how is method reference working even when signature of apply method does not match

Question

I was learning method reference topic in java8. I learnt that method reference provide reference to already existing method (with similar argument types). But I want to know, how is the below code RUNNING successfully even when method arguments are different.

import java.util.function.*;


interface Sayable{
    String say();
}


public class Test {
    public static void main(String[] args) {
        Test test = new Test();

        System.out.println(test.doThis(Sayable::say));

    }

    private <T> T doThis(Function<Sayable,T> func) {
        SaySomething ra = new SaySomething();
        return func.apply(ra);
    }

}


class SaySomething implements Sayable{

    @Override
    public String say() {
        return "hi";
    }

}

If I am not wrong here Sayable::say will get to Function<Sayable,T> func argument of doThis method. But say() method has no argument yet how it gets referenced to R apply(T) method of Function interface. If String say() method is different than R apply(T) then how is Sayable::say not showing any error?