GADT inference suffer from polymorphic functions

[DoSOfRedRiver]

minimized code

trait Smt[F[_], A] {
  def fa: F[A]

  def smt[G[_]](f: [X] => F[X] => G[X]): G[A] = f(fa)
}

enum Console[A] {
  case ReadLine extends Console[String]
  case PrintLine extends Console[Unit]
}

val smtListInts = new Smt[Console, String] {
  def fa = Console.ReadLine
}

val res: List[String] = smtListInts.smt[List]([X] => (con: Console[X]) => con match {
  case Console.ReadLine =>
    List("string")
  case Console.PrintLine =>
    List(())
})

expectation

I expect compiler to infer [X] type, but instead, I have to cast results manually:

val res: List[String] = smtListInts.smt[List]([X] => (con: Console[X]) => con match {
  case Console.ReadLine =>
    List("string").asInstanceOf[List[X]]
  case Console.PrintLine =>
    List(()).asInstanceOf[List[X]]
})

[abgruszecki]

Thanks for reporting this issue!

We already track it as #5828, so I'm going to close this one.