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    CWG Issue 1158</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="1158"></A><H4>1158.
  
Recursive instantiation via alias template
</H4>
<B>Section: </B>13.7.8&#160; [<A href="https://wg21.link/temp.alias">temp.alias</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>US
 &#160;&#160;&#160;

 <B>Date: </B>2010-08-03<BR><BR>


<P>[Voted into the WP at the November, 2010 meeting.]</P>

<A href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3296.html#US73">N3092 comment
  US&#160;73<BR></A>

<P>The current wording of 9.2.4 [<A href="https://wg21.link/dcl.typedef#2">dcl.typedef</A>] paragraph 2
requires that the identifier in an
<I>alias-declaration</I> &#8220;...shall not appear in the
<I>type-id</I>.&#8221; With template aliases, however, the
name of the alias can be used indirectly:</P>

<PRE>
    template&lt;typename T&gt; struct A;
    template&lt;typename T&gt; using B=typename A&lt;T&gt;::U;
    template&lt;typename T&gt; struct A {
      typedef B&lt;T&gt; U;
    };
    B&lt;short&gt; b;
</PRE>

<P>Here the instantiation of <TT>B&lt;short&gt;</TT> causes
the instantiation of <TT>A&lt;short&gt;</TT>, and the
typedef in <TT>A&lt;short&gt;</TT> ends up attempting to use
<TT>B&lt;short&gt;</TT>, which is still in the process of
being instantiated.</P>

<P>There should be an explicit prohibition of such uses.</P>

<P><B>Proposed resolution (August, 2010):</B></P>

<P>Add the following as a new paragraph at the end of
13.7.8 [<A href="https://wg21.link/temp.alias">temp.alias</A>]:</P>

<BLOCKQUOTE>

<P><INS>The <I>type-id</I> in an alias template declaration
shall not refer to the alias template being declared. The
type produced by an alias template specialization shall not
directly or indirectly make use of that
specialization. [<I>Example:</I></INS></P>

<PRE>
<INS>  template &lt;class T&gt; struct A;
  template &lt;class T&gt; using B = typename A&lt;T&gt;::U;
  template &lt;class T&gt; struct A {
    typedef B&lt;T&gt; U;
  };
  B&lt;short&gt; b; //<SPAN CLASS="cmnt"> Error: instantiation of </SPAN>B&lt;short&gt;<SPAN CLASS="cmnt"> uses own type via </SPAN>A&lt;short&gt;::U.</INS>
</PRE>

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

</BLOCKQUOTE>

<I>[Note: this wording assumes the change from &#8220;template
alias&#8221; to &#8220;alias template&#8221; requested by comment
FI 11 on FCD N3092.]</I>

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