Private Super-Trait Inconsistency with 2.13

Compiler version

3.0.1

Minimized code

package foo

private[foo] trait Super {
  private[this] val dummy = "42"
}

trait Sub extends Super {
  def foo: Unit
}

// meanwhile, in another part of town

package bar

object Baz extends Sub {
  def foo = ()
}

Output

Scala 2.13 compiles the above just fine. In 3.0.1, this will result in an error saying that class Super cannot be accessed in module Baz.

Expectation

Consistency with 2.13.

[smarter]

Quoting from the spec (https://www.scala-lang.org/files/archive/spec/2.13/05-classes-and-objects.html#templates):

It is possible to write a list of parents that starts with a trait reference, e.g. mt_1​ with … with mt_n​. In that case the list of parents is implicitly extended to include the supertype of mt_1​ as the first parent type

In other words, when we write:

object Baz extends foo.Sub

because Sub is a trait whose parent class is Super, it gets expanded to:

object Baz extends foo.Super with Sub

which does not typecheck because foo.Super is inaccessible. So I think that the error is legitimate, and Scala 2 just happens to not check for accessibility here, so I’d say this is a bug in Scala 2 rather than Scala 3 (but this is also the option that requires the least amount of work for me, so I’m biased 😃).

[djspiewak]

which does not typecheck because foo.Super is inaccessible. So I think that the error is legitimate, and Scala 2 just happens to not check for accessibility here, so I’d say this is a bug in Scala 2 rather than Scala 3 (but this is also the option that requires the least amount of work for me, so I’m biased 😃).

Sounds good to me!