AssertionError in dotty.tools.dotc.transform.ResolveSuper

[dgruntz]

Compiler version

Welcome to Scala 3.1.1 (17, Java Java HotSpot(TM) 64-Bit Server VM).
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Minimized code and output

I executed the following lines in the Scala REPL:

trait A {
    def m(a:Int): Int
}

trait B extends A {
    override def m(a:Int): Int = { return a; }
}

trait C extends A {
    abstract override def m(a:Int):Int = { return super.m(a); }
}

trait D extends B with C {
    override def m(a:Int):Int = { return super.m(a); }
}

trait E extends C with B {
    abstract override def m(a:Int):Int = { return super.m(a); }
}

class X extends E with D

and with entering the last line I see the following stack trace (and the shell exits).

Click to expand!

Exception in thread "main" java.lang.AssertionError: assertion failed: cannot rebind method repl$$rs$line$3$C$$super$m, repl$$rs$line$3$C$$super$m m
    at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
    at dotty.tools.dotc.transform.ResolveSuper$.rebindSuper(ResolveSuper.scala:115)
    at dotty.tools.dotc.transform.ResolveSuper.superAccessors$2$$anonfun$2(ResolveSuper.scala:52)
    at scala.collection.immutable.List.map(List.scala:246)
    at dotty.tools.dotc.transform.ResolveSuper.superAccessors$3(ResolveSuper.scala:53)
    at dotty.tools.dotc.transform.ResolveSuper.$anonfun$1(ResolveSuper.scala:55)
    at scala.collection.immutable.List.flatMap(List.scala:293)
    at dotty.tools.dotc.transform.ResolveSuper.transformTemplate(ResolveSuper.scala:55)
    at dotty.tools.dotc.transform.ResolveSuper.transformTemplate(ResolveSuper.scala:43)
    at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:1004)
    at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:363)
    at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
    at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:256)
    at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
    at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
    at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
    at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
    at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
    at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:362)
    at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
    at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:256)
    at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:427)
    at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:437)
    at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
    at dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:442)
    at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:442)
    at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:382)
    at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:385)
    at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:429)
    at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:448)
    at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:460)
    at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:308)
    at scala.collection.immutable.List.map(List.scala:246)
    at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:309)
    at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:261)
    at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
    at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
    at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
    at dotty.tools.dotc.Run.runPhases$5(Run.scala:272)
    at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:280)
    at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
    at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
    at dotty.tools.dotc.Run.compileUnits(Run.scala:289)
    at dotty.tools.dotc.Run.compileUnits(Run.scala:228)
    at dotty.tools.repl.ReplCompiler.runCompilationUnit(ReplCompiler.scala:155)
    at dotty.tools.repl.ReplCompiler.compile(ReplCompiler.scala:165)
    at dotty.tools.repl.ReplDriver.compile(ReplDriver.scala:250)
    at dotty.tools.repl.ReplDriver.interpret(ReplDriver.scala:218)
    at dotty.tools.repl.ReplDriver.loop$1(ReplDriver.scala:152)
    at dotty.tools.repl.ReplDriver.runUntilQuit$$anonfun$1(ReplDriver.scala:155)
    at dotty.tools.repl.ReplDriver.withRedirectedOutput(ReplDriver.scala:174)
    at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:155)
    at dotty.tools.repl.ReplDriver.tryRunning(ReplDriver.scala:117)
    at dotty.tools.repl.Main$.main(Main.scala:6)
    at dotty.tools.repl.Main.main(Main.scala)

Expectation

Well, the definition of class X is not correct and therefore I expected an error message. The linearization of X is X D E B C A i.e. class C is not mixed in correctly as method C.m is declared abstract (i.e. needs to be mixed in in such a way that a concrete implementation follows class C in the linearization). The problem seems to be the definition of method D.m which should have been declared as abstract override.

