scala: compare types of inner class objects

Question

How do I know if two objects of inner class have the same runtime type? In the example below I expect to see the class of aa.getClass == a.Inner and ba.getClass == b.Inner, but in fact they are both Outer.Inner and equal.

class Outer{
      class Inner{}
    }

    val a = new Outer
    val b = new Outer

    val aa = new a.Inner
    val ab = new a.Inner
    val ba = new b.Inner

    val res1 = aa.getClass == ba.getClass
    val res2 = aa.isInstanceOf[ab.type ]

    scala>      |      | defined class Outer

    scala> a: Outer = Outer@550a1967

    scala> b: Outer = Outer@5f9678e1

    scala> aa: a.Inner = Outer$Inner@70a36a66

    scala> ab: a.Inner = Outer$Inner@1dd6d4b7

    scala> ba: b.Inner = Outer$Inner@2e61d218

    scala> res1: Boolean = true

    scala> res2: Boolean = false

Answer 1

... aa.getClass == a.Inner and ba.getClass == b.Inner, but in fact they are both Outer.Inner and equal

This is not true. Inner is a class member and is unique to its parent instance of Outer. This means that both a and b have their own unique version of Inner, which are incompatible types. So a.Inner is not the same type as b.Inner, and therefore an a.Inner can never be equal to a b.Inner. I cannot assign one for the other:

scala> val z: a.Inner = aa       // aa is a.Inner, so this is ok
z: a.Inner = Outer$Inner@575d06dd

scala> val z: b.Inner = aa       // aa is not b.Inner, so it fails to compile
<console>:14: error: type mismatch;
 found   : a.Inner
 required: b.Inner
       val z: b.Inner = aa
                        ^

getClass just isn't very useful here.

We can prove this with reflection:

import scala.reflect.runtime.universe._

def tpeOf[A](a: A)(implicit tt: TypeTag[A]) = tt.tpe

scala> tpeOf(aa) =:= tpeOf(ba) // different Outer parents
res24: Boolean = false

scala> tpeOf(aa) =:= tpeOf(aa) // Same instance
res25: Boolean = true

scala> tpeOf(aa) =:= tpeOf(ab) // Same Outer parent
res26: Boolean = true

On the other hand, you can use Outer#Inner to specify that you don't care which Outer your Inner type belongs to.

val x: Outer#Inner = aa
val x: Outer#Inner = ab
val x: Outer#Inner = ba

So as stated by @BenReich, you could use aa.isInstanceOf[Outer#Inner] to check if you have any of those types, and they would all return true.

ab.type means something completely different. ab.type is a singleton type that contains nothing but ab. So naturally then, aa.isInstanceOf[ab.type] must be false, because aa is not ab, regardless of whether or not they are both a.Inner.