Compiler accepts a class as non-abstract, then the runtime system fails reporting that class as abstract

[jphmrst]

reproduction steps

using Scala 2.13.6,

import scala.collection.mutable.Builder

class DDD[S,T,A]

trait NN[S, T, A, K[_], +D <: DDD[Set[S],T,K[A]]]
class NNN[S, T, K[_], A] extends NN[S, T, A, K, DDD[Set[S],T,K[A]]]

object NN {
  def newBuilder[S, T, A, K[_]] =
    new NNNbuilder[S, T, K, A]
}

// Remove the type parameter E, hardcoding in E := Unit, and the issue
// goes away.
trait NNbuilder
  [S, T, K[_], A, +D <: DDD[Set[S],T,K[A]], +N <: NN[S,T,A,K,D], E]
    extends Builder[Unit, N] {
  def clear(): Unit = throw new UnsupportedOperationException()
  final def addOne(builder: E): this.type = this
}

// Unfold this class defn, and the issue goes away
abstract class AbstractNNNbuilder
  [S, T, K[_], A, +D <: DDD[Set[S],T,K[A]], +N <: NN[S,T,A,K,D], E]
    extends NNbuilder[S,T,K,A,D,N,E]

class NNNbuilder[S, T, K[_], A] extends AbstractNNNbuilder[
  S, T, K, A, DDD[Set[S], T, K[A]], NNN[S, T, K, A], Unit
] {
  override def result(): NNN[S, T, K, A] = new NNN[S, T, K, A]
}

object Test {
  def main(args: Array[String]): Unit = {
    val builder = NN.newBuilder[String, Char, Int, Set]
    builder += ()
    builder.result()
  }
}

problem

I believe the compiler is correct that the class does not need to be marked abstract; I would it expect it to generate code which the RTS accepts.

See also dotty issue

[jphmrst]

I should add --- the comments refer to things I'd tried in Scala3. I didn't try them here; just reporting the bug here as requested in the dotty ticket.

[joroKr21]

Related? #8681, #12209, scala/scala3#10079

[SethTisue]

the error is:

java.lang.AbstractMethodError: Method NNNbuilder.addOne(Ljava/lang/Object;)Lscala/collection/mutable/Growable; is abstract
  at NNNbuilder.addOne(<console>)
  at scala.collection.mutable.Growable.$plus$eq(Growable.scala:36)
  at scala.collection.mutable.Growable.$plus$eq$(Growable.scala:36)
  at AbstractNNNbuilder.$plus$eq(<console>:23)
  at Test$.main(<console>:36)
  ... 32 elided

[SethTisue]

(Surely this isn't anywhere near minimized...?)