Open
Description
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