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    CWG Issue 590</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="590"></A><H4>590.
  
Nested classes and the &#8220;current instantiation&#8221;
</H4>
<B>Section: </B>13.8.3.2&#160; [<A href="https://wg21.link/temp.dep.type">temp.dep.type</A>]
 &#160;&#160;&#160;

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

 <B>Submitter: </B>James Widman
 &#160;&#160;&#160;

 <B>Date: </B>23 August 2006<BR>




<P>In 13.8.3.2 [<A href="https://wg21.link/temp.dep.type#5">temp.dep.type</A>] paragraph 5 we have:</P>

<BLOCKQUOTE>

A name is a <I>member of an unknown specialization</I> if the name is a
<I>qualified-id</I> in which the <I>nested-name-specifier</I>
names a dependent type that is not the current instantiation.

</BLOCKQUOTE>

<P>So given:</P>

<PRE>
    template&lt;class T&gt; struct A {
        struct B {
            struct C {
                A&lt;T&gt;::B::C f();
            };
        };
    };
</PRE>

<P>it appears that the name <TT>A&lt;T&gt;::B::C</TT> should be
taken as a member of an unknown specialization, because the WP
refers to &#8220;the&#8221; current instantiation, implying that
there can be at most one at any given time.  At the declaration
of <TT>f()</TT>, the current instantiation is <TT>C</TT>,
so <TT>A&lt;T&gt;::B</TT> is not the current instantiation.</P>

<P>Would it be better to refer to &#8220;a known instantiation&#8221;
instead of &#8220;the current instantiation?&#8221;</P>

<P>
<U>Mike Miller</U>:</P>

<P>I agree that there is a problem here, but I don't think the
&#8220;current instantiation&#8221; terminology needs to be
replaced.  By way of background, paragraph 1 makes it clear that
<TT>A&lt;T&gt;::B</TT> &#8220;refers to&#8221; the current
instantiation:</P>

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

</UL>

</BLOCKQUOTE>

<P>
<TT>A&lt;T&gt;::B</TT> satisfies bullet 3.  Paragraph 4 says,</P>

<BLOCKQUOTE>

<P>A name is a member of the current instantiation if it is</P>

<UL>
<LI><P>An unqualified name that, when looked up, refers to a
member of a class template. [<I>Note:</I> this can only occur
when looking up a name in a scope enclosed by the definition of a
class template. &#8212;<I>end note</I>]</P></LI>

<LI><P>A <I>qualified-id</I> in which
the <I>nested-name-specifier</I> refers to the current
instantiation.</P></LI>

</UL>

</BLOCKQUOTE>

<P>So clearly by paragraphs 1 and 4, <TT>A&lt;T&gt;::B::C</TT> is
a member of the current instantiation.  The problem is in the
phrasing of paragraph 5, which incorrectly requires that the
<I>nested-name-specifier</I> &#8220;be&#8221; the current
instantiation rather than simply &#8220;referring to&#8221; the
current instantiation, which would be the correct complement to
paragraph 4.  Perhaps paragraph 5 could simply be rephrased as,
&#8220;...a dependent type and it is not a member of the current
instantiation.&#8221;</P>

<P>(Paragraph 1 may require a bit more wordsmithing to make it
truly recursive across multiple levels of nested classes; as it
stands, it's not clear whether the name of a nested class of a
nested class of a class template is covered or not.)</P>

<P><B>Additional note (April, 2011):</B></P>

<P>It appears that these concerns are addressed by the resolution
of <A HREF="1043.html">issue 1043</A> in document N3283.</P>

<P><B>Proposed resolution (December, 2011):</B></P>

<P>This issue is resolved by the resolution of <A HREF="1043.html">issue 1043</A>.</P>

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