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    CWG Issue 996</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="996"></A><H4>996.
  
Ambiguous partial specializations of member class templates
</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>C++11
 &#160;&#160;&#160;

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

 <B>Date: </B>28 Oct, 2009<BR>


<P>[Voted into the WP at the March, 2011 meeting as part of paper N3262.]</P>



<P>Given an example like</P>

<PRE>
  template&lt;typename T, typename U&gt;
  struct Outer {
    template&lt;typename X, typename Y&gt; struct Inner;
    template&lt;typename Y&gt; struct Inner&lt;T, Y&gt; {};
    template&lt;typename Y&gt; struct Inner&lt;U, Y&gt; {};
  };
  Outer&lt;int, int&gt; outer;                      // #1
  Outer&lt;int, int&gt;::Inner&lt;int, float&gt; inner;   // #2
</PRE>

<P>Is #1 ill-formed because of the identical partial specializations?
If not, presumably #2 is ill-formed because of the resulting
ambiguity (13.7.6.2 [<A href="https://wg21.link/temp.spec.partial.match#1">temp.spec.partial.match</A>] paragraph 1).</P>

<P><B>Notes from the November, 2010 meeting:</B></P>

<P>The instantiation of <TT>Outer&lt;int,int&gt;</TT> results in
duplicate declarations of the partial specialization, which are
ill-formed by 11.4 [<A href="https://wg21.link/class.mem#1">class.mem</A>] paragraph 1. No normative
change is required, but it might be helpful to add an example like
this somewhere.</P>

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