Compat Scala 2: Abstract override

class Seq

trait IterableOnceOps {
  def toSeq: Seq = new Seq
}

trait IterableOps extends IterableOnceOps

trait SeqOps extends IterableOps {
  def toSeq: Seq
}

class Foo extends SeqOps

The code snippet above compiles with Scala2 but not Dotty:

-- Error: tests/allan/Test.scala:53:6 ------------------------------------------
53 |class Foo extends SeqOps
   |      ^
   | class Foo needs to be abstract, since def toSeq: => Seq is not defined 
one error found

This pattern is used in the 2.13 collection library

[Jasper-M]

Only discussion related to this that I can remember is https://contributors.scala-lang.org/t/ability-to-make-a-parents-concrete-method-abstract/1255

It’s also pointed out in that thread that Scala 2 already does the “Java thing” when it comes to abstract classes.

scala> class Baz { def foo: String = "string" }
class Baz

scala> trait Foo extends Baz{
     |   override def foo: String
     | }
trait Foo

scala> new Foo{}
val res5: Baz with Foo = $anon$1@4374e0

scala> abstract class Bar extends Baz{
     |   override def foo: String
     | }
class Bar

scala> new Bar{}
           ^
       error: object creation impossible. No implementation found in a subclass for deferred declaration
       override def foo: String (defined in class Bar)

In case that’s news to anyone.

[lrytz]

Scala 2 allows widening the type in an abstract override, but that seems a pretty obscure feature

scala> trait T { def f: String = "" }; trait U extends T { override def f: Object }
trait T
trait U