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    CWG Issue 1399</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="1399"></A><H4>1399.
  
Deduction with multiple function parameter packs
</H4>
<B>Section: </B>13.10.3.2&#160; [<A href="https://wg21.link/temp.deduct.call">temp.deduct.call</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD3
 &#160;&#160;&#160;

 <B>Submitter: </B>Jason Merrill
 &#160;&#160;&#160;

 <B>Date: </B>2011-09-29<BR>


<P>[Moved to DR at the October, 2012 meeting.]</P>



<P>Consider:</P>

<PRE>
    template &lt;class... T&gt;
    void f(T..., int, T...) { }

    int main() {
       f(0);          // OK
       f&lt;int&gt;(0,0,0); // OK
       f(0,0,0);      // error
    }
</PRE>

<P>It seems clear that the third call is ill-formed because by
the time we get to the second function parameter pack we've
already assumed that <TT>T</TT> is empty, so deducing anything
for <TT>T</TT> would be nonsensical.  But I don't think this is
expressed anywhere in the standard.</P>

<P>One way to handle this would be to say that a template
parameter pack is not deducible if it is used in a function
parameter pack not at the end of the parameter list.</P>

<P><B>Proposed resolution (February, 2012):</B></P>

<P>This issue is resolved by the resolution of
<A HREF="1388.html">issue 1388</A>.</P>

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