Question

I’m running into a weird issue with reflection in Scala 2.10.0 Milestone 4 that I can’t wrap my head around. First for the stuff that works the way I’d expect:

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> trait A[X]; trait B[Y] extends A[Y]
defined trait A
defined trait B

scala> typeOf[B[String]].parents
res0: List[reflect.runtime.universe.Type] = List(java.lang.Object, A[String])

scala> typeOf[B[String]].parents contains typeOf[A[String]]
res1: Boolean = true

Similarly (in the same session):

scala> trait D; trait E extends A[D]
defined trait D
defined trait E

scala> typeOf[E].parents
res2: List[reflect.runtime.universe.Type] = List(java.lang.Object, A[D])

scala> typeOf[E].parents contains typeOf[A[D]]
res3: Boolean = true

No surprises here: I can ask for a type’s parents and get exactly what I expect. Now I essentially combine the two examples above:

scala> trait F extends A[String]
defined trait F

scala> typeOf[F].parents
res4: List[reflect.runtime.universe.Type] = List(java.lang.Object, A[String])

scala> typeOf[F].parents contains typeOf[A[String]]
res5: Boolean = false

I don’t understand how this could be false. The same thing happens if I have F extend A[Seq[D]], A[Int], etc. What’s the generalization I’m missing that would make this behavior make sense?

Answer 1

That’s a bug. Right this morning I was going to investigate and fix it.

Edit. this appears to be an implementation detail of Scala reflection API leaking to the userland. It’s not easy to fix, so for now we leave it as it is, but will look into the possibilities to improve.

In the meanwhile, to get correct results, one should always use =:= to compare types, not ==.