NoSuchMethodError when accessing an override val via super

[Atry]

Compiler version

3.2.0

Minimized code

trait A {
  def f: String
}

trait B extends A {
  def f = "B";
}

trait C extends A {
  override val f = "C"
}

trait D extends A with B {
  def d = super.f
}

object O extends B with C with D

O.f
O.d

Output

java.lang.NoSuchMethodError: 'java.lang.String Playground$C.f$(Playground$C)'
	at Playground$O$.Playground$D$$super$f(main.scala:19)
	at Playground$D.d(main.scala:16)
	at Playground$D.d$(main.scala:15)
	at Playground$O$.d(main.scala:19)
	at Playground$.<clinit>(main.scala:22)
	at Main$.<clinit>(main.scala:27)
	at Main.main(main.scala)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:78)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at sbt.Run.invokeMain(Run.scala:143)
	at sbt.Run.execute$1(Run.scala:93)
	at sbt.Run.$anonfun$runWithLoader$5(Run.scala:120)
	at sbt.Run$.executeSuccess(Run.scala:186)
	at sbt.Run.runWithLoader(Run.scala:120)
	at sbt.Run.run(Run.scala:127)
	at com.olegych.scastie.sbtscastie.SbtScastiePlugin$$anon$1.$anonfun$run$1(SbtScastiePlugin.scala:38)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.util.InterfaceUtil$$anon$1.get(InterfaceUtil.scala:17)
	at sbt.ScastieTrapExit$App.run(ScastieTrapExit.scala:258)
	at java.base/java.lang.Thread.run(Thread.java:831)

Note that this bug can be reproduced in Scala 2, too. See scala/bug#12715

Expectation

No error

[odersky]

Observations:

• The val in trait C is essential. It works with def.
• The translations of both versions in the transformer pipeline are basically the same. They both produce this implemention of D$$super$f:

   def D$$super$f(): String = super[C].f()

• It looks like the backend translates the mixin call super[C].f() to a call to an interface method

  C.f$:(LC;)Ljava/lang/String
  

  I did not see a correspondent of this method with either def or val anywhere in the transformation pipeline, so maybe it's the backend that generates the interface method, and fails to do so in the case of val?

[odersky]

@sjrd Maybe you have an idea about what could be the cause of this? Does the JS backend have the same problem?

[lrytz]

I can take a look at this one (and its cousin on Scala 2)

[sjrd]

Scala.js has the same issue (most likely from the same cause in an earlier phase):

 Referring to non-existent method C.f()java.lang.String
   called from O$.D$$super$f()java.lang.String
   called from D.d()java.lang.String
   called from O$.d()java.lang.String
   called from Main$.main([java.lang.String)void
   called from static Main.main([java.lang.String)void
   called from core module module initializers
 involving instantiated classes:
   O$
   Main$

[Atry]

By the way, I think an override method that does not include a super call should be a lint error, because it silently suppresses the implementation from any super classes, even when the super classes are dynamically determined.

trait A {
  def f: String
}

trait C extends A {
  // What is the purpose? Implementing A.f, or transforming A.f?
  override val f = "C" 
}

If there is a lint error is raised for the above code, the author of C should either remove override:

trait A {
  def f: String
}

trait C extends A {
  // The purpose is implementing A.f
  val f = "C"
}

or add the missing call to super class:

trait A {
  def f: String
}

trait C extends A {
  // The purpose is transforming A.f
  abstract override val f = s"${super.f}/C"
}

[lrytz]

I added a comment here: scala/bug#12715 (comment)