Internal error with case classes without a base case

[Frostweeds]

CC: @jad-hamza

object Main {
  sealed abstract class D
  case class A(d: D) extends D
  case class B(a: A) extends D
  
  def f(d: D) = ()
}

[Internal] Not well formed definition @invariant(inv)

Changing the program as follow resolves the issue

object Main {
  sealed abstract class D
  case class C() extends D
  case class A(d: D) extends D
  case class B(a: A) extends D
  
  def f(d: D) = ()
}