Bridge method missing in presence of type parameter + existentials types

[trobert]

reproduction steps

using Scala 2.13.3, 2.12.12, 2.11.12

Welcome to Scala 2.13.3 (OpenJDK 64-Bit Server VM, Java 1.8.0_222).
Type in expressions for evaluation. Or try :help.

scala> :paste
// Entering paste mode (ctrl-D to finish)

object Test extends App {
  trait Box[A] { self =>
    type T
    def zip(f: Box[A]) = new Box[A] { type T = (self.T, f.T) }
  }
  def opaqueBox[A]: Box[A] = new Box[A] { type T = Int }
  def intBox[A]            = new Box[A] { type T = Int }

  trait Base {
    val box: Box[String] // works: Box[Int]
    def m(x: box.T): Int
  }
  class Impl extends Base {
    import language.existentials

    val box = opaqueBox[String].zip(intBox[String]) // works: opaqueBox[Int].zip(intBox[Int])
    //box: Box[String]{type S = (_1.S, Int)} forSome { val _1: Box[String] }
    def m(x: box.T) = x._2
  }

  val a: Base = new Impl
  val x       = (0, 0).asInstanceOf[a.box.T]
  println(a.m(x))
}



// Exiting paste mode, now interpreting.

object Test

scala> Test.main(Array.empty)
java.lang.AbstractMethodError: Test$Impl.m(Ljava/lang/Object;)I
  at Test$.delayedEndpoint$Test$1(<pastie>:23)
  at Test$delayedInit$body.apply(<pastie>:1)
  at scala.Function0.apply$mcV$sp(Function0.scala:39)
  at scala.Function0.apply$mcV$sp$(Function0.scala:39)
  at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
  at scala.App.$anonfun$main$1(App.scala:73)
  at scala.App.$anonfun$main$1$adapted(App.scala:73)
  at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:553)
  at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:551)
  at scala.collection.AbstractIterable.foreach(Iterable.scala:920)
  at scala.App.main(App.scala:73)
  at scala.App.main$(App.scala:71)
  at Test$.main(<pastie>:1)
  ... 40 elided

problem

Bridge method for m(x: Object) in Impl is not generated, causing the AbstractMethodError.
Changing to Box[Int] makes it work, which is surprising because this does not change the erasure of m (actually A in Box[A] is never used here, but its presence is mandatory to trigger the issue).