Wrong erasure for SeqLiteral elements

[sir-wabbit]

object App {
  type Id[A] >: A <: A
  val a: List[_ >: Id[_]] = List(0)
  def main(args: Array[String]): Unit = {
    println(a)
  }
}

java.lang.AssertionError: assertion failed: Expected primitive types I - Ljava/lang/Object;
	at scala.Predef$.assert(Predef.scala:219)
	at scala.tools.nsc.backend.jvm.BTypes$BType.conformsTo(BTypes.scala:151)
	at scala.tools.nsc.backend.jvm.BTypes$BType.conformsTo$(BTypes.scala:112)
	at scala.tools.nsc.backend.jvm.BTypes$INT$.conformsTo(BTypes.scala:320)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.adapt(BCodeBodyBuilder.scala:897)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:435)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genArray(BCodeBodyBuilder.scala:783)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genArrayValue(BCodeBodyBuilder.scala:768)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:419)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.$anonfun$genLoadArguments$1(BCodeBodyBuilder.scala:1018)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.$anonfun$genLoadArguments$1$adapted(BCodeBodyBuilder.scala:1018)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoadArguments(BCodeBodyBuilder.scala:1018)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genNormalMethodCall$1(BCodeBodyBuilder.scala:725)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genApply(BCodeBodyBuilder.scala:754)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:337)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.$anonfun$genLoadArguments$1(BCodeBodyBuilder.scala:1018)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.$anonfun$genLoadArguments$1$adapted(BCodeBodyBuilder.scala:1018)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoadArguments(BCodeBodyBuilder.scala:1018)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genNormalMethodCall$1(BCodeBodyBuilder.scala:725)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genApply(BCodeBodyBuilder.scala:754)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:337)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genStat(BCodeBodyBuilder.scala:77)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.$anonfun$genBlock$1(BCodeBodyBuilder.scala:860)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.$anonfun$genBlock$1$adapted(BCodeBodyBuilder.scala:860)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genNormalBlock$1(BCodeBodyBuilder.scala:860)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genBlock(BCodeBodyBuilder.scala:892)
	at scala.tools.nsc.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:408)
	at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.emitNormalMethodBody$1(BCodeSkelBuilder.scala:602)
	at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genDefDef(BCodeSkelBuilder.scala:635)
	at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:506)
	at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.$anonfun$gen$1(BCodeSkelBuilder.scala:508)
	at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.$anonfun$gen$1$adapted(BCodeSkelBuilder.scala:508)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:508)
	at scala.tools.nsc.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genPlainClass(BCodeSkelBuilder.scala:116)
	at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.visit(GenBCode.scala:224)
	at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.run(GenBCode.scala:191)
	at dotty.tools.backend.jvm.GenBCodePipeline.buildAndSendToDisk(GenBCode.scala:421)
	at dotty.tools.backend.jvm.GenBCodePipeline.run(GenBCode.scala:387)
	at dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:69)
	at dotty.tools.dotc.core.Phases$Phase.$anonfun$runOn$1(Phases.scala:295)
	at scala.collection.immutable.List.map(List.scala:283)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:293)
	at dotty.tools.dotc.core.Phases$Phase.runOn$(Phases.scala:292)
	at dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:74)
	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:47)
	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:191)
	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:88)
	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:29)
	at dotty.tools.dotc.Driver.process(Driver.scala:127)
	at dotty.tools.dotc.Driver.process(Driver.scala:96)
	at dotty.tools.dotc.Driver.process(Driver.scala:108)
	at dotty.tools.dotc.Driver.main(Driver.scala:135)
	at dotty.tools.dotc.Main.main(Main.scala)

[Blaisorblade]

Minimizable further to

object App {
  type Id[A] >: A <: A
  val a: List[_ >: Id[_]] = List(0)
}

First impression: Per restrictions on existentials, we should probably reject Id[_] altogether — it can't be translated to a type using wildcard — or approximate it, by simplifying it to _ and approximating according to variance. If we did that, we'd get

object App {
  type Id[A] >: A <: A
  val a: List[_ >: Nothing] = List(0) // bound comes from >: Id[_] and could be erased
  def main(args: Array[String]): Unit = {
    println(a)
  }
}

which is accepted by the REPL.

Worse, if we replace the definition of Id by an equivalent one, the program is already rejected:

object App {
  type Id[A] = A
  val a: List[_ >: Id[_]] = List(0)
}
3 |    val a: List[_ >: Id[_]] = List(0)
  |                     ^^^^^
  |unreducible application of higher-kinded type [A] => A to wildcard arguments

[smarter]

The Id[_] isn't needed to reproduce the crash, it's a bug in erasure.

object App {
  type Id[A] >: A
  val a: List[_ >: Id[Int]] = List(1,2)
}

I'm on it.

[smarter]

@Blaisorblade However, the fact that Id[_] does not lead to an unreducible application error might be a separate issue too.