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<title>Issue 3632: unique_ptr "Mandates: This constructor is not selected by class template argument deduction"</title>
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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#C++23">C++23</a> status.</em></p>
<h3 id="3632"><a href="lwg-defects.html#3632">3632</a>. <code>unique_ptr</code> "<i>Mandates:</i> This constructor is not selected by class template argument deduction"</h3>
<p><b>Section:</b> 20.3.1.3.2 <a href="https://wg21.link/unique.ptr.single.ctor">[unique.ptr.single.ctor]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Arthur O'Dwyer <b>Opened:</b> 2021-11-03 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#unique.ptr.single.ctor">issues</a> in [unique.ptr.single.ctor].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<a href="https://wg21.link/P1460R1" title=" Mandating the Standard Library: Clause 20 - Utilities library">P1460R1</a> changed the wording for <code>unique_ptr</code>'s constructor <code>unique_ptr(pointer)</code>.
In C++17, it said in [unique.ptr.single.ctor] p5:
</p>
<blockquote>
<pre>
explicit unique_ptr(pointer p) noexcept;
</pre>
<blockquote>
<p>
<i>Preconditions:</i> [&hellip;]
<p/>
<i>Effects:</i> [&hellip;]
<p/>
<i>Postconditions:</i> [&hellip;]
<p/>
<b><i>Remarks:</i></b> If <code>is_pointer_v&lt;deleter_type&gt;</code> is <code>true</code> or 
<code>is_default_constructible_v&lt;deleter_type&gt;</code> is <code>false</code>, this constructor shall not 
participate in overload resolution. <b>If class template argument deduction would select the function 
template corresponding to this constructor, then the program is ill-formed</b>.
</p>
</blockquote>
</blockquote>
<p>
In C++20, it says in [unique.ptr.single.ctor] p5:
</p>
<blockquote>
<pre>
explicit unique_ptr(pointer p) noexcept;
</pre>
<blockquote>
<p>
<i>Constraints:</i> <code>is_pointer_v&lt;deleter_type&gt;</code> is <code>false</code> and 
<code>is_default_constructible_v&lt;deleter_type&gt;</code> is <code>true</code>.
<p/>
<b><i>Mandates:</i> This constructor is not selected by class template argument deduction.</b>
<p/>
<i>Preconditions:</i> [&hellip;]
<p/>
<i>Effects:</i> [&hellip;]
<p/>
<i>Postconditions:</i> [&hellip;]
</p>
</blockquote>
</blockquote>
<p>
Normally, we use "<i>Mandates:</i>" for <code>static_assert</code>-like stuff, not just to indicate that 
some constructor doesn't contribute to CTAD. Both libstdc++ and Microsoft (and soon libc++, see 
<a href="https://reviews.llvm.org/D112904">LLVM issue</a>) seem to agree about the intent of this wording: 
It's basically asking for the constructor to be implemented with a CTAD firewall, as
</p>
<blockquote>
<pre>
explicit unique_ptr(type_identity_t&lt;pointer&gt; p) noexcept;
</pre>
</blockquote>
<p>
and there is no actual <code>static_assert</code> corresponding to this "<i>Mandates:</i>" element. In particular, 
the following program is well-formed on all vendors:
</p>
<blockquote>
<pre>
// <a href="https://godbolt.org/z/h5bb1WbTz">godbolt link</a>
template&lt;class T&gt; auto f(T p) -&gt; decltype(std::unique_ptr(p));
template&lt;class T&gt; constexpr bool f(T p) { return true; }  
static_assert(f((int*)nullptr));
</pre>
</blockquote>
<p>
I claim that this is a confusing and/or wrong use of "<i>Mandates:</i>". My proposed resolution is 
simply to respecify the constructor as
</p>
<blockquote>
<pre>
explicit unique_ptr(<b>type_identity_t&lt;pointer&gt;</b> p) noexcept;
</pre>
<blockquote>
<p>
<i>Constraints:</i> <code>is_pointer_v&lt;deleter_type&gt;</code> is <code>false</code> and 
<code>is_default_constructible_v&lt;deleter_type&gt;</code> is <code>true</code>.
<p/>
<i>Preconditions:</i> [&hellip;]
<p/>
<i>Effects:</i> [&hellip;]
<p/>
<i>Postconditions:</i> [&hellip;]
</p>
</blockquote>
</blockquote>
<p>
with no <i>Mandates:</i> or <i>Remarks:</i> elements at all.
</p>

<p><i>[2022-01-29; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3632"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

<ol>

<li><p>Modify 20.3.1.3.1 <a href="https://wg21.link/unique.ptr.single.general">[unique.ptr.single.general]</a>, class template <code>unique_ptr</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 20.3.1.3.2 <a href="https://wg21.link/unique.ptr.single.ctor">[unique.ptr.single.ctor]</a>, constructors</i>
constexpr unique_ptr() noexcept;
explicit unique_ptr(<ins>type_identity_t&lt;</ins>pointer<ins>&gt;</ins> p) noexcept;
unique_ptr(<ins>type_identity_t&lt;</ins>pointer<ins>&gt;</ins> p, <i>see below</i> d1) noexcept;
unique_ptr(<ins>type_identity_t&lt;</ins>pointer<ins>&gt;</ins> p, <i>see below</i> d2) noexcept;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 20.3.1.3.2 <a href="https://wg21.link/unique.ptr.single.ctor">[unique.ptr.single.ctor]</a> as indicated:</p>

<blockquote>
<pre>
explicit unique_ptr(<ins>type_identity_t&lt;</ins>pointer<ins>&gt;</ins> p) noexcept;
</pre>
<blockquote>
<p>
-5- <i>Constraints:</i> <code>is_pointer_v&lt;deleter_type&gt;</code> is <code>false</code> and 
<code>is_default_constructible_v&lt;deleter_type&gt;</code> is <code>true</code>.
<p/>
<del>-6- <i>Mandates:</i> This constructor is not selected by class template argument 
deduction (12.2.2.9 <a href="https://wg21.link/over.match.class.deduct">[over.match.class.deduct]</a>).</del>
<p/>
-7- <i>Preconditions:</i> [&hellip;]
<p/>
-8- <i>Effects:</i> [&hellip;]
<p/>
-9- <i>Postconditions:</i> [&hellip;]
</p>
</blockquote>
<pre>
unique_ptr(<ins>type_identity_t&lt;</ins>pointer<ins>&gt;</ins> p, const D&amp; d) noexcept;
unique_ptr(<ins>type_identity_t&lt;</ins>pointer<ins>&gt;</ins> p, remove_reference_t&lt;D&gt;&amp;&amp; d) noexcept;
</pre>
<blockquote>
<p>
-10- <i>Constraints:</i> <code>is_constructible_v&lt;D, decltype(d)&gt;</code> is <code>true</code>.
<p/>
<del>-11- <i>Mandates:</i> These constructors are not selected by class template argument 
deduction (12.2.2.9 <a href="https://wg21.link/over.match.class.deduct">[over.match.class.deduct]</a>).</del>
<p/>
-12- <i>Preconditions:</i> [&hellip;]
<p/>
-13- <i>Effects:</i> [&hellip;]
<p/>
-14- <i>Postconditions:</i> [&hellip;]
<p/>
-15- <i>Remarks:</i> If <code>D</code> is a reference type, the second constructor is defined as deleted.
<p/>
-16- [<i>Example 1</i>: [&hellip;] &mdash; <i>end example</i>]
</p>
</blockquote>
</blockquote>
</li>

</ol>





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