Ycheck failure in covariant java varargs

[abeln]

This involves Scala code calling a Java varargs methods.

Java

  class Animal {}
  class Dog extends Animal {}
  class J {
    void foo(Animal... animal) {}
  }

Scala

class S {
  val j = new J()
  val x: Array[Dog] = ???
  j.foo(x: _*)
}

Compiling without -Ycheck succeeds, but -Ycheck:refchecks fails:

exception while typing this.j.foo(this.x) of class class dotty.tools.dotc.ast.Trees$Apply # 802
exception while typing @scala.annotation.internal.SourceFile("tests/neg/java-varargs-cov/S.scala") 
  class
 S() extends Object() { 
  val j: J = new J()
  val x: Array[Dog] = ???
  this.j.foo(this.x)
} of class class dotty.tools.dotc.ast.Trees$TypeDef # 804
exception while typing package <empty> {
  @scala.annotation.internal.SourceFile("tests/neg/java-varargs-cov/S.scala") 
    class
   S() extends Object() { 
    val j: J = new J()
    val x: Array[Dog] = ???
    this.j.foo(this.x)
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # 805
*** error while checking tests/neg/java-varargs-cov/S.scala after phase refchecks ***
exception occurred while compiling tests/neg/java-varargs-cov/S.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: found:    Array[Dog](S.this.x)
required: Array[Animal]


tree = this.x
	at scala.Predef$.assert(Predef.scala:219)
	at dotty.tools.dotc.transform.TreeChecker$Checker.adapt(TreeChecker.scala:460)
	at dotty.tools.dotc.typer.ProtoTypes$FunProto.typedArg(ProtoTypes.scala:302)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:663)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.typedArg(Applications.scala:661)
	at dotty.tools.dotc.typer.Applications$Application.addTyped$1(Applications.scala:434)
	at dotty.tools.dotc.typer.Applications$Application.matchArgs(Applications.scala:472)
	at dotty.tools.dotc.typer.Applications$Application.init(Applications.scala:255)
	at dotty.tools.dotc.typer.Applications$TypedApply.<init>(Applications.scala:564)
	at dotty.tools.dotc.typer.Applications$ApplyToUntyped.<init>(Applications.scala:662)
	at dotty.tools.dotc.typer.Applications.simpleApply$1(Applications.scala:722)
	at dotty.tools.dotc.typer.Applications.$anonfun$typedApply$7(Applications.scala:747)
	at dotty.tools.dotc.typer.Typer.tryEither(Typer.scala:1983)
	at dotty.tools.dotc.typer.Applications.$anonfun$typedApply$1(Applications.scala:748)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
	at dotty.tools.dotc.typer.Applications.realApply$1(Applications.scala:692)
	at dotty.tools.dotc.typer.Applications.typedApply(Applications.scala:793)
	at dotty.tools.dotc.typer.Applications.typedApply$(Applications.scala:690)
	at dotty.tools.dotc.typer.Typer.typedApply(Typer.scala:82)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1817)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1868)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:112)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:282)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1899)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1895)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1911)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:270)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1951)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1964)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:446)
	at dotty.tools.dotc.typer.Typer.$anonfun$typedClassDef$1(Typer.scala:1564)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1487)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedClassDef(TreeChecker.scala:399)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1808)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1867)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:112)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:282)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1899)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1895)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1911)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:270)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1930)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1964)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedStats(TreeChecker.scala:446)
	at dotty.tools.dotc.typer.Typer.$anonfun$typedPackageDef$1(Typer.scala:1674)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1667)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1847)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1868)
	at dotty.tools.dotc.typer.ReTyper.typedUnadapted(ReTyper.scala:112)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:282)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1899)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1895)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1911)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:270)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1975)
	at dotty.tools.dotc.transform.TreeChecker.check(TreeChecker.scala:132)
	at dotty.tools.dotc.transform.TreeChecker.run(TreeChecker.scala:104)
	at dotty.tools.dotc.core.Phases$Phase.$anonfun$runOn$1(Phases.scala:298)
	at scala.collection.immutable.List.map(List.scala:283)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:296)
	at dotty.tools.dotc.core.Phases$Phase.runOn$(Phases.scala:295)
	at dotty.tools.dotc.transform.TreeChecker.runOn(TreeChecker.scala:48)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$3(Run.scala:174)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.trackTime(Stats.scala:49)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$2(Run.scala:171)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$2$adapted(Run.scala:169)
	at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
	at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
	at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:194)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:169)
	at dotty.tools.dotc.Run.$anonfun$compileUnits$1(Run.scala:194)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:90)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:149)
	at dotty.tools.dotc.Run.compileSources(Run.scala:136)
	at dotty.tools.dotc.Run.compile(Run.scala:120)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:31)
	at dotty.tools.dotc.Driver.process(Driver.scala:134)
	at dotty.tools.dotc.Driver.process(Driver.scala:103)
	at dotty.tools.dotc.Driver.process(Driver.scala:115)
	at dotty.tools.dotc.Driver.main(Driver.scala:142)

[abeln]

The first question here is what is the intended behaviour. Specifically, is this supposed to compile? Notice it's unsound, because we're passing an Array[Dog] that Java can later populate with an Animal.

Looking at the typer, the following happens while typing j.foo(x: _*):

1. x is typed with a prototype of Seq[Any]
2. for that work, an implicit conversion is added: this.j.foo(wrapRefArray[Dog](this.x): Dog*)
3. an ascription of : Dog*, which is RepeatedParamType[Dog] is also added
4. RP[Dog] is checked against the param type, which is RP[Animal]
5. since RP is covariant, the checks succeeds
6. however, somehow Ycheck is able to see through the RP type into the "real" type of Array[Dog] and Array[Animal], and that only happens after refChecks (in particular, setting -Ycheck:elimRepeated succeeds)

I don't yet understand why step 6 happens. This is relevant to the nullability work we're doing, because we add |Null inside Java-declared array types, so
void foo(String... x)
becomes
void foo(String|Null... x) => void foo(x: RepeatedParam[String|Null]|Null)

so this type of covariance + arrays + java interop comes up more often in the non-null world.

[allanrenucci]

Does scalac reject this program?

There is an attempt at changing how we typecheck wildcard star args in #4787. Ideally we should not type them as Seq if the method is java defined

[odersky]

I believe if it fails to Ycheck after refchecks it should not get the area:typer flag.

[allanrenucci]

I believe if it fails to Ycheck after refchecks it should not get the area:typer flag.

It happens to fail Ycheck later in the pipeline but this is a problem in typer. The program should not typecheck

[abeln]

@allanrenucci scala 2.12.6 accepts the program. Do we know the behaviour we want? Fix the Ycheck error or disallow in the typer?