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<title>Issue 3233: Broken requirements for shared_ptr converting constructors</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++20">C++20</a> status.</em></p>
<h3 id="3233"><a href="lwg-defects.html#3233">3233</a>. Broken requirements for <code>shared_ptr</code> converting constructors</h3>
<p><b>Section:</b> 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2019-07-10 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#util.smartptr.shared.const">active issues</a> in [util.smartptr.shared.const].</p>
<p><b>View all other</b> <a href="lwg-index.html#util.smartptr.shared.const">issues</a> in [util.smartptr.shared.const].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Issue <a href="lwg-defects.html#2875" title="shared_ptr::shared_ptr(Y*, D, [&hellip;]) constructors should be constrained (Status: C++17)">2875</a><sup><a href="https://cplusplus.github.io/LWG/issue2875" title="Latest snapshot">(i)</a></sup> added 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> paragraph 13:
<blockquote>
<i>Remarks</i>: When <code>T</code> is an array type, this constructor shall not participate in overload
resolution unless <code>is_move_constructible_v&lt;D&gt;</code> is <code>true</code>, the expression
<code>d(p)</code> is well-formed, and either <code>T</code> is <code>U[N]</code> and <code>Y(*)[N]</code> is
convertible to <code>T*</code>, or <code>T</code> is <code>U[]</code> and <code>Y(*)[]</code> is convertible to
<code>T*</code>. When <code>T</code> is not an array type, this constructor shall not participate in
overload resolution unless <code>is_move_constructible_v&lt;D&gt;</code> is <code>true</code>, the
expression <code>d(p)</code> is well-formed, and <code>Y*</code> is convertible to <code>T*</code>.
</blockquote>
which pertains to the four constructor overloads:
<blockquote><pre>
template&lt;class Y, class D&gt; shared_ptr(Y* p, D d);
template&lt;class Y, class D, class A&gt; shared_ptr(Y* p, D d, A a);
template&lt;class D&gt; shared_ptr(nullptr_t p, D d);
template&lt;class D, class A&gt; shared_ptr(nullptr_t p, D d, A a);
</pre></blockquote>
which is fine (ignoring for now that two occurrences of "this constructor" should read "these
constructors") for the two overloads with a template parameter <code>Y</code>, but not so much for the
two with no such template parameter.
</p>
<p>
MSVC ignores the constraints on <code>Y</code> for the overloads with no such template parameter,
whereas libstdc++ and libc++ seem to ignore <em>all</em> of the constraints for those overloads (See
<a href="https://gcc.godbolt.org/z/ECdop6">Compiler Explorer</a>). We should fix the broken wording,
ideally by requiring the MSVC interpretation - the <code>nullptr_t</code> constructors participate in
overload resolution only when <code>is_movable_v&lt;D&gt;</code> is <code>true</code> and <code>d(p)</code> is
well-formed - so concepts and traits that check constructibility are correct.
</p>

<p><i>[2019-11-17 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.</p>


<p id="res-3233"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4835">N4835</a>.</p>

<ol>
<li><p>Modify 20.3.2.2.2 <a href="https://wg21.link/util.smartptr.shared.const">[util.smartptr.shared.const]</a> as indicated:</p>

<blockquote>
<blockquote><pre>
template&lt;class Y, class D&gt; shared_ptr(Y* p, D d);
template&lt;class Y, class D, class A&gt; shared_ptr(Y* p, D d, A a);
template&lt;class D&gt; shared_ptr(nullptr_t p, D d);
template&lt;class D, class A&gt; shared_ptr(nullptr_t p, D d, A a);
</pre></blockquote>
<p>
<ins>-?- <i>Constraints:</i> <code>is_move_constructible_v&lt;D&gt;</code> is <code>true</code>, and
<code>d(p)</code> is a well-formed expression. For the first two overloads:</ins>
</p>
<ul>
  <li><p><ins>
    If <code>T</code> is an array type, then either <code>T</code> is <code>U[N]</code> and <code>Y(*)[N]</code> is
    convertible to <code>T*</code>, or <code>T</code> is <code>U[]</code> and <code>Y(*)[]</code> is convertible to
    <code>T*</code>.
  </ins></p></li>
  <li><p><ins>
    If <code>T</code> is not an array type, then <code>Y*</code> is convertible to <code>T*</code>.
  </ins></p></li>
</ul>
<p>
-9- <i><del>Requires</del><ins>Expects</ins>:</i> Construction of <code>d</code> and a deleter of type [&hellip;]
</p>
<p>[&hellip;]</p>
<p>
<del>-13- <i>Remarks:</i> When <code>T</code> is an array type, this constructor shall not participate
in overload resolution unless <code>is_move_constructible_v&lt;D&gt;</code> is <code>true</code>, the
expression <code>d(p)</code> is well-formed, and either <code>T</code> is <code>U[N]</code> and <code>Y(*)[N]</code>
is convertible to <code>T*</code>, or <code>T</code> is <code>U[]</code> and <code>Y(*)[]</code> is convertible to
<code>T*</code>. When <code>T</code> is not an array type, this constructor shall not participate in
overload resolution unless <code>is_move_constructible_v&lt;D&gt;</code> is <code>true</code>, the
expression <code>d(p)</code> is well-formed, and <code>Y*</code> is convertible to <code>T*</code>.</del>
</p>
</blockquote>
</li>
</ol>




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