Codegen crashes on implicit lookup of Type[T], where T is type member of variable from match case

[fhackett]

Note: this is a New and Improved™ version of this issue after I discovered a much more precise description of the problem while trying to develop a temporary workaround. I edited the existing issue since it is still the exact same problem, hopefully that's ok.

// we need this to get something with a type member
// and a corresponding implicitly unreachable but known Type node
import scala.quoted._
trait Trait {
  type t
  val tType : Type[t]
}

object O {
  def fn[T](c : Trait) given QuoteContext : Unit = c match {
    case v => {
      implicit val _ : Type[v.t] = v.tType
      type VT = v.t
      val a = '[List[VT]]
      ()
    }
  }
}

This, as long as the '[...] is a non-trivial expression in terms of VT, crashes Dotty's codegen ('[VT] is fine, probably because it does not need to generate anything). This can be reproduced as long as v comes from a pattern match, and VT is derived from a type member of v. This includes implicit Type nodes generated by calling a function that takes Type[...VT...] as an implicit argument.

The crash can be avoided by explicitly writing out the type splices:

object O {
  def fn[T](c : Trait) : Unit = c match {
    case v => {
      implicit val _ : Type[v.t] = v.tType
      type VT = v.t
      val tp : Type[List[VT]] = '[List[${v.tType}]]
      ()
    }
  }
}

The error will look something like this:

java.util.NoSuchElementException: class Type
        at scala.collection.mutable.AnyRefMap$ExceptionDefault.apply(AnyRefMap.scala:437)
        at scala.collection.mutable.AnyRefMap$ExceptionDefault.apply(AnyRefMap.scala:436)
        at scala.collection.mutable.AnyRefMap.apply(AnyRefMap.scala:192)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder$locals$.load(BCodeSkelBuilder.scala:396)
        at dotty.tools.backend.jvm.BCodeBodyBuilder$PlainBodyBuilder.genLoad(BCodeBodyBuilder.scala:392)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.emitNormalMethodBody$1(BCodeSkelBuilder.scala:
597)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genDefDef(BCodeSkelBuilder.scala:630)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:501)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen$$anonfun$1(BCodeSkelBuilder.scala:503)
        at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:392)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.gen(BCodeSkelBuilder.scala:503)
        at dotty.tools.backend.jvm.BCodeSkelBuilder$PlainSkelBuilder.genPlainClass(BCodeSkelBuilder.scala:109)
        at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.visit(GenBCode.scala:213)
        at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.run(GenBCode.scala:180)
        at dotty.tools.backend.jvm.GenBCodePipeline.buildAndSendToDisk(GenBCode.scala:510)
        at dotty.tools.backend.jvm.GenBCodePipeline.run(GenBCode.scala:476)
        at dotty.tools.backend.jvm.GenBCode.run(GenBCode.scala:54)
        at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:313)
        at scala.collection.immutable.List.map(List.scala:286)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:315)
        at dotty.tools.backend.jvm.GenBCode.runOn(GenBCode.scala:59)
        at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:158)
        at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:36)
        at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:33)
        at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:198)
        at dotty.tools.dotc.Run.runPhases$5(Run.scala:170)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:178)
        at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:102)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:185)
        at dotty.tools.dotc.Run.compileSources(Run.scala:120)
        at dotty.tools.dotc.Run.compile(Run.scala:104)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:33)
        at dotty.tools.dotc.Driver.process(Driver.scala:170)
        at dotty.tools.dotc.Driver.process(Driver.scala:139)
        at dotty.tools.dotc.Driver.process(Driver.scala:151)
        at dotty.tools.dotc.Driver.main(Driver.scala:178)
        at dotty.tools.dotc.Main.main(Main.scala)
Error while emitting Polyparse.scala
class Type

[fhackett]

I've updated the issue above with a much more precise description - hopefully that helps.

[nicolasstucki]

It looks like the issue comes from the _ of implicit val _ : Type[v.t] = v.tType, giving it any other name fixes it.

[nicolasstucki]

Can be minimized to

object O {
  def foo = {
    type T
    implicit val _ : scala.quoted.Type[T] = ???
    '[List[T]]
    ()
  }
}