Description
The smallest example I was able to distill involves a type lambda:
sealed trait Foo[F[_]]
case class Bar[F[_]]() extends Foo[F]
object Test {
val foo: Foo[({ type Out[X] = String })#Out] = ???
foo match {
case Bar() => // error: constructor cannot be instantiated to expected type
// found : Bar[F]
// required: Foo[[X]String]
}
}It is reproducible also without type lambdas, but the example is slightly more involved (some simpler cases do work):
sealed trait Foo[F[_]]
case class Bar[F[_]]() extends Foo[F]
trait TC[A, B] {
type F[X] = B
}
object TC {
implicit val intInstance: TC[Int, String] =
new TC[Int, String] {}
implicit class Ops[A, B](a: A)(implicit val tc: TC[A, B]) {
def getFoo: Foo[tc.F] = ???
}
1.getFoo match {
case Bar() => // error: constructor cannot be instantiated to expected type
// found : Bar[F]
// required: Foo[[X]String]
}
}In both cases, it seems it got the shape of type arguments right, but doesn't unify F[_] with [X]String.