Ambiguous access of unaccessible virtual function

Question

Consider this code:

struct A
{
    virtual void foo() {};
};

struct B : private A {};

struct C : private A {};

struct D : A, B, C {};

int main()
{
    D d;
    d.foo(); // Error: ambiguous access of foo (is it A::foo(), B::foo() or C::foo() ?)
}

The compiler is considering d.foo() an ambiguous call although both d.B::foo() and d.C::foo() are inaccessible and hidden to the user. The only valid call should be d.A::foo(), why is it ambiguous for the compiler?