Subtype information getting lost

[ultrasecreth]

Hi there, I recenctly got this bug mockito/mockito-scala/issues/229 reported and after some digging it seems to be caused by what I think it may be a bug in the compiler.

reproduction steps

In the following scenario, the inheritance information is not present on the compiled Class (Tested in both 2.12.11 and 2.13.2)

abstract class Abstract
trait TraitExtendsAbstract extends Abstract

println(classOf[TraitExtendsAbstract].getSuperclass)
println(classOf[Abstract].isAssignableFrom(classOf[TraitExtendsAbstract]))

problem

Running that code will print

null
false

expectation

It should print

class Abstract
true

extra details

If I run the following code it would actually print the expected result

val anInstance: TraitExtendsAbstract = new TraitExtendsAbstract {}

println(anInstance.getClass.getSuperclass)
println(classOf[Abstract].isAssignableFrom(anInstance.getClass.getSuperclass))

[sjrd]

That is unfortunately unavoidable. On the JVM, an interface cannot extend any class but Object. So if you look at Java reflection, which only uses the JVM type system, and not the Scala type system, you have no way of telling that the trait initially extended a class.

[ultrasecreth]

@sjrd Fair enough, many thanks for your answer!