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    CWG Issue 1275</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="1275"></A><H4>1275.
  
Incorrect comment in example of template parameter pack restriction
</H4>
<B>Section: </B>13.2&#160; [<A href="https://wg21.link/temp.param">temp.param</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>Johannes Schaub
 &#160;&#160;&#160;

 <B>Date: </B>2011-03-25<BR>


<P>[Voted into the WP at the February, 2012 meeting;
moved to DR at the October, 2012 meeting.]</P>



<P>The following example from 13.2 [<A href="https://wg21.link/temp.param#11">temp.param</A>] paragraph 11
is incorrect:</P>

<PRE>
  //<SPAN CLASS="cmnt"> </SPAN>U<SPAN CLASS="cmnt"> cannot be deduced or specified</SPAN>
  template&lt;class... T, class... U&gt; void f() { }
  template&lt;class... T, class U&gt; void g() { }
</PRE>

<P>In fact, <TT>U</TT> can be deduced to an empty sequence by
13.10.2 [<A href="https://wg21.link/temp.arg.explicit#3">temp.arg.explicit</A>] paragraph 3:</P>

<BLOCKQUOTE>

A trailing template parameter pack (13.7.4 [<A href="https://wg21.link/temp.variadic">temp.variadic</A>]) not
otherwise deduced will be deduced to an empty sequence of template
arguments.

</BLOCKQUOTE>

<P><B>Proposed resolution (August, 2011):</B></P>

<P>Change 13.2 [<A href="https://wg21.link/temp.param#11">temp.param</A>] paragraph 11 as follows:</P>

<BLOCKQUOTE>

<P>...A template parameter pack of a function template shall not be
followed by another template parameter unless that template parameter
can be deduced <INS>from the <I>parameter-type-list</I> of the
function template</INS> or has a default argument (13.10.3 [<A href="https://wg21.link/temp.deduct">temp.deduct</A>]).  [<I>Example:</I>
</P>

<PRE>
  template&lt;class T1 = int, class T2&gt; class B;   //<SPAN CLASS="cmnt"> error</SPAN>

  //<SPAN CLASS="cmnt"> </SPAN>U<SPAN CLASS="cmnt"> cannot be deduced <INS>from the <I>parameter-type-list</I></INS> or specified</SPAN>
  template&lt;class... T, class... U&gt; void f() { } <INS>//<SPAN CLASS="cmnt"> error</SPAN></INS>
  template&lt;class... T, class U&gt; void g() { }    <INS>//<SPAN CLASS="cmnt"> error</SPAN></INS>
</PRE>

<P>&#8212;<I>end example</I>]</P>

</BLOCKQUOTE>

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