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    CWG Issue 1056</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="1056"></A><H4>1056.
  
Template aliases, member definitions, and the current instantiation
</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>John Spicer
 &#160;&#160;&#160;

 <B>Date: </B>2010-03-17<BR>


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

<P>According to 13.8.3.2 [<A href="https://wg21.link/temp.dep.type#1">temp.dep.type</A>] paragraph 1, in a primary
class template a name refers to the current instantiation if it is the
injected-class-name or the name of the class template followed by the
template argument list of the template.  Although a <I>template-id</I>
referring to a specialization of a template alias is described as
&#8220;equivalent to&#8221; the associated type, a specialization of
a template alias is neither of the things that qualifies as naming the
current instantiation, so presumably the <TT>typename</TT> keyword in
the following example is required:</P>

<PRE>
    template &lt;class T&gt; struct A;
    template &lt;class T&gt; using B = A&lt;T&gt;;

    template &lt;class T&gt; struct A {
        struct C {};
        typename B&lt;T&gt;::C bc;  // typename needed
    };
</PRE>

<P>(However, the list in 13.8.3.2 [<A href="https://wg21.link/temp.dep.type">temp.dep.type</A>] may not be
exactly what we want; it doesn't allow use of a typedef denoting
the type of the current instantiation, either, but that presumably
should be accepted.)</P>

<P>For analogous reasons, it should not be permitted to use a
template alias as a <I>nested-name-specifier</I> when defining the
members of a class template:</P>

<PRE>
    template &lt;class T&gt; struct A {
        void g();
    };
    template &lt;class T&gt; using B = A&lt;T&gt;;
    template &lt;class T&gt; void B&lt;T&gt;::g() {} // error
</PRE>

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

<P>The CWG disagreed with the suggested direction, feeling that aliases
should work like typedefs and that the examples should be accepted.</P>

<P><B>Proposed resolution (November, 2010) [SUPERSEDED]:</B></P>

<OL>
<LI><P>Change 13.8.3.2 [<A href="https://wg21.link/temp.dep.type#1">temp.dep.type</A>] paragraph 1 as
follows:</P></LI>

<BLOCKQUOTE>

<P>In the definition of a class template, a nested class of a class
template, a member of a class template, or a member of a nested class
of a class template, a name refers to the <I>current instantiation</I>
if it is</P>

<UL>
<LI><P>the injected-class-name (Clause 11 [<A href="https://wg21.link/class">class</A>]) of
the class template or nested class,</P></LI>

<LI><P>in the definition of a primary class template, the name of the
class template followed by the template argument list of the primary
template (as described below) enclosed in <TT>&lt;&gt;</TT> <INS>(or
a template alias specialization equivalent to same)</INS>,</P></LI>

<LI><P>in the definition of a nested class of a class template, the
name of the nested class referenced as a member of the current
instantiation, or</P></LI>

<LI><P>in the definition of a partial specialization, the name of the
class template followed by the template argument list of the partial
specialization enclosed in <TT>&lt;&gt;</TT> <INS>(or a template
alias specialization equivalent to same)</INS>. If the <I>n</I>th
template parameter is a parameter pack, the <I>n</I>th template
argument is a pack expansion (13.7.4 [<A href="https://wg21.link/temp.variadic">temp.variadic</A>]) whose
pattern is the name of the parameter pack.</P></LI>

</UL>

</BLOCKQUOTE>

<P>Change 13.8.3.2 [<A href="https://wg21.link/temp.dep.type#3">temp.dep.type</A>] paragraph 3 as follows:</P>

A template argument that is equivalent to a template parameter (i.e.,
has the same constant value or the same type as the template
parameter) can be used in place of that template parameter in a
reference to the current instantiation<INS>, except that a
<I>decltype-specifier</I> that denotes a dependent type is always
considered non-equivalent</INS>. In the case of a non-type
template argument, the argument must have been given the value of the
template parameter and not an expression in which the template
parameter appears as a subexpression. [<I>Example:</I>...

</OL>

<P>This resolution also resolves <A HREF="1057.html">issue 1057</A>.</P>

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