AssertionError when typechecking a match with variance

[adamw]

Minimized code

object Test {
  sealed trait Base[-X]
  case class Child[X]() extends Base[X]

  def apply[A, B](r: Base[A with B]): Unit = {
    r match {
      case Child() => ()
    }
  }
}

This is simplified version of code from sttp. Using dotty 0.26.0-RC1.

Output

java.lang.AssertionError: assertion failed while compiling /Users/adamw/projects/sttp/core/src/main/scala/sttp/client/monad/Test.scala
[error] ## Exception when compiling 45 sources to /Users/adamw/projects/sttp/core/target/jvm-0.26/classes
[error] java.lang.AssertionError: assertion failed
[error] scala.Predef$.assert(Predef.scala:267)
[error] scala.Predef$Ensuring$.ensuring$extension(Predef.scala:362)
[error] dotty.tools.dotc.core.ProperGadtConstraint.fullBounds(GadtConstraint.scala:176)
[error] dotty.tools.dotc.typer.Typer$$anon$1.transform(Typer.scala:1395)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform$$anonfun$2(Trees.scala:1371)
[error] scala.collection.immutable.List.mapConserve(List.scala:472)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1371)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1274)
[error] dotty.tools.dotc.typer.Typer$$anon$1.transform(Typer.scala:1388)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1334)
[error] dotty.tools.dotc.typer.Typer$$anon$1.transform(Typer.scala:1388)
[error] dotty.tools.dotc.ast.Trees$Instance$TreeMap.transform(Trees.scala:1280)
[error] dotty.tools.dotc.typer.Typer$$anon$1.transform(Typer.scala:1388)
[error] dotty.tools.dotc.typer.Typer.caseRest$1(Typer.scala:1426)
[error] dotty.tools.dotc.typer.Typer.typedCase(Typer.scala:1437)
[error] dotty.tools.dotc.typer.Typer.typedCases$$anonfun$1(Typer.scala:1374)
[error] dotty.tools.dotc.core.Decorators$ListDecorator$.loop$1(Decorators.scala:69)
[error] dotty.tools.dotc.core.Decorators$ListDecorator$.mapconserve$extension(Decorators.scala:85)
[error] dotty.tools.dotc.typer.Typer.typedCases(Typer.scala:1376)
[error] dotty.tools.dotc.typer.Typer.$anonfun$17(Typer.scala:1366)
[error] dotty.tools.dotc.typer.Applications.harmonic(Applications.scala:2072)
[error] dotty.tools.dotc.typer.Applications.harmonic$(Applications.scala:233)
[error] dotty.tools.dotc.typer.Typer.harmonic(Typer.scala:91)
[error] dotty.tools.dotc.typer.Typer.typedMatchFinish(Typer.scala:1366)
[error] dotty.tools.dotc.typer.Typer.typedMatch(Typer.scala:1331)
[error] dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2436)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2484)
[error] dotty.tools.dotc.typer.Typer.op$1(Typer.scala:2553)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2554)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2557)
[error] dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2671)
[error] dotty.tools.dotc.typer.Typer.typedBlock(Typer.scala:871)
[error] dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2431)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2484)
[error] dotty.tools.dotc.typer.Typer.op$1(Typer.scala:2553)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2554)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2557)
[error] dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2671)
[error] dotty.tools.dotc.typer.Typer.$anonfun$32(Typer.scala:1946)
[error] dotty.tools.dotc.typer.PrepareInlineable$.dropInlineIfError(PrepareInlineable.scala:217)
[error] dotty.tools.dotc.typer.Typer.typedDefDef(Typer.scala:1946)
[error] dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2400)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2483)
[error] dotty.tools.dotc.typer.Typer.op$1(Typer.scala:2553)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2554)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2557)
[error] dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2579)
[error] dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2627)
[error] dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:2081)
[error] dotty.tools.dotc.typer.Typer.typedTypeOrClassDef$2(Typer.scala:2411)
[error] dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2415)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2483)
[error] dotty.tools.dotc.typer.Typer.op$1(Typer.scala:2553)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2554)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2557)
[error] dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2579)
[error] dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2627)
[error] dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:2207)
[error] dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2455)
[error] dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2484)
[error] dotty.tools.dotc.typer.Typer.op$1(Typer.scala:2553)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2554)
[error] dotty.tools.dotc.typer.Typer.typed(Typer.scala:2557)
[error] dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2671)
[error] dotty.tools.dotc.typer.FrontEnd.liftedTree1$1(FrontEnd.scala:78)
[error] dotty.tools.dotc.typer.FrontEnd.typeCheck$$anonfun$1(FrontEnd.scala:83)
[error] dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
[error] dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:42)
[error] dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:84)
[error] dotty.tools.dotc.typer.FrontEnd.runOn$$anonfun$3(FrontEnd.scala:119)
[error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.immutable.List.foreach(List.scala:333)
[error] dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:119)
[error] dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:181)
[error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
[error] dotty.tools.dotc.Run.runPhases$5(Run.scala:191)
[error] dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:199)
[error] dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
[error] dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:64)
[error] dotty.tools.dotc.Run.compileUnits(Run.scala:206)
[error] dotty.tools.dotc.Run.compileSources(Run.scala:143)
[error] dotty.tools.dotc.Run.compile(Run.scala:125)
[error] dotty.tools.dotc.Driver.doCompile(Driver.scala:38)
[error] dotty.tools.dotc.Driver.process(Driver.scala:195)
[error] dotty.tools.dotc.Main.process(Main.scala)
[error] xsbt.CachedCompilerImpl.run(CachedCompilerImpl.java:69)

Expectation

Compiled code, no error ;)

[IndiscriminateCoding]

I'm also hitting this (but with union types instead of with).
Workaround is matching on a case class type w/ existentials and then asInstanceOf it to a (hopefully proper) type - which is ugly and easy to get wrong.
Any chances it would be fixed soon?