Monomorphism check missing on anonymous fns

[japgolly]

Consider this snippet:

object TypeParam {

  sealed trait F[A]
  case object FI extends F[Int]
  case object FS extends F[String]

  trait FFn { def apply[A](f: F[A]): A }

  val test4: FFn = {
    case FI => 1
    case FS => ""
  }
}

In scalac it fails saying:

  // [error] missing parameter type for expanded function
  // [error] The argument types of an anonymous function must be fully known. (SLS 8.5)
  // [error] Expected type was: TypeParam.FFn
  // [error]   val test4: FFn = {
  // [error]                    ^

In dotc it fails saying:

-- Error: <console>:9:6 --------------------------------------------------------
9 |      case FI => 1
  |      ^
  |     missing parameter type for parameter x$1 of expanded function x$1 => 
  |       x$1 match 
  |         {
  |           case FI => 
  |             1
  |           case FS => 
  |             ""
  |         }, expected = FFn

which is much more confusing a message. It seems a monomorphism check is present in Scala and provides a specific error message where as in Dotty it's missing then fails further down the line.