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    CWG Issue 2717</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="2717"></A><H4>2717.
  
Pack expansion for <I>alignment-specifier</I>
</H4>
<B>Section: </B>13.7.4&#160; [<A href="https://wg21.link/temp.variadic">temp.variadic</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD7
 &#160;&#160;&#160;

 <B>Submitter: </B>Jim X
 &#160;&#160;&#160;

 <B>Date: </B>2023-04-11<BR>


<P>[Accepted as a DR at the June, 2023 meeting.]</P>

<P>Subclause 13.7.4 [<A href="https://wg21.link/temp.variadic#11">temp.variadic</A>] paragraph 11 specifies:</P>

<BLOCKQUOTE>

The instantiation of any other pack expansion produces a list of
elements E1, E2, ... , EN.

</BLOCKQUOTE>

<P>Consider this example:</P>

<PRE>
  template&lt;class ...T&gt;
  struct Align{
   alignas(T...) unsigned char buffer[128];
  };
  Align&lt;int, short&gt; a;
</PRE>

<P>The pack expansion of the <I>alignment-specifier</I>
yields <TT>alignas(int), alignas(short)</TT>, an ill-formed list per
the grammar in 9.13.1 [<A href="https://wg21.link/dcl.attr.grammar">dcl.attr.grammar</A>].</P>

<P><B>Proposed resolution (approved by CWG 2023-04-28):</B></P>

<P>Insert a new paragraph after 13.7.4 [<A href="https://wg21.link/temp.variadic#9">temp.variadic</A>] paragraph 9 as follows:</P>

<BLOCKQUOTE>

<P>The instantiation of a sizeof... expression
(7.6.2.5 [<A href="https://wg21.link/expr.sizeof">expr.sizeof</A>]) produces an integral constant with
value N.</P>

<P class="ins">
The instantiation of an <I>alignment-specifier</I> with an ellipsis
produces <TT>E<sub>1</sub> E<sub>2</sub>
... E<sub><I>N</I></sub></TT>.
</P>

</BLOCKQUOTE>

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