Scala 2 reports an error on definition of method D.m:

`abstract override` modifiers required to override:
absoverride def m(a: Int): Int (defined in trait C)
  with override def m(a: Int): Int (defined in trait D)

If method D.m were declared as abstract override, then the Scala compiler would report the following error on creation of class X without crashing:

scala> class X extends E with D
-- Error: ----------------------------------------------------------------------
1 |class X extends E with D
  |      ^
  |class X needs to be abstract, since def m(a: Int): Int in trait A is not defined
  |(The class implements abstract override def m(a: Int): Int in trait D but that definition still needs an implementation)
1 error found

[smarter]

The abstract override check is skipped because of the use of inLinearizationOrder in:
https://github.com/lampepfl/dotty/blob/4c7ec9b7411b6d8e979e1e5409b72fd6d90c0914/compiler/src/dotty/tools/dotc/typer/RefChecks.scala#L211-L219

More precisely, when we check as seen from class X if method m in D is a valid override of method m in C, the check is skipped because C is a parent of D, but it turns out that for abstract override checks this isn't enough: the linearization order of D is D C B A whereas in the linearization order of X ends with B C A as you noted. So we need a stricter check at least when sym2.is(AbsOverride), but I don't know if that's the only special case here /cc @odersky

[dgruntz]

I think the problem is not the check triggered by the definition of class X, but rather the fact that the Scala3 compiler does not report an error on the definition of method D.m. If D.m were declared as an abstract override, then the Scala3 compiler would report an error on the definition of class X (similar to Scala2):

scala> class X extends E with D
-- Error: ----------------------------------------------------------------------
1 |class X extends E with D
  |      ^
  |class X needs to be abstract, since def m(a: Int): Int in trait A is not defined
  |(The class implements abstract override def m(a: Int): Int in trait D but that definition still needs an implementation)
1 error found

The difference to Scala2 is, that the Scala3 compiler does not report an error on the definition of method D.m which should be declared as abstract override. This then leads to the java.lang.AssertionError on the definition of class X.

[smarter]

The difference to Scala2 is, that the Scala3 compiler does not report an error on the definition of method D.m

Not sure what you mean, Scala 2 does not report an error if I try to typecheck D and its parent classes but not X:

trait A {
    def m(a:Int): Int
}

trait B extends A {
    override def m(a:Int): Int = { return a; }
}

trait C extends A {
    abstract override def m(a:Int):Int = { return super.m(a); }
}

trait D extends B with C {
    override def m(a:Int):Int = { return super.m(a); }
}

The error you mention is only reported in X, because it's only when clazz=X, other=C.m that other.isIncompleteIn(clazz) returns true, this check is identical in both Scala 2 and 3, the only difference is that we skip it in Scala 3 because of the reasons I mentioned above, see:

• https://github.com/lampepfl/dotty/blob/74c29542f9008e0b8cd7848d9c9383a986200f39/compiler/src/dotty/tools/dotc/typer/RefChecks.scala#L465-L466
• https://github.com/scala/scala/blob/d49c6a7db220cfeb2cd0d6c2b014284f81400ea8/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala#L405-L406

[dgruntz]

Sorry for the confusion. You are right. Scala 2 also only reports an error on the definition on X (as you wrote) and not on the definition of method D.m:

Scala 2 (2.13.8) shows the following behavior:

• In case that D.m is defined as an override:

    scala> class X extends E with D
                 ^
           error: `abstract override` modifiers required to override:
           absoverride def m(a: Int): Int (defined in trait C)
             with override def m(a: Int): Int (defined in trait D)
  

• In case that D.m is defined as an abstract override:

    scala> class X extends E with D
                 ^
           error: Member method m of mixin trait iw$C is missing a concrete super implementation.
  

Scala 3 (3.1.1) shows the following behavior:

• In case that D.m is defined as an override:

  scala> class X extends E with D
  Exception in thread "main" java.lang.AssertionError: assertion failed: cannot rebind method repl$$rs$line$3$C$$super$m, repl$$rs$line$3$C$$super$m m
          at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
          at dotty.tools.dotc.transform.ResolveSuper$.rebindSuper(ResolveSuper.scala:122)
          ...
  

• In case that D.m is defined as an abstract override:

  scala> class X extends E with D
  -- Error: ----------------------------------------------------------------------
  1 |class X extends E with D
    |      ^
    |class X needs to be abstract, since def m(a: Int): Int in trait A is not defined
    |(The class implements abstract override def m(a: Int): Int in trait D but that definition still needs an implementation)
  1 error found
  

[odersky]

@smarter Your analysis is spot on.