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    CWG Issue 1495</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="1495"></A><H4>1495.
  
Partial specialization of variadic class template
</H4>
<B>Section: </B>13.7.6&#160; [<A href="https://wg21.link/temp.spec.partial">temp.spec.partial</A>]
 &#160;&#160;&#160;

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

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

 <B>Date: </B>2012-04-16<BR>


<P>[Moved to DR at the April, 2013 meeting.]</P>



<P>Consider an example like</P>

<PRE>
  template &lt;int B, typename Type1, typename... Types&gt;
  struct A;

  template&lt;typename... Types&gt;
  struct A&lt;0, Types...&gt; { };

  A&lt;0,int,int&gt; t;
</PRE>

<P>In this case, the partial specialization seems well-formed by the
rules in 13.7.6 [<A href="https://wg21.link/temp.spec.partial">temp.spec.partial</A>], but it is not more specialized
than the primary template.  However, 13.7.6.2 [<A href="https://wg21.link/temp.spec.partial.match">temp.spec.partial.match</A>]
says that if exactly one matching specialization is found, it is used,
which suggests that the testcase is well-formed.  That seems
undesirable; I think a partial specialization that is not more
specialized than the primary template should be ill-formed.
</P>

<P>If the example is rewritten so that both versions are partial
specializations, i.e.,</P>

<PRE>
  template &lt;int B, typename... Types&gt;
  struct A;

  template &lt;int B, typename Type1, typename... Types&gt;
  struct A&lt;B, Type1, Types...&gt; { }

  template&lt;typename... Types&gt;
  struct A&lt;0, Types...&gt; { };

  A&lt;0,int,int&gt; t;
</PRE>

<P>There is implementation variance, with gcc and clang reporting an
ambiguity and EDG choosing the second specialization.</P>

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

<P>Add the following as a new bullet in 13.7.6.1 [<A href="https://wg21.link/temp.spec.partial.general#9">temp.spec.partial.general</A>] paragraph 9:
</P>

<UL>
<LI><P>...</P></LI>

<LI><P>The argument list of the specialization shall not be identical to the
implicit argument list of the primary template.</P></LI>

<LI><P><INS>The specialization shall be more specialized than the primary
template (13.7.6.3 [<A href="https://wg21.link/temp.spec.partial.order">temp.spec.partial.order</A>]).</INS></P></LI>

<LI><P>...</P></LI>

</UL>

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