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<title>Issue 3486: is_constructible&lt;T[], T...&gt; may be misleading in C++20</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#LEWG">LEWG</a> status.</em></p>
<h3 id="3486"><a href="lwg-active.html#3486">3486</a>. <code>is_constructible&lt;T[], T...&gt;</code> may be misleading in C++20</h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> <b>Status:</b> <a href="lwg-active.html#LEWG">LEWG</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2020-10-01 <b>Last modified:</b> 2020-10-01</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#meta.unary.prop">active issues</a> in [meta.unary.prop].</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.unary.prop">issues</a> in [meta.unary.prop].</p>
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<p><b>Discussion:</b></p>
<p>
According to the current wording,
<code>std::is_constructible&lt;int[], int&gt;</code>
should be true, because the preconditions are met
(all types are complete types or unbounded arrays)
and the variable definition is well-formed since C++20:
</p>
<blockquote><pre>
using T = int[];
T t(declval&lt;int&gt;()); <i>// equiv. to</i> int t[] = {1};
</pre></blockquote>

<p>
However, this doesn't construct an object of type <code>int[]</code>
because it deduces the array bound from the initializers,
and so constructs <code>int[1]</code>, which is not the type being asked about.
It seems more logical for the trait to give a false result
for an unbounded array, because it's an incomplete type,
and no <code>int[]</code> can ever be constructed.
</p>

<p>
On the reflector Tim Song noted:
</p>
<blockquote>
On the other hand, the result is something to which an <code>int(&amp;)[]</code>
can be bound directly thanks to another C++20 change,
so a lot of things might Just Work (for some definition of "Work")
despite the type difference.
</blockquote>

<p>
This seems to me a reasonable rationale for
<code>is_constructible&lt;int(&amp;&amp;)[], int&gt;</code>
to be true (which it is today),
but not for <code>int[]</code>.
</p>

<p>
Peter Dimov replied:
</p>
<blockquote>
Placement new, which is often the way to construct we're interested in,
is not going to work even for <code>T[2]</code>.
</blockquote>

<p>For example:</p>
<blockquote><pre>
using T2 = int[2];
T2 x;
new(x) T2(1, 2); <i>// ill-formed</i>
</pre></blockquote>

<p>
We need to decide what behaviour we want here.
Do we just want <code>is_constructible</code>
to reflect the <code>T(declval&lt;Args...&gt;);</code> construct
as currently specified in 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> p8,
or do we want to give a more useful/meaningful answer here?
</p>

<p>
Should we revisit 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> p8
in the light of parenthesized aggregate init,
so that <code>is_constructible&lt;T[], T&gt;</code>
and <code>is_constructible&lt;T[1], T&gt;</code> are false?
</p>

<p>
There may be some interaction with LWG <a href="lwg-defects.html#3436" title="std::construct_at should support arrays (Status: WP)">3436</a><sup><a href="https://cplusplus.github.io/LWG/issue3436" title="Latest snapshot">(i)</a></sup>.
</p>


<p id="res-3486"><b>Proposed resolution:</b></p>
<p>
</p>





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