ClassCastExcpetion when inheriting from two traits that define conflicting vals

[alex-aua]

Compiler version

3.3.5, 3.6.4

Minimized code

trait Actor

class LoggingAdapter {
  def info: String = "LoggingAdapter"
}

class DiagnosticLoggingAdapter extends LoggingAdapter {
  override def info: String = "DiagnosticLoggingAdapter"
}

trait ActorLogging {
  val log: LoggingAdapter = new LoggingAdapter 
}

trait DiagnosticActorLogging extends Actor {
  val log: DiagnosticLoggingAdapter = new DiagnosticLoggingAdapter {}
}

trait MyDiagnosticActorLogging extends Actor with DiagnosticActorLogging {
  def redefLog: DiagnosticLoggingAdapter = log
}

trait FSM extends Actor with ActorLogging

class MyActor extends FSM with MyDiagnosticActorLogging {
  override val log: DiagnosticLoggingAdapter = redefLog
}

(new MyActor).log.info

Output

java.lang.ExceptionInInitializerError
....
Caused by: java.lang.ClassCastException: class Playground$LoggingAdapter cannot be cast to class Playground$DiagnosticLoggingAdapter (Playground$LoggingAdapter and Playground$DiagnosticLoggingAdapter are in unnamed module of loader sbt.internal.BottomClassLoader @79576d95)
	at Playground$MyActor.Playground$ActorLogging$_setter_$log_$eq(main.scala:27)
	at Playground$ActorLogging.$init$(main.scala:14)
	at Playground$MyActor.<init>(main.scala:27)
	at Playground$.<clinit>(main.scala:31)

Expectation

I would have expected a compile error somewhere instead of the Runtime issue.

(when compiling under 3.6.4 with -Wsafe-init the compiler warns at the line in MyActor about an unitialized access to log, compare this scastie https://scastie.scala-lang.org/kTVVbeCwSlOdwqO58ldS9w )

Background: what we would have originally liked to do is fix a compile error by disambiguating the inherited log) via

class MyActor extends FSM with MyDiagnosticActorLogging {
  override val log: DiagnosticLoggingAdapter = super[MyDiagnosticActorLogging].log
}

Combining super with vals, however, does not seem to be allowed (full scastie: https://scastie.scala-lang.org/eiogKgSTTnyCB3CFCggCXA)

(Side note: We stumbled across this while trying to minimize a slightly different case of a failed disambiguation attempt; I'll link to it as soon as my colleague reports it)

[som-snytt]

Edit: apologies, I see I missed that you said this already!

Under -Wsafe-init:

➜  snips scala-cli run --server=false -S 3.7.0 i23061.scala
-- Warning: /home/amarki/snips/i23061.scala:30:15 ------------------------------
30 |  override val log: DiagnosticLoggingAdapter = redefLog
   |               ^
   |Access non-initialized value log. Calling trace:
   |├── class MyActor extends FSM with MyDiagnosticActorLogging {       [ i23061.scala:29 ]
   |│   ^
   |├── override val log: DiagnosticLoggingAdapter = redefLog   [ i23061.scala:30 ]
   |│                                                ^^^^^^^^
   |└── def redefLog: DiagnosticLoggingAdapter = log    [ i23061.scala:24 ]
   |                                             ^^^
1 warning found

Under Scala 2, the log setters are overridden in MyActor such that the main expression fails with

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "DiagnosticLoggingAdapter.info()" because the return value of "MyActor.log()" is null