This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


590. Nested classes and the “current instantiation”

Section: 13.8.3.2  [temp.dep.type]     Status: C++11     Submitter: James Widman     Date: 23 August 2006

In 13.8.3.2 [temp.dep.type] paragraph 5 we have:

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

So given:

    template<class T> struct A {
        struct B {
            struct C {
                A<T>::B::C f();
            };
        };
    };

it appears that the name A<T>::B::C should be taken as a member of an unknown specialization, because the WP refers to “the” current instantiation, implying that there can be at most one at any given time. At the declaration of f(), the current instantiation is C, so A<T>::B is not the current instantiation.

Would it be better to refer to “a known instantiation” instead of “the current instantiation?”

Mike Miller:

I agree that there is a problem here, but I don't think the “current instantiation” terminology needs to be replaced. By way of background, paragraph 1 makes it clear that A<T>::B “refers to” the current instantiation:

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 current instantiation if it is

A<T>::B satisfies bullet 3. Paragraph 4 says,

A name is a member of the current instantiation if it is

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

(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.)

Additional note (April, 2011):

It appears that these concerns are addressed by the resolution of issue 1043 in document N3283.

Proposed resolution (December, 2011):

This issue is resolved by the resolution of issue 1043.