Mismatched existential skolem type error

[retronym]

import scala.async.internal.AsyncId._
class F[A]
class S[A]
def result[A](f: F[A]): S[A with String] = ???
val f: F[_] = ???
val s /* F[$1 with String] */ = result(f)
async {
  val res = await(s)
  if(true) {
    await(0)
  }
  res
}

More test cases and analysis in #77