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    CWG Issue 1893</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="1893"></A><H4>1893.
  
Function-style cast with <I>braced-init-list</I>s and empty pack expansions
</H4>
<B>Section: </B>7.6.1.4&#160; [<A href="https://wg21.link/expr.type.conv">expr.type.conv</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD5
 &#160;&#160;&#160;

 <B>Submitter: </B>Richard Smith
 &#160;&#160;&#160;

 <B>Date: </B>2014-03-13<BR>


<P>[Accepted as a DR at the March, 2018 (Jacksonville) meeting.]</P>



<P>According to 7.6.1.4 [<A href="https://wg21.link/expr.type.conv#1">expr.type.conv</A>] paragraph 1,</P>

<BLOCKQUOTE>

A <I>simple-type-specifier</I> (9.2.9.3 [<A href="https://wg21.link/dcl.type.simple">dcl.type.simple</A>])
or <I>typename-specifier</I> (13.8 [<A href="https://wg21.link/temp.res">temp.res</A>]) followed by a
parenthesized <I>expression-list</I> constructs a value of the specified
type given the expression list. If the expression list is a single
expression, the type conversion expression is equivalent (in definedness,
and if defined in meaning) to the corresponding cast expression
(7.6.3 [<A href="https://wg21.link/expr.cast">expr.cast</A>]). If the type specified is a class type, the
class type shall be complete. If the expression list specifies more than a
single value, the type shall be a class with a suitably declared
constructor (9.5 [<A href="https://wg21.link/dcl.init">dcl.init</A>], 11.4.5 [<A href="https://wg21.link/class.ctor">class.ctor</A>]), and the
expression <TT>T(x1, x2, ...)</TT> is equivalent in effect to the
declaration <TT>T t(x1, x2, ...);</TT> for some invented temporary
variable <TT>t</TT>, with the result being the value of <TT>t</TT> as a
prvalue.

</BLOCKQUOTE>

<P>This does not cover the cases when the <I>expression-list</I>
contains a single <I>braced-init-list</I> (which is neither an
expression nor more than a single value) or if it contains no
expressions as the result of an empty pack expansion.</P>

<P><b>Proposed resolution (June, 2014): [SUPERSEDED]</b></P>

<P>This issue is resolved by the resolution of
<A HREF="1299.html">issue 1299</A>.</P>

<P><B>Proposed resolution (November, 2017)</B></P>

<P>Change 7.6.1.4 [<A href="https://wg21.link/expr.type.conv#2">expr.type.conv</A>] paragraph 2 as follows:</P>

<BLOCKQUOTE>

If the initializer is a parenthesized single expression, the type
conversion expression is equivalent to the corresponding cast expression
(7.6.3 [<A href="https://wg21.link/expr.cast">expr.cast</A>]). Otherwise, if the type is <I>cv</I>
<TT>void</TT> and the initializer is <TT>()</TT> <INS>(after pack
expansion, if any)</INS>, the expression is a prvalue of the specified type
that performs no initialization. Otherwise, the expression is a prvalue of
the specified type whose result object is direct-initialized
(9.5 [<A href="https://wg21.link/dcl.init">dcl.init</A>]) with the initializer. <DEL>For an expression of
the form <TT>T()</TT>, <TT>T</TT></DEL> <INS>If the initializer is a
parenthesized optional <I>expression-list</I>, the specified type</INS>
shall not be an array type.

</BLOCKQUOTE>

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