The code below doesn't compile. Maybe because GCC with std=c++2a is still not completely up to date with the most recent draft

Question

[class.mem]/6 :

A complete-class context of a class is a

(6.1) function body, (6.2) default argument, (6.3) noexcept-specifier ([except.spec]), (6.4) contract condition, or (6.5) default member initializer

within the member-specification of the class. [ Note: A complete-class context of a nested class is also a complete-class context of any enclosing class, if the nested class is defined within the member-specification of the enclosing class. — end note ]

This paragraph was introduced in the draft with pull-request #2231 .

As far as I can understand, the following code should compile, according to the Note above. But it doesn't . I'm assuming that the GCC compiler is still not up to date with the most recent draft. Am I correct, or is it the case that my understanding about this note is incorrect?

struct A {
    int i = 1;
    struct B { 
        int j = 2;
        int f() {
            return i + j;
        }
    };
};

Which fails with:

source>: In member function 'int A::B::f()':
<source>:6:20: error: invalid use of non-static data member 'A::i'
    6 |             return i + j;
      |                    ^
<source>:2:9: note: declared here
    2 |     int i = 1;
      |         ^

Answer 1

I think the confusion stems here from what the point of complete-class context is and how is it intended to be used.

Importantly, name lookup will find i there. So I can write this:

struct A {
    struct B { 
        int j = 2;
        int f() {
            using T = decltype(i); // ok, even though 'i' declared later lexically
            return T{} + j;        // ok
        }
    };
    int i = 1;
};

As well as:

struct A {
    struct B { 
        int j = 2;
        int f() {
            return i + j; // ok, even though 'i' declared later lexically
        }
    };
    static const int i = 1;
};

Indeed, this was ok all the way back in C++11.

But i is still a non-static member, so you can only access it from the context of an object of type A . Within the body of a member function of B , we don't implicitly have an A object. So such a free use of i is still ill-formed.

---

That said, this:

I'm assuming that the GCC compiler is still not up to date with the most recent draft.

Is certainly true, and will remain true for quite some time.

Answer 2

Then what exactly is that note saying?

Names declared within the class A are in scope within the complete-class context even if before the point of declaration of the name.

The note means that the complete-class context of the enclosing class (noting the pre-condition) extends to the complete-class context of the nested class.

Therefore following is well-formed:

struct A {
    struct B{
        void foo() {
            // names are in scope despite being before point of declaration
            I i;
            var;
        }
    };

     using I = int;
     static int var;
};

You cannot access a non-static data member in a (non-static) member function of another class just like you cannot access non-static members in a static member function of the same class. Complete-class context does not change that.