According to 6.3 [basic.def.odr] paragraph 2,

A variable or non-overloaded function whose name appears as a potentially-evaluated expression is used... A virtual member function is used if it is not pure.

However, that does not adequately address when a pure virtual function is used or not used. For example,

    struct S {
      virtual void pure1() = 0;
      virtual void pure2() = 0;
    };
    void f(S* p) {
      p->pure1();
      p->S::pure2();
    };

Both pure1 and pure2 satisfy the criterion that their name appears in a potentially-evaluated expression, but pure1 should not be considered “used” (which would require that it be defined); pure2 is “used” and must be defined.