NotFantastic.scala

// this code demonstrates unsoundness in scala (tested in 2.11 and 2.12)
//
// it seems like scala is checking that the return type is a Pipe[_]
// but not a Pipe[E]; maybe due to erasure?

sealed abstract class Pipe[E]
case class Single[E](elem: E) extends Pipe[E]
case class Double[A, B](a: Pipe[A], b: Pipe[B]) extends Pipe[(A, B)]

object Pipe {
  def prepare[E](p: Pipe[E]): Pipe[E] =
    p match {
      case Single(_) =>
        p
      case Double(a, b) =>
        // any Double works -- it's type doesn't have to match E.
        Double(Single(1), Single(1))
    }

  prepare[(String, String)](Double(Single("foo"), Single("bar")))
}

Wow, yeah, those Dotty errors look really great @olafurpg!

We ran into a similar problem with record pulled from Datomic (or was it DynamoDB? Not sure). Fields that were supposed to be List[A] were indeed lists, but didn't contains As. I like your example, though: it's minimal in the sense that it gets right to the root of the issue.