FP in S4276: interfaces with generic wildcard types can't be specialized

When an interface takes two generic wildcard types (i.e. "?"), it is not (always) possible to use a specialized Functional Interface instead.

import java.util.function.Function;

public class FunTypes {
    public static final Function<?, ?> NEW_OBJECT = anything -> new Object(); // FP
}

Message states:

Refactor this code to use the more specialized Functional Interface ‘UnaryOperator<?>’

It is not possible, the first "?" is not necessarily the same as the second "?".