Pickler crashes with unresolved local type in closure

Compiler version

Fails in 3.1.1 Works in 3.1.2 Works In 3.1.3-RC4 Fails in 3.1.3-RC5 Fails in 3.2.0-RC1

Minimized code

Based on https://scala3.westeurope.cloudapp.azure.com/blue/organizations/jenkins/buildCommunityProject/detail/buildCommunityProject/561/pipeline

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
  }
}

Output (click arrow to expand)

java.lang.AssertionError: assertion failed: unresolved symbols: type R (line 11) #6547 when pickling /home/wmazur/projects/virtuslab/community-build3/test.scala
        at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
        at dotty.tools.dotc.core.tasty.TreePickler.pickle(TreePickler.scala:783)
        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$$anonfun$adapted$1(Run.scala:262)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:262)
        at dotty.tools.dotc.Run.compileSources(Run.scala:186)
        at dotty.tools.dotc.Run.compile(Run.scala:170)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
        at dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
        at dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
        at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.compileSources$1(BloopHighLevelCompiler.scala:133)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$7(BloopHighLevelCompiler.scala:151)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$1(BloopHighLevelCompiler.scala:71)
        at bloop.tracing.BraveTracer.traceInternal(BraveTracer.scala:65)
        at bloop.tracing.BraveTracer.trace(BraveTracer.scala:39)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.timed$1(BloopHighLevelCompiler.scala:70)
        at sbt.internal.inc.bloop.internal.BloopHighLevelCompiler.$anonfun$compile$6(BloopHighLevelCompiler.scala:151)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
        at monix.eval.internal.TaskRunLoop$.monix$eval$internal$TaskRunLoop$$loop$1(TaskRunLoop.scala:187)
        at monix.eval.internal.TaskRunLoop$RestartCallback$1.onSuccess(TaskRunLoop.scala:119)
        at monix.eval.Task$.$anonfun$forkedUnit$2(Task.scala:1463)
        at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1395)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)
Error: Unexpected error when compiling project_6d5831c594_6d5831c594-15348604ae: 'assertion failed: unresolved symbols: type R (line 11) #6547 when pickling /home/wmazur/projects/virtuslab/community-build3/test.scala'

Code after typer phase (click arrow to expand)

Differences in enclosing packages and imports originate from minimalizing issue in the original repo

package <empty> {
  import util.WithTemporaryCollection.*
  import org.specs2.matcher.MatchResult
  final class ChangeStreamSpec() extends Object() {
    def resultOf[T >: Nothing <: Any](v: T): org.specs2.matcher.MatchResult[T]
       = 
    ???
    trait Coll() extends Object {}
    def convert[T >: Nothing <: Seq[? >: Nothing <: Any], 
      U >: Nothing <: org.specs2.matcher.MatchResult[? >: Nothing <: Any]
    ](fn: T => U)(implicit x: Seq[ChangeStreamSpec.this.Coll]): U = ???
    def convert$default$2[T >: Nothing <: Seq[? >: Nothing <: Any], 
      U >: Nothing <: org.specs2.matcher.MatchResult[? >: Nothing <: Any]
    ](fn: T => U): Seq[ChangeStreamSpec.this.Coll] @uncheckedVariance = 
      Seq.empty[ChangeStreamSpec.this.Coll]
    {
      val fn$1: Seq[?] => org.specs2.matcher.MatchResult[R] = 
        {
          {
            def $anonfun(_$1: Seq[?]): org.specs2.matcher.MatchResult[R] = 
              {
                type R = String
                this.resultOf[R](???):org.specs2.matcher.MatchResult[String]
              }
            closure($anonfun)
          }
        }
      this.convert[Seq[?], org.specs2.matcher.MatchResult[R]](fn$1)(
        this.convert$default$2[Seq[?], org.specs2.matcher.MatchResult[R]](fn$1)
      )
    }
  }
}

[griggt]

Seems like this started working after #14026.

The breakage between 3.1.3-RC4 and RC5 was caused by #15388

The corresponding PR #15343 is what broke it on main

[dwijnand]

Hmm. Same result, but perhaps they’re different causes, different issue numbers.