trait Collection[Self, Element]:
  type Index
  extension (self: Self)
    def start: Index

sealed trait Tree[+T]
object Tree:
  case object Empty extends Tree[Nothing]
  case class Node[+T](value: T, lhs: Tree[T], rhs: Tree[T]) extends Tree[T]

enum Direction:
  case Left, Right, Here

given [T]: Collection[Tree[T], T] with
  type Index = List[Direction]
  extension (self: Tree[T])
    def start: List[Direction] = match self
      case Empty => Nil
      case Node(_, l, _) => l.start :+ Left

Exception in thread "main" java.lang.AssertionError: assertion failed: position error, parent span does not contain child span
parent      = [T] extends Collection[Tree[T], T] {
  type Index = List[Direction]
  extension (self: Tree[T])
    {
      def start: List[Direction] = _root_.scala.Predef.???
      def <error> = _root_.scala.Predef.???
      def <error> = _root_.scala.Predef.???
    }
} # -1,
parent span = <1682..1884>,
child       = extension (self: Tree[T])
  {
    def start: List[Direction] = _root_.scala.Predef.???
    def <error> = _root_.scala.Predef.???
    def <error> = _root_.scala.Predef.???
  } # -1,
child span  = [1747..2046]
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
	at dotty.tools.dotc.ast.Positioned.check$1(Positioned.scala:177)
	at dotty.tools.dotc.ast.Positioned.check$1$$anonfun$3(Positioned.scala:207)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:334)
	at dotty.tools.dotc.ast.Positioned.check$1(Positioned.scala:207)
	at dotty.tools.dotc.ast.Positioned.checkPos(Positioned.scala:228)
	at dotty.tools.dotc.ast.Positioned.check$1(Positioned.scala:202)
	at dotty.tools.dotc.ast.Positioned.checkPos(Positioned.scala:228)
	at dotty.tools.dotc.ast.Positioned.check$1(Positioned.scala:202)
	at dotty.tools.dotc.ast.Positioned.check$1$$anonfun$3(Positioned.scala:207)
	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:334)
	at dotty.tools.dotc.ast.Positioned.check$1(Positioned.scala:207)
	at dotty.tools.dotc.ast.Positioned.checkPos(Positioned.scala:228)
	at dotty.tools.dotc.parsing.Parser.parse$$anonfun$1(ParserPhase.scala:39)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at dotty.tools.dotc.core.Phases$Phase.monitor(Phases.scala:510)
	at dotty.tools.dotc.parsing.Parser.parse(ParserPhase.scala:40)
	at dotty.tools.dotc.parsing.Parser.$anonfun$2(ParserPhase.scala:52)
	at scala.collection.Iterator$$anon$6.hasNext(Iterator.scala:479)
	at scala.collection.Iterator$$anon$9.hasNext(Iterator.scala:583)
	at scala.collection.immutable.List.prependedAll(List.scala:152)
	at scala.collection.immutable.List$.from(List.scala:685)
	at scala.collection.immutable.List$.from(List.scala:682)
	at scala.collection.IterableOps$WithFilter.map(Iterable.scala:900)
	at dotty.tools.dotc.parsing.Parser.runOn(ParserPhase.scala:51)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:343)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:336)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:383)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:395)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:69)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:395)
	at dotty.tools.dotc.Run.compileSources(Run.scala:282)
	at dotty.tools.dotc.Run.compile(Run.scala:267)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
	at dotty.tools.dotc.Driver.process(Driver.scala:201)
	at dotty.tools.dotc.Driver.process(Driver.scala:169)
	at dotty.tools.dotc.Driver.process(Driver.scala:181)
	at dotty.tools.dotc.Driver.main(Driver.scala:211)
	at dotty.tools.dotc.Main.main(Main.scala)
Compilation failed