As part of solving defect report #39, name lookup in 10.2[class-
member.lookup] was changed, but it seems to have missed to handle the
following case

struct A { int a; };
struct B : A { };
struct C : A { };
struct D : B, C { };
int main() { &D::a; }

I believe that the problem can be solved by adding a similar paragraph to
5.3.1[expr.unary.op] that was also added to 5.2.5[expr.ref]/5:

For a qualified-id, if the member is a non-static data member or a non-
static member function, the program is ill-formed if the class of which it
is directly a member is an ambiguous base (10.2) of the naming class (11.2)
of the qualified-id.

And refer to it by the note in 10.2[class.member.lookup]/13

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