C++ Forward Declaring Classes Within Classes

The following simple piece of code compiles, although I don't understand why:

class C {
    class B;

    class A {
        B getB() { return B(); }
    };

    class B {
    };
};

int main(int, char**)
{
    return 0;
}

If I then comment out the "class C" stuff, so that the forward declaration of B, the definition of A and the definition of B are no longer nested within a class, the code does not compile, since B is of an incomplete type:

main.cpp: In member function 'B A::getB()':
main.cpp:6: error: return type 'struct B' is incomplete
main.cpp:6: error: invalid use of incomplete type 'struct B'
main.cpp:3: error: forward declaration of 'struct B'

I understand what it means for a type to be incomplete, namely that it has not been defined yet and so the compiler can't possibly know how much space to allocate for it. But why is B not considered incomplete in the code above, where A and B are both declared and defined inside of C?