Are Java parents supposed to come with their own arguments or delegate that to their lawyers?

[Blaisorblade]

One of these two things is wrong:

scala> class i0 extends java.util.ArrayList
defined class i0

scala> class i0 extends java.util.List
<console>:11: error: trait List takes type parameters
       class i0 extends java.util.List
                                  ^

I vote for "they're both illegal and Dotty will forbid both, absent some extremely strong compatibility requirement".

The first happens to be tested by test/files/run/t3613.scala, which still lives in Dotty, but maybe the code was legal back then, either because of different semantics with raw types, or because AbstractListModel had no type argument in Java 6 and only got one in Java 7. (Thanks @hrhino for pointing this out and helping diagnose this on Gitter).

[Blaisorblade]

So @hrhino suggests/found out that a type argument is being inferred for ArrayList. But since there are no arguments, even trying sounds fishy... Dotty's parser can tell if arguments are passed or not and produces different trees.

[Blaisorblade]

We should ideally agree on behavior with scala/scala3#5027.

[Jasper-M]

For the record, this is a difference between classes and traits/interfaces and not related to Java or raw types.

scala> abstract class MyClass[A]
defined class MyClass

scala> class A extends MyClass
defined class A

scala> trait MyTrait[A]
defined trait MyTrait

scala> class B extends MyTrait
                       ^
       error: trait MyTrait takes type parameters

There's also this funny trick you can exploit it for:

scala> :pa
// Entering paste mode (ctrl-D to finish)

sealed trait Default[F[_]]
object Default {
  type Id[A] = A
  implicit val default = new Default[Id] {}
  implicit def fallback[F[_]] = new Default[F] {}
}

// Exiting paste mode, now interpreting.

scala> abstract class MyAbstraction[F[_]: Default] { def foo[A](a: A): F[A] }
defined class MyAbstraction

scala> class SimpleImpl extends MyAbstraction { def foo[A](a: A) = a }
defined class SimpleImpl

scala> class FPImpl extends MyAbstraction[IO] { def foo[A](a: A) = IO(a) }
defined class FPImpl

[SethTisue]

Scala 3 gives a really cryptic error:

Welcome to Scala 3.4.0-RC1-bin-20231027-c0eae68-NIGHTLY-git-c0eae68 (17.0.9, Java OpenJDK 64-Bit Server VM).

scala> class i0 extends java.util.List
-- Error: ----------------------------------------------------------------------
1 |class i0 extends java.util.List
  |      ^
  |      bad parameter reference i0.this.E at protectedAccessors
  |      the parameter is type E in trait List but the prefix (i0.this : i0)
  |      does not define any corresponding arguments.
  |      idx = 0, args = ,
  |      constraint =  uninstantiated variables:
  |       constrained types:
  |       bounds:
  |       ordering:
  |       co-deps:
  |       contra-deps: