Better type inference for Scala: send us your problematic cases!

Language Design
166

I’m not sure if this is intended to work, but implicit search fails here

trait Typeclass[Self] {
  extension (self: Self) def name: String
}

object Hidden {
  given Typeclass[String] with {
    extension (self: String) def name: String = self
  }

  given Typeclass[Int] with {
    extension (self: Int) def name: String = self.toString
  }
}

class TypeclassBox[T: Typeclass](t: T) {
  def apply[U](f: T => Typeclass[T] ?=> U): U = f(t)
}

def branch(b: Boolean): TypeclassBox[_] = {
  import Hidden.given
  if (b) TypeclassBox("4") else TypeclassBox(3)
}
val box = branch(true)
box(_.name) // error: value name is not a member of Playground.box.T

It works fine if branch statically returns TypeclassBox[String] or TypeclassBox[Int]. I’m not sure if there’s any expectation of implicit search on wildcard types, but it’d be nice!