Surprising equivalences and non-equivalences regarding this.type

Question

It appears to make a difference whether you refer to this.type from inside a Trait or from the scope where the object is created, with surprising results.

import scala.reflect.runtime.universe._

trait Trait {
  val ttag = typeOf[this.type]
  println(s"Trait constructor: $this")
}

object Instance1 extends Trait

object Instance2 extends Trait

println(typeOf[Instance1.type] =:= typeOf[Instance2.type])  // Should be false
println(Instance1.ttag =:= Instance2.ttag)                  // Should be false
println(Instance1.ttag =:= typeOf[Instance1.type])          // Should be true

Here's the output:

false    // As expected: the singleton types of two objects are different.
Trait constructor: $line9.$read$$iw$$iw$$iw$$iw$Instance1$@58c46295
Trait constructor: $line10.$read$$iw$$iw$$iw$$iw$Instance2$@452451ba
true     // But the this.type tags are equivalent
false    // and the this.type tag is not equivalent to the singleton type.

So, there are two distinct objects, but apparently each is getting an equivalent type tag for its this.type, which is not equivalent to the .type of the object as seen from an enclosing scope.

Is this a compiler bug, or, if not, could you explain why this behavior makes sense?

(I'm running Scala 2.11.2. I've tried it with a self alias for this, with the same result.)

Answer 1

The following program prints false and then true. To my mind, there should be no material difference between these two cases (though that's really more of an opinion; I can't really say if there's a reason or not):

import scala.reflect.runtime.universe._

object Test2 extends App {
  def foo(): Type = {
    object a
    typeOf[a.type]
  }

  println(foo() =:= foo()) // false

  trait Trait {
    val ttag = typeOf[this.type]
  }

  object Instance1 extends Trait

  object Instance2 extends Trait

  println(Instance1.ttag =:= Instance2.ttag) // true
}