Thanks for the quick explanation. I understand that class scopes don't
extend and this is documented behavior.

However, the question is why the if clause is executed in a scope of its
own and where this is documented.

You would expect that the problematic generator expression is equivalent
to the following code which works fine:

class A:
    a = 'test'
    def __g(a):
        for c in a:
            if c in a:
                yield c
    tuple(__g(a))