Rationale (November, 2010):

The consensus of the CWG was that the using-declaration does, indeed, hide B2::f() and thus D should be ill-formed.

The intent appears to be that the following example is well-formed, even though D::f(int) hides B2::f():

    struct B1 { void f(); };
    struct B2 { void f(); };
    struct[[base_check]] D: B1, B2 {
      using B1::f;
      void f(int);
    };

However, this is not reflected in the current wording.