Dotty: No warning/error when matching unrelated object types

[olofwalker]

Minimized code

Dotty 0.27-RC1

object Main:

  abstract class RecoveryCompleted
  case object RecoveryCompleted extends RecoveryCompleted

  abstract class TypedRecoveryCompleted 
  case object TypedRecoveryCompleted extends TypedRecoveryCompleted

  TypedRecoveryCompleted match {
    case RecoveryCompleted => println("Recovery completed")
    case TypedRecoveryCompleted => println("Typed recovery completed")
  }

Output

Nothing

Expectation

Something like:

pattern type is incompatible with expected type;
 found   : ammonite.$sess.cmd5.RecoveryCompleted.type
 required: ammonite.$sess.cmd7.TypedRecoveryCompleted.type
    case RecoveryCompleted => println("Recovery completed")

[liufengyun]

This seems to be the correct behavior. See the following example:

class Error {
  abstract class RecoveryCompleted
  case object RecoveryCompleted extends RecoveryCompleted {
    override def equals(that: Any): Boolean = that == TypedRecoveryCompleted
  }

  abstract class TypedRecoveryCompleted
  case object TypedRecoveryCompleted extends TypedRecoveryCompleted


  TypedRecoveryCompleted match {
    case RecoveryCompleted => println("Recovery completed")
    case TypedRecoveryCompleted => println("Typed recovery completed")
  }
}

object Test {
  def main(args: Array[String]): Unit = new Error
}

The first branch matches.

[liufengyun]

Related: #3200 https://github.com/scala/bug/issues/1503

[olofwalker]

@liufengyun

This fails in 2.13, I just assumed this would also fail in Dotty.

pattern type is incompatible with expected type
[error]  found   : Error.this.RecoveryCompleted.type
[error]  required: Error.this.TypedRecoveryCompleted.type

The context for this is that I never got a RecoveryCompleted signal to fire in an Akka persistent actor, there are two of those classes, one for classic actors and one for typed, and I had imported and used the wrong one.

[liufengyun]

@olofwalker Thanks for clarifying the context. It's a difference between Scala 2 and Scala 3.

Maybe Scala 3 should be more conservative here to prevent such mistakes.

[olofwalker]

@liufengyun I think I would have been helped by a warning, its a bit like Scala 2, you unknowingly import an implicit, and all of a sudden things work differently.

[smarter]

We can't just copy what 2.13 is doing here as demonstrated by Fengyun's example in https://github.com/lampepfl/dotty/issues/9740#issuecomment-688157605, scalac wrongly emits an error even though this code is correct. We would have to check if equals in RecoveryCompleted is compiler-generated first

[liufengyun]

We can't just copy what 2.13 is doing here as demonstrated by Fengyun's example in #9740 (comment), scalac wrongly emits an error even though this code is correct. We would have to check if equals in RecoveryCompleted is compiler-generated first

It is incomplete, but it seems sound/safe to reject the code. The additional expressiveness seems to be not useful in practice, and usually implies something goes wrong.

[smarter]

It's safe but it might be annoying to someone :). But I think if we only emit the warning if scrutinee.is(Case) && scrutineeEqualsMethod.is(Synthetic) then it's fine and catch the cases we're interested in.

[smarter]

Oops actually we want to check the symbol of the case, not the symbol of the scrutinee.