Internal compiler assertion failure on extension method

[XDracam]

Compiler version

3.0.1

Minimized code

Hint: when I comment out the reversed method, everything compiles just fine

object Implementing_Tuples:

  sealed trait Tup
  case class ConsTup[T, H <: Tup](head: T, tail: H) extends Tup
  case object EmptyTup extends Tup

  val *: = ConsTup                     // for unapply
  type *:[H, T <: Tup] = ConsTup[H, T] // for type matching
  type EmptyTup = EmptyTup.type        // for type matching

  extension [H](head: H)
    def *:[T <: Tup](tail: T) = ConsTup(head, tail)

  type Fold[T <: Tup, Seed, F[_,_]] = T match
    case EmptyTup => Seed
    case h *: t => Fold[t, F[Seed, h], F]

  extension [T <: Tup](v: T)
    def fold[Seed, F[_,_]](seed: Seed)(
      fn: [C, Acc] => (C, Acc) => F[C, Acc]
    ): Fold[T, Seed, F] =
      (v match
        case EmptyTup => seed
        case h *: t => t.fold(fn(h, seed))(fn)
      ).asInstanceOf[Fold[T, Seed, F]]

  extension [T <: Tup](v: T) def reversed =
    v.fold[EmptyTup, [C, Acc] =>> Acc match {
      case h *: t => C *: h *: t
    }](EmptyTup)(
      [C, Acc] => (c: C, acc: Acc) => acc match
        case _@(_ *: _) => c *: acc
    )

  @main def testProperFold =
    val t = (1 *: '2' *: "foo" *: EmptyTup)
    val reversed: (String *: Char *: Int *: EmptyTup) = t.reversed
    println(reversed)

end Implementing_Tuples

Output (click arrow to expand)

Heavily cropped because the stack trace is quite large

java.lang.AssertionError: assertion failed
	at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:11)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.extensionParam(SymDenotations.scala:341)
	at dotty.tools.dotc.typer.Typer.isLocalExtensionMethodRef$3$$anonfun$2(Typer.scala:524)
	at dotty.tools.dotc.core.Denotations$SingleDenotation.hasAltWith(Denotations.scala:637)
	at dotty.tools.dotc.core.Denotations$MultiDenotation.hasAltWith(Denotations.scala:1251)
	at dotty.tools.dotc.typer.Typer.isLocalExtensionMethodRef$1(Typer.scala:524)
	at dotty.tools.dotc.typer.Typer.typedIdent(Typer.scala:530)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2664)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2757)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2823)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2827)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2943)
	at dotty.tools.dotc.typer.Typer.typeSelectOnTerm$1(Typer.scala:609)
	at dotty.tools.dotc.typer.Typer.typedSelect(Typer.scala:659)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2665)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2757)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2823)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2827)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2943)
	at dotty.tools.dotc.typer.Applications.tryWithProto$1(Applications.scala:1256)
........