Breaking Dotty (Scala 3) and Java 16 Decidability
As I was preparing my talk for the upcoming IFIP WG 2.4, I reflected on trying to explain the foundational work done by my recently graduated PhD student Julian Mackay as published in our POPL 2020 paper and APLAS 2020 paper ) last year. For this example, I used the latest release of Dotty (a.k.a. Scala 3) and the latest release of Java 16 : Imagine you want to have an interface or trait allowing you to compare items called Eq with parameter E . Here is both Java and Scala code alongside each other: Now imagine a Set that insists on containing the elements that are comparable with other Set instances using Eq and themselves contain elements that are comparable with Eq with both Java and Scala code alongside each other as follows: Finally, lets create a useful class Tree that implements such Set : If you are to ask either Dotty or Java a simple looking question: "is Tree a subtype of Eq<? super Tree> in Java or a subtype of Eq {type E >: Tree} in Sc