AssertionError: assertion failed: orphan parameter reference: TypeParamRef(T) - when matching type

[mwisnicki]

Compiler version

3.2.0-RC1

Minimized code

type Var[T]
type Set[T, D <: Var[T], V <: T]

type Get[S] = S match {
  case Set[?, ?, v] => v
}

https://scastie.scala-lang.org/DswQvPTZTou8wqLq9zAXDw

Output (click arrow to expand)

java.lang.AssertionError: assertion failed: orphan parameter reference: TypeParamRef(T)
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:291)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:160)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$2$$anonfun$1(TreePickler.scala:181)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)

java.lang.AssertionError: assertion failed: orphan parameter reference: TypeParamRef(T)
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:291)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:160)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$2$$anonfun$1(TreePickler.scala:181)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$2(TreePickler.scala:181)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$adapted$2(TreePickler.scala:181)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:181)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:160)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$5(TreePickler.scala:258)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$adapted$5(TreePickler.scala:260)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:260)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:160)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$20(TreePickler.scala:531)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$adapted$18(TreePickler.scala:534)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:534)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$32$$anonfun$1(TreePickler.scala:625)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$32(TreePickler.scala:625)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$adapted$29(TreePickler.scala:625)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:625)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$13(TreePickler.scala:501)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$adapted$12(TreePickler.scala:501)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:501)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$33$$anonfun$1(TreePickler.scala:631)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$33(TreePickler.scala:631)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$adapted$30(TreePickler.scala:632)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:632)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$35(TreePickler.scala:641)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$adapted$32(TreePickler.scala:641)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:641)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTpt(TreePickler.scala:318)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef$$anonfun$1(TreePickler.scala:336)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef$$anonfun$adapted$1(TreePickler.scala:340)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef(TreePickler.scala:340)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:567)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats$$anonfun$2(TreePickler.scala:365)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats(TreePickler.scala:365)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$26(TreePickler.scala:591)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$adapted$24(TreePickler.scala:592)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:592)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef$$anonfun$1(TreePickler.scala:335)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef$$anonfun$adapted$1(TreePickler.scala:340)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef(TreePickler.scala:340)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:567)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats$$anonfun$2(TreePickler.scala:365)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats(TreePickler.scala:365)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$29(TreePickler.scala:607)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$adapted$27(TreePickler.scala:607)
	at scala.Function0.apply$mcV$sp(Function0.scala:39)
	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:58)
	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:607)
	at dotty.tools.dotc.core.tasty.TreePickler.pickle$$anonfun$1(TreePickler.scala:779)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.core.tasty.TreePickler.pickle(TreePickler.scala:779)
	at dotty.tools.dotc.transform.Pickler.run$$anonfun$1$$anonfun$1(Pickler.scala:72)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.transform.Pickler.run$$anonfun$1(Pickler.scala:110)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.transform.Pickler.run(Pickler.scala:110)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:311)
	at scala.collection.immutable.List.map(List.scala:246)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:312)
	at dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:115)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:234)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:245)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:253)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:262)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:262)
	at dotty.tools.dotc.Run.compileSources(Run.scala:186)
	at dotty.tools.dotc.Run.compile(Run.scala:170)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
	at dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
	at dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
	at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:192)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:247)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:182)
	at sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163)
	at sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
	at sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163)
	at sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:210)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:528)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:528)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:177)
	at sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:175)
	at sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:461)
	at sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
	at sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
	at sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
	at sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:263)
	at sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:416)
	at sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:503)
	at sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:403)
	at sbt.internal.inc.Incremental$.apply(Incremental.scala:169)
	at sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:528)
	at sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:482)
	at sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
	at sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:420)
	at sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
	at sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2366)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2316)
	at sbt.internal.server.BspCompileTask$.$anonfun$compute$1(BspCompileTask.scala:30)
	at sbt.internal.io.Retry$.apply(Retry.scala:46)
	at sbt.internal.io.Retry$.apply(Retry.scala:28)
	at sbt.internal.io.Retry$.apply(Retry.scala:23)
	at sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:30)
	at sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2314)
	at scala.Function1.$anonfun$compose$1(Function1.scala:49)
	at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
	at sbt.std.Transform$$anon$4.work(Transform.scala:68)
	at sbt.Execute.$anonfun$submit$2(Execute.scala:282)
	at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
	at sbt.Execute.work(Execute.scala:291)
	at sbt.Execute.$anonfun$submit$1(Execute.scala:282)
	at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
	at sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1135)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:831)

[mwisnicki]

Unrelated to the crash but I would appreciate suggestions how to:

1. avoid explicit T parameter in Set - ideally infer it from D and V
2. a simpler way to extract V from Set in Get

[prolativ]

A slightly smaller minimization:

type Set[T, V <: T]

type Get[S] = S match
  case Set[?, v] => v

[prolativ]

Also the problem disappears if you define Set as a trait instead of a type

[odersky]

The error is likely in PatternTypeComparer.constrainPatternType.

[dwijnand]

Duplicated by #15595, where the minimisation is:

trait MatchResult[+T]

@main def Test() = {
  def convert[T <: Seq[_], U <: MatchResult[_]](fn: T => U)(implicit x: Seq[_] = Seq.empty): U = ???
  def resultOf[T](v: T): MatchResult[T] = ???

  convert { _ =>
    type R = String
    resultOf[R](???)
    // this would not lead to crash: 
    // val x = resultOf[R](???)
    // x
  }
}

which doesn't involve any patterns.

(edit: Changed my mind: it's the same result, but possibly distinct enough it's not a duplicate)

[odersky]

How do we know it's a regression? I am actually not sure this is a duplicate of #15595 after all.

[odersky]

@mwisnicki Try to convert all your types to classes. Matching against types like that is bound to be full of surprises.