

<sref ref="[unique.ptr.single.observers]"/>

</section>

<submitter>Hewill Kang</submitter>

<date>24 Aug 2025</date>

<priority>99</priority>

<discussion>

<p>

LWG <iref ref="2762"/> added a conditional `noexcept` specification to `unique_ptr::operator*`

to make it consistent with `shared_ptr::operator*`:

</p>

<pre>

constexpr add_lvalue_reference_t&lt;T&gt; operator*() const <ins>noexcept(noexcept(*declval&lt;pointer&gt;()))</ins>;

</pre>

<p>

This unexpectedly makes <code>unique_ptr&lt;void&gt;::operator*</code> no longer SFINAE-friendly,

<a href="https://godbolt.org/z/xd15jYPhz">for example</a>:

</p>

<blockquote><pre>

#include &lt;memory&gt;

template&lt;class T&gt; concept dereferenceable = requires(T&amp; t) { *t; };

static_assert( dereferenceable&lt;int *&gt;);

static_assert(!dereferenceable&lt;void*&gt;);

static_assert( dereferenceable&lt;std::shared_ptr&lt;int &gt;&gt;);

static_assert(!dereferenceable&lt;std::shared_ptr&lt;void&gt;&gt;);

static_assert( dereferenceable&lt;std::unique_ptr&lt;int &gt;&gt;);

static_assert( dereferenceable&lt;std::unique_ptr&lt;void&gt;&gt;); // <span style="color:#C80000;font-weight:bold">hard error</span>

</pre></blockquote>

<p>

Given that the standard intends for `operator*` of `shared_ptr` and `unique_ptr` to be

SFINAE-friendly based on <sref ref="[util.smartptr.shared.obs]"/>, regardless of the value of

`static_assert`, it is reasonable to assume that there should be no hard error here.

</p>

</discussion>

<resolution>

<p>

This wording is relative to <paper num="N5014"/>.

</p>

<ol>

<li><p>Modify <sref ref="[unique.ptr.single.observers]"/> as indicated:</p>

<blockquote>

<pre>

constexpr add_lvalue_reference_t&lt;T&gt; operator*() const noexcept(<del>noexcept(*declval&lt;pointer&gt;())</del><ins><i>see below</i></ins>);

</pre>

<blockquote>

<p>

-1- <i>Mandates</i>: <tt>reference_converts_from_temporary_v&lt;add_lvalue_reference_t&lt;T&gt;,

decltype(*declval&lt;pointer&gt;())&gt;</tt> is `false`.

<p/>

-2- <i>Preconditions</i>: `get() != nullptr` is `true`.

<p/>

-3- <i>Returns</i>: `*get()`.

<p/>

<ins>-?- <i>Remarks:</i>: The exception specification is equivalent to:</ins>

</p>

<blockquote><pre>

<ins>!requires { *declval&lt;pointer&gt;(); } || requires { { *declval&lt;pointer&gt;() } noexcept; }</ins>

</pre></blockquote>

</blockquote>

</blockquote>

</li>

</ol>

</resolution>

</issue>