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    CWG Issue 692</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="692"></A><H4>692.
  
Partial ordering of variadic class template partial specializations
</H4>
<B>Section: </B>13.10.3.6&#160; [<A href="https://wg21.link/temp.deduct.type">temp.deduct.type</A>]
 &#160;&#160;&#160;

 <B>Status: </B>C++11
 &#160;&#160;&#160;

 <B>Submitter: </B>Doug Gregor
 &#160;&#160;&#160;

 <B>Date: </B>16 April, 2008<BR>


<P>[Voted into the WP at the March, 2011 meeting as paper N3281.]</P>



<P>13.10.3.6 [<A href="https://wg21.link/temp.deduct.type#22">temp.deduct.type</A>] paragraph 22 describes how we
cope with partial ordering between two function templates that
differ because one has a function parameter pack while the other
has a normal function parameter.  However, this paragraph was
meant to apply to template parameter packs as well, e.g., to help
with partial ordering of class template partial specializations:</P>

<PRE>
   template &lt;class T1, class ...Z&gt; class S; // #1
   template &lt;class T1, class ...Z&gt; class S&lt;T1, const Z&amp;...&gt; {}; // #2
   template &lt;class T1, class T2&gt; class S&lt;T1, const T2&amp;&gt; {};; // #3
   S&lt;int, const int&amp;&gt; s; // both #2 and #3 match; #3 is more specialized
</PRE>

<P>(See also <A HREF="818.html">issue 818</A>.)</P>

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

<P>Change 13.10.3.6 [<A href="https://wg21.link/temp.deduct.type">temp.deduct.type</A>] paragraphs 9-10 as follows
(and add the example above to paragraph 9):</P>

<BLOCKQUOTE>

<P>If <TT>P</TT> has a form that contains &lt;<TT>T</TT>&gt; or
&lt;<TT>i</TT>&gt;, then each argument
<TT>P</TT><SUB><I>i</I></SUB> of the respective template argument
list of <TT>P</TT> is compared with the corresponding argument
<TT>A</TT><SUB><I>i</I></SUB> of the corresponding template
argument list of <TT>A</TT>. If the template argument list of
<TT>P</TT> contains a pack expansion that is not the last
template argument, the entire template argument list is a
non-deduced context. If <TT>P</TT><SUB><I>i</I></SUB> is a pack
expansion, then the pattern of <TT>P</TT><SUB><I>i</I></SUB> is
compared with each remaining argument in the template argument
list of <TT>A</TT>. Each comparison deduces template arguments
for subsequent positions in the template parameter packs expanded
by <TT>P</TT><SUB><I>i</I></SUB>. <INS>During partial ordering
(13.10.3.5 [<A href="https://wg21.link/temp.deduct.partial">temp.deduct.partial</A>]), if <TT>A</TT><SUB><I>i</I></SUB>
was originally a pack expansion and <TT>P</TT><SUB><I>i</I></SUB>
is not a pack expansion, or if <TT>P</TT> does not contain a
template argument corresponding to <TT>A</TT><SUB><I>i</I></SUB>,
argument deduction fails.</INS>
</P>

<P>Similarly, if <TT>P</TT> has a form that contains
<TT>(T)</TT>, then each parameter type
<TT>P</TT><SUB><I>i</I></SUB> of the respective
<I>parameter-type-list</I> of <TT>P</TT> is compared with the
corresponding parameter type <TT>A</TT><SUB><I>i</I></SUB> of the
corresponding <I>parameter-type-list</I> of <TT>A</TT>. If the
<I>parameter-declaration</I> corresponding to
<TT>P</TT><SUB><I>i</I></SUB> is a function parameter pack, then
the type of its <I>declarator-id</I> is compared with each
remaining parameter type in the <I>parameter-type-list</I> of
<TT>A</TT>. Each comparison deduces template arguments for
subsequent positions in the template parameter packs expanded by
the function parameter pack. <INS>During partial ordering
(13.10.3.5 [<A href="https://wg21.link/temp.deduct.partial">temp.deduct.partial</A>]), if <TT>A</TT><SUB><I>i</I></SUB>
was originally a function parameter pack and
<TT>P</TT><SUB><I>i</I></SUB> is not a function parameter
pack, or if <TT>P</TT> does not contain a function parameter type
corresponding to <TT>A</TT><SUB><I>i</I></SUB>, argument deduction
fails.</INS> [<I>Note:</I> A function parameter pack
can only occur at the end of a <I>parameter-declaration-list</I>
(9.3.4.6 [<A href="https://wg21.link/dcl.fct">dcl.fct</A>]). &#8212;<I>end note</I>]</P>

</BLOCKQUOTE>

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