The following code (from 3.4.5-4 - class member access) is rejected:

struct A { int a; };

struct B: virtual A { };

struct C: B { };

struct D: B { };

struct E : public C, public D { };

void f()
{
  E e;
  e.B::a = 0;
}


f1.cxx: In function 'void f()':
f1.cxx:25:8: error: 'B' is an ambiguous base of 'E'

According to 10.2.5 (Member name lookup), member access e.B::a = 0 is OK because there is only one "a" (virtual)