My apologies. We are behind in making our code Kotlin friendly.
In Java, public members of non-public super classes are visible from the subclass. This is not the case in Kotlin. The “sum” method is declared in a package protected superclass of Function (AbstractFunction). That means that Kotlin can’t see it.
I am afraid that this will not change in the next release; perhaps not even in the one after that. You will have to write your query creation code in Java.
Again, my apologies. I am trying to get this fixed.