Repro.java

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Objects;

public class Repro {
    public static void main(String[] args) {
        Holder someHolder = new Holder();

        List<C> Cs = new LinkedList<>();
// works fine
        someHolder.getAs()
                  .stream()
                  .map(Converter::convertAToC)
                  .forEach(Cs ::add);
        someHolder.getBs()
                  .stream()
                  .map(Converter::convertBToC) // bad return type in method reference: cannot convert C to R
                  .filter(Objects::nonNull)
                  .forEach(Cs ::add); // Cannot resolve method 'add'
    }
}

class A {}
class B {}
class C {}
class Converter {
    public static C convertAToC(A a) { return new C(); }
    public static C convertBToC(B b) { return new C(); }
}
class Holder {
    List<A> getAs() {
        return Collections.emptyList();
    }

    List<B> getBs() {
        return Collections.emptyList();
    }
}