assertion failed: unresolved symbols while pickling quote with type bounds

[xuwei-k]

Compiler version

• 3.1.2-RC2
• 3.1.1
• 3.1.3-RC1-bin-20220317-17e46ad-NIGHTLY

Minimized code

import scala.quoted.*

object Main {
  def foo(a: Expr[Any])(using Quotes) = {
    a match {
      case '{ ($x: Set[t]).toSet } =>
      case _ =>
    }
  }
}

workaround(?)

import scala.quoted.*

object Main {
  def foo(a: Expr[Any])(using Quotes) = {
    a match {
      case '{
              type t1
              type t2 >: `t1`
              ($x: Set[`t1`]).toSet[`t2`]
            } =>
      case _ =>
    }
  }
}

Output (click arrow to expand)

[error] java.lang.AssertionError: assertion failed: unresolved symbols: value t$given1 (line 5) #14166 when pickling /Users/kenji/set-expr-match/Main.scala
[error] scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error] dotty.tools.dotc.core.tasty.TreePickler.pickle(TreePickler.scala:777)
[error] dotty.tools.dotc.transform.Pickler.run$$anonfun$1$$anonfun$1(Pickler.scala:69)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.immutable.List.foreach(List.scala:333)
[error] dotty.tools.dotc.transform.Pickler.run$$anonfun$1(Pickler.scala:106)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.immutable.List.foreach(List.scala:333)
[error] dotty.tools.dotc.transform.Pickler.run(Pickler.scala:106)
[error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:308)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:309)
[error] dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:111)
[error] dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:261)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] dotty.tools.dotc.Run.runPhases$1(Run.scala:272)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:280)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:289)
[error] dotty.tools.dotc.Run.compileSources(Run.scala:222)
[error] dotty.tools.dotc.Run.compile(Run.scala:206)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:39)
[error] dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
[error] dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
[error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:192)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:247)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:182)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:210)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:177)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:175)
[error] sbt.internal.inc.Incremental$$anon$2.run(Incremental.scala:461)
[error] sbt.internal.inc.IncrementalCommon$CycleState.next(IncrementalCommon.scala:116)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:56)
[error] sbt.internal.inc.IncrementalCommon$$anon$1.next(IncrementalCommon.scala:52)
[error] sbt.internal.inc.IncrementalCommon.cycle(IncrementalCommon.scala:263)
[error] sbt.internal.inc.Incremental$.$anonfun$incrementalCompile$8(Incremental.scala:416)
[error] sbt.internal.inc.Incremental$.withClassfileManager(Incremental.scala:503)
[error] sbt.internal.inc.Incremental$.incrementalCompile(Incremental.scala:403)
[error] sbt.internal.inc.Incremental$.apply(Incremental.scala:169)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileInternal(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileIncrementally$1(IncrementalCompilerImpl.scala:482)
[error] sbt.internal.inc.IncrementalCompilerImpl.handleCompilationError(IncrementalCompilerImpl.scala:332)
[error] sbt.internal.inc.IncrementalCompilerImpl.compileIncrementally(IncrementalCompilerImpl.scala:420)
[error] sbt.internal.inc.IncrementalCompilerImpl.compile(IncrementalCompilerImpl.scala:137)
[error] sbt.Defaults$.compileIncrementalTaskImpl(Defaults.scala:2366)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$2(Defaults.scala:2316)
[error] sbt.internal.server.BspCompileTask$.$anonfun$compute$1(BspCompileTask.scala:30)
[error] sbt.internal.io.Retry$.apply(Retry.scala:46)
[error] sbt.internal.io.Retry$.apply(Retry.scala:28)
[error] sbt.internal.io.Retry$.apply(Retry.scala:23)
[error] sbt.internal.server.BspCompileTask$.compute(BspCompileTask.scala:30)
[error] sbt.Defaults$.$anonfun$compileIncrementalTask$1(Defaults.scala:2314)
[error] scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error] sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:62)
[error] sbt.std.Transform$$anon$4.work(Transform.scala:68)
[error] sbt.Execute.$anonfun$submit$2(Execute.scala:282)
[error] sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:23)
[error] sbt.Execute.work(Execute.scala:291)
[error] sbt.Execute.$anonfun$submit$1(Execute.scala:282)
[error] sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[error] sbt.CompletionService$$anon$2.call(CompletionService.scala:64)
[error] java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
[error] java.util.concurrent.FutureTask.run(FutureTask.java:266)
[error] java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
[error] java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
[error] java.lang.Thread.run(Thread.java:750)

[nicolasstucki]

At first glance of the stacktrace, it seems this is happening in the Pickler phase before the changes in the later phases of #12540.
The Typer, in QuotesAndSplices might be generating type reference that the pickler does not know how to handle.

[odersky]

The unresolved symbol gets created in typedQuoteTypeVar.
Looking at the code after Typer, something seems to be off:

[[syntax trees at end of                     typer]] // i14708.scala
package <empty>#1677 {
  import scala#25.quoted#422.*
  final lazy module val Main#2822: Main#2823 = new Main#2823()
  final module class Main#2823() extends Object() { this: Main#2822.type =>
    def foo#6754(a#6786: quoted#422.Expr#4925[Any#456])(using 
      x$2#7232: quoted#422.Quotes#4937
    ): Unit#155 = 
      {
        a#6786 match 
          {
            case 
              x$2#7232.asInstanceOf#2632[
                scala#25.quoted#422.runtime#4949.QuoteMatching#7251
              ].ExprMatch#7287.unapply#15094[Tuple1#302[t#14796 @ _], 
                (scala#25.quoted#422.Type#4943[t#14796], 
                  scala#25.quoted#422.Expr#4925[Set[t#14796]]
                )
              ](
                Tuple2#135.unapply#13801[scala#25.quoted#422.Type#4943[t#14796]
                  , 
                scala#25.quoted#422.Expr#4925[Set[t#14796]]](
                  given t$given1#14935 @ _
                , x#14936 @ _:scala#25.quoted#422.Expr#4925[Set#6796[t#14796]])
              )(
                '{
                  {
                    @patternType type t#14202
                    scala#25.quoted#422.runtime#4949.Patterns#7249.
                      patternHole#14206
                    [Set[t#14202]].toSet#14580[t$given1#14221.Underlying#14218]
                  }
                }.apply#14273(x$2#7232)
              )
             => 
              ()
            case _ => 
              ()
          }
      }
  }

Here, we do bind a t$given1 in the pattern of ExprMatch.unapply, but then we use it in the implicit argument to the same unapply. The second usage does not have a corresponding binding. Is this a case
of an illegal recursion between pattern and argument?

[nicolasstucki]

After type checking we have

...
       '{
                  {
                    @patternType type t
                    scala.quoted.runtime.Patterns.patternHole[Set[t]].toSet[t$given1.Underlying]
                  }
                }.apply(x$2)
...

but we should have

       '{
                  {
                    @patternType type t
                    scala.quoted.runtime.Patterns.patternHole[Set[t]].toSet[t]
                  }
                }.apply(x$2)

This must be a bug in QuotesAndSplices.scala. Probably in splitQuotePattern.

[nicolasstucki]

Simpler workaround

-      case '{ ($x: Set[t]).toSet } =>
+      case '{ type t; ($x: Set[`t`]).toSet } =>