Scala 2 type inference questions

Language Design
1

The following code does not compile with Scala 2.13.3 but with Dotty. Is there a chance that this can be fixed in the Scala 2.13 line? Is there a workaround without giving explicit type annoations?

object TypeDeriviationTest {

  trait Record

  trait Table[R <: Record]

  def test[R <: Record, T <: Table[R]](t: T) = ???

  class Rec extends Record

  class Tab extends Table[Rec]

  // Error:(13, 3) inferred type arguments [Nothing,TypeDeriviationTest.Tab] do not conform to method test's type parameter bounds [R <: TypeDeriviationTest.Record,T <: TypeDeriviationTest.Table[R]]
  test(new Tab)

}
2

You have to get the R in the parameter list somehow:

def test[R <: Record, T <: Table[R]](t: T with Table[R]) = ???
3

Is there no type member trickery? In this particular case there is no record but only a table definition available.

4

The trick is to type t as T with Table[R] instead of T.

I guess you could also make it work by making R a type member of Table instead of a type parameter. But whether that’s a good idea probably depends on the rest of your design and not on a single method.

5

Many thanks! Will apply the with trick.

6

Another one: The following code does not compile with Scala 2.13.3 but with Dotty. (The LUB is not derived.) Is there a chance that this can be fixed in the Scala 2.13 line? Is there a workaround without giving explicit type annoations?

object TDT {

  trait Base

  class A extends Base
  class B extends Base

  implicit class OptMap[X](x: X) {
    def optMap[O, Y >: X](o: Option[O])(f: (X, O) => Y): Y = o match {
      case Some(o) => f(x, o)
      case None => x
    }
  }

//  Error:(32, 37) type mismatch;
//  found   : TDT.B
//  required: TDT.A
  (new A).optMap(Some(1))((a, o) => new B)
}