Thread: Generics solve as one function but not two

So I am trying to write an internal DSL and having a hard time to get generics work the way I want. I know I can write one method call like

collectingAndThen(toSet(), Set::size);

but I want to write

toSet().andThen(Set::size)

which works OK here where I am passing the result into a method parameter, like so

var grouped = collect(groupingBy(i->i % 3, toSet().andThen(Set::size)), that);

but here where I try to return it

    public static <X> Collector<X,Set<X>,Integer> countDistinct() {
        // return collectingAndThen(toSet(), Set::size);
        return Collectors.<X>toSet().andThen(Set::size);
    }

I have to write

Collectors.<X>toSet().andThen(Set::size);

to tell the compiler what the type parameter is. In my mind it should be able to figure it out, but the compiler tells me

incompatible types: no instance(s) of type variable(s) FinalResult exist so that com.ontology2.pidove.checked.Collector<java.lang.Object,java.util.Set<java.lang.Object>,FinalResult> conforms to com.ontology2.pidove.checked.Collector<X,java.util.Set<X>,java.lang.Integer>

Here are the definitions:

    public static <X> SimpleCollector<X, Set<X>> toSet() {
        return toCollection(HashSet::new);
    }
 
public record SimpleCollector<Input, Result> (
    Supplier<Result> supplier,
    BiConsumer<Input, Result> accumulator
) implements AnyCollector<Input, Result> {
    public <FinalResult> Collector<Input,Result,FinalResult> andThen(Function<Result,FinalResult> finisher) {
        return Collector.of(supplier(), accumulator(), finisher);
    }
}

if I don't put in the <X>. I like the way Function.andThen() works and I'd like to make my andThen() work the same way. Am I missing something or I am butting my head against type erasure or both?

Can I make this work or should I just settle for writing static functions that always get applied on the left?