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TypeTag[this.type]'s of distinct objects are equivalent #9439

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scabug opened this issue Aug 19, 2015 · 3 comments
Closed

TypeTag[this.type]'s of distinct objects are equivalent #9439

scabug opened this issue Aug 19, 2015 · 3 comments
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@scabug
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scabug commented Aug 19, 2015

Inside a trait, typeOf[this.type] results in equivalent Types for distinct objects, as shown below:

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

Actual output:

Trait constructor: $line9.$read$$iw$$iw$$iw$$iw$Instance1$@58c46295
Trait constructor: $line10.$read$$iw$$iw$$iw$$iw$Instance2$@452451ba
false    // As expected: the singleton types of two objects are different
true     // But the this.type tags are equivalent!

First reported at http://stackoverflow.com/q/32083332/1393162
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scabug commented Aug 19, 2015

Imported From: https://issues.scala-lang.org/browse/SI-9439?orig=1
Reporter: Ben Kovitz (bkovitz)
Affected Versions: 2.11.2

@scabug
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scabug commented Aug 19, 2015

@retronym said:
Unfortunately there isn't much we can do about this one.

Summoning a type tag for this.type gives a "context sensitive" type, ThisType(symbolOf[T]).

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

scala> class T { def t = typeTag[this.type] }; showRaw(new T().t.tpe)
defined class T
res36: String = ThisType(T)

The code that does this is the expansion of the typeTag macro. It has no knowledge of the prefix that you will later use to call this method.

@scabug scabug closed this as completed Aug 19, 2015
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scabug commented Aug 19, 2015

Owen Healy (ellbur) said:
Jason, how does it handle the case

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

which seems to have essentially the same difficulty but works correctly?

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