Extracting contravariant type parameter in match breaks a quote

[prolativ]

Compiler version

3.2.0-RC3 and earlier

Minimized code

Encoder.scala

import scala.quoted._
import scala.deriving.Mirror

trait Encoder[-A]

trait PrimitiveEncoder[A] extends Encoder[A]

given intOpt: PrimitiveEncoder[Option[Int]] with {}
  
given primitiveNotNull[T](using e: Encoder[Option[T]]): PrimitiveEncoder[T] =
  new PrimitiveEncoder[T] {}

transparent inline given fromMirror[A]: Any = ${ fromMirrorImpl[A] }

def fromMirrorImpl[A : Type](using q: Quotes): Expr[Any] =
  Expr.summon[Mirror.Of[A]].get match
    case '{ ${mirror}: Mirror.ProductOf[A] { type MirroredElemTypes = elementTypes } } =>
      val encoder = Type.of[elementTypes] match
        case '[tpe *: EmptyTuple] =>
          Expr.summon[Encoder[tpe]].get

      encoder match
        case '{ ${encoder}: Encoder[tpe] } => println("AAA")
        case _ => println("BBB")

      encoder match
        case '{ ${encoder}: Encoder[tpe] } => println("AAA")
        case _ => println("BBB")

      encoder

Main.scala

case class JustInt(i: Int)

val x = fromMirror[JustInt]

Output

AAA
BBB
java.lang.AssertionError: assertion failed: unresolved symbols: type tpe (line 22) #29193 when pickling Main.scala while compiling Encoder.scala, Main.scala
[error] ## Exception when compiling 2 sources to dotty-test/tmp3/target/scala-3.2.0-RC3/classes
[error] java.lang.AssertionError: assertion failed: unresolved symbols: type tpe (line 22) #29193 when pickling Main.scala
[error] scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error] dotty.tools.dotc.core.tasty.TreePickler.pickle(TreePickler.scala:783)
[error] dotty.tools.dotc.transform.Pickler.run$$anonfun$1$$anonfun$1(Pickler.scala:72)
[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$$anonfun$1(Pickler.scala:110)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.immutable.List.foreach(List.scala:333)
[error] dotty.tools.dotc.transform.Pickler.run(Pickler.scala:110)
[error] dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:311)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:312)
[error] dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:115)
[error] dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:234)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
[error] dotty.tools.dotc.Run.runPhases$1(Run.scala:245)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:253)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:262)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:262)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:192)
[error] dotty.tools.dotc.Driver.finish(Driver.scala:56)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:36)
[error] dotty.tools.xsbt.CompilerBridgeDriver.run(CompilerBridgeDriver.java:88)
[error] dotty.tools.xsbt.CompilerBridge.run(CompilerBridge.java:22)
[error] sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:91)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$7(MixedAnalyzingCompiler.scala:192)
[error] scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
[error] sbt.internal.inc.MixedAnalyzingCompiler.timed(MixedAnalyzingCompiler.scala:247)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4(MixedAnalyzingCompiler.scala:182)
[error] sbt.internal.inc.MixedAnalyzingCompiler.$anonfun$compile$4$adapted(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.JarUtils$.withPreviousJar(JarUtils.scala:239)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compileScala$1(MixedAnalyzingCompiler.scala:163)
[error] sbt.internal.inc.MixedAnalyzingCompiler.compile(MixedAnalyzingCompiler.scala:210)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.IncrementalCompilerImpl.$anonfun$compileInternal$1$adapted(IncrementalCompilerImpl.scala:528)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5(Incremental.scala:177)
[error] sbt.internal.inc.Incremental$.$anonfun$apply$5$adapted(Incremental.scala:175)
[error] sbt.internal.inc.Incremental$$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:748)
[error]            
[error] stack trace is suppressed; run last tmp3 / Compile / compileIncremental for the full output
[error] (tmp3 / Compile / compileIncremental) java.lang.AssertionError: assertion failed: unresolved symbols: type tpe (line 22) #29193 when pickling Main.scala

Expectation

This should compile successfully, without breaking the pickler.

It looks like an attempt to extract a type parameter from the type of a quoted expression in a match somehow mutates the quote: The first occurrence of

      encoder match
        case '{ ${encoder}: Encoder[tpe] } => println("AAA")
        case _ => println("BBB")

prints AAA, the second one prints BBB.

On the other hand nothing breaks if at least one of the modifications below is performed:

• Encoder is declared as invariant
• PrimitiveEncoder is declared as contravariant
• the definition of JustInt gets moved to Encoder.scala
• case '{ ${encoder}: Encoder[tpe] } => println("AAA") branch gets removed from both matches
• the return value of fromMirrorImpl is something else than encoder