a type defined in a trait is "not found" in an object that extends the trait (under certain circumstances)

[yurique]

Compiler version

3.3.3, 3.4.2, 3.5.0-RC7, 3.5.1-RC1

Preface

I'm not sure if this is a bug or works as intended. The same code compiles in Scala 2 and I don't know why it shouldn't in Scala 3 - but I might be just unaware of a particular limitation/restriction introduced in dotty.

Minimized code

(https://scastie.scala-lang.org/a2YCoAw8TOyOXi2Q7vl5Gg)

case class Table(owner: Owner.Id)

case class Owner(owner: Owner.Id) // type Id is not a member of object Playground.Owner

trait Typed[Tag] {
  type Id = String
}

object Owner extends Typed[Owner] {
  //type Id = String
}

Output

(from scastie)

type Id is not a member of object Playground.Owner

Expectation

Expectation is for this code to compile.

---

Observations:

• this code compiles in Scala 2 (https://scastie.scala-lang.org/VgS8ZtZZS1uI10SV6DbOGQ)
• if only one of the case classes references the Owner.Id - it compiles (https://scastie.scala-lang.org/X4cRDtzcTUa0ZrIBkowU6w)
• if type Id is defined directly in the object (the commented out line) - it compiles (https://scastie.scala-lang.org/noCBIv1TQUGeNBuSHoFdKQ)
• if the type parameter for Typed is not the Owner class - it compiles (https://scastie.scala-lang.org/0Y0hTN0oQtuQjT7YVBfpVg)

[NyuB]

Played a bit and found some more details, hope it can help to understand/debug:

• The bug is not related to the object being a 'companion object' of Owner, the name does not matter (the following does not compile)

case class A(owner: Obj.Id)

case class B(owner: Obj.Id)

trait Typed[Tag] {
  type Id = String
}

object Obj extends Typed[B]

• Only the second case class declaration fails to compile, a third one does not raise any compilation error (only an error for B declaration is reported in scastie for the following snippet) (https://scastie.scala-lang.org/NyuB/45vwz4atRLGpOwmsffERZg/2)

case class A(owner: Obj.Id)

case class B(owner: Obj.Id)

case class C(owner: Obj.Id)

trait Typed[Tag] {
  type Id = String
}

object Obj extends Typed[B]

• Even stranger, if you interleave a valid declaration between the to declaration referring to Obj.Id, everything compiles fine (https://scastie.scala-lang.org/NyuB/fNKPKdCqTEyj4mUCSZ1WVw/5)

case class A(owner: Obj.Id)

case class B(notAndObjId: String)

case class C(owner: Obj.Id)

trait Typed[Tag] {
  type Id = String
}

object Obj extends Typed[B]