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    CWG Issue 777</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="777"></A><H4>777.
  
Default arguments and parameter packs
</H4>
<B>Section: </B>9.3.4.7&#160; [<A href="https://wg21.link/dcl.fct.default">dcl.fct.default</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD2
 &#160;&#160;&#160;

 <B>Submitter: </B>Michael Wong
 &#160;&#160;&#160;

 <B>Date: </B>13 February, 2009<BR>


<P>[Voted into WP at March, 2010 meeting.]</P>

<P>9.3.4.7 [<A href="https://wg21.link/dcl.fct.default#4">dcl.fct.default</A>] paragraph 4 says,</P>

<BLOCKQUOTE>

In a given function declaration, all parameters subsequent to a
parameter with a default argument shall have default arguments
supplied in this or previous declarations.

</BLOCKQUOTE>

<P>It is not clear whether this applies to parameter packs or not.
For example, is the following well-formed?</P>

<PRE>
    template &lt;typename... T&gt; void f(int i = 0, T ...args) { }
</PRE>

<P>Note for comparison the corresponding wording in
13.2 [<A href="https://wg21.link/temp.param#11">temp.param</A>] paragraph 11 regarding template parameter
packs:</P>

<BLOCKQUOTE>

If a <I>template-parameter</I> of a class template has a default
<I>template-argument</I>, each subsequent <I>template-parameter</I>
shall either have a default <I>template-argument</I> supplied or be a
template parameter pack.

</BLOCKQUOTE>

<P><B>Proposed resolution (October, 2009):</B></P>

<P>Change 9.3.4.7 [<A href="https://wg21.link/dcl.fct.default#4">dcl.fct.default</A>] paragraph 4:</P>

<BLOCKQUOTE>

...In a given function declaration, <DEL>all</DEL> <INS>each</INS>
parameter<DEL>s</DEL> subsequent to a parameter with a default
argument shall have <INS>a</INS> default argument<DEL>s</DEL> supplied
in this or <INS>a</INS> previous declaration<DEL>s</DEL> <INS>or shall
be a function parameter pack</INS>. A default argument...

</BLOCKQUOTE>

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