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Does Existential Quantification over Values work as described in reference? #4914

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scabug opened this issue Aug 15, 2011 · 5 comments
Closed

Does Existential Quantification over Values work as described in reference? #4914

scabug opened this issue Aug 15, 2011 · 5 comments
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@adriaanm
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2.11.8

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@scabug
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scabug commented Aug 15, 2011

If I understand §3.2.10 of the language reference correctly then

Map[x.type#S, x.type#S] forSome { val x: T }

should be equivalent to

Map[t#S, t#S] forSome { type t <: T with Singleton }

but they do seem to behave different in practice. When compiling

trait Type {
  type S
}

class ConcreteType extends Type {
  type S = Double
}

trait Base {
  type T <: Type
  val m: Map[t#S, t#S] forSome { type t <: T with Singleton }
  val n: Map[x.type#S, x.type#S] forSome { val x: T }
}

abstract class Derived extends Base {
  override type T = ConcreteType
  override val m = Map[Double, Double]()
  /** This does not work. §3.2.10 indicates that types n is shorthand for type of m. */
  override val n = Map[Double, Double]()
}

There is an error for overriding n, but no error for overriding m. Am I misunderstanding the language reference?
@scabug
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scabug commented Aug 15, 2011

Imported From: https://issues.scala-lang.org/browse/SI-4914?orig=1
Reporter: Mathias Körner (mkoerner)
Affected Versions: 2.9.0-1, 2.9.1

@scabug
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scabug commented Aug 15, 2011

@paulp said:
This compiles, which I imagine is sufficient evidence that you're looking at a bug.

trait Type {
  type S
}

trait Base {
  type T <: Type
  type M = Map[t#S, t#S] forSome { type t <: T with Singleton }
  type N = Map[x.type#S, x.type#S] forSome { val x: T }
}

abstract class Derived extends Base {
  type T = Type { type S = Double }
  override type M = Map[Double, Double]
  override type N = Map[Double, Double]
}

abstract class Derived2 extends Derived {
  val m: M = Map()
  val n: N = Map()
}

@scabug
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scabug commented Feb 12, 2012

@paulp said:
This is a duplicate of #482, but I'm closing that one in favor of this one.

@scabug
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scabug commented Oct 15, 2013

@gkossakowski said:
Unassigning and rescheduling to M7 as previous deadline was missed.

@scabug
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scabug commented Jul 22, 2016

@milessabin said:
This is fixed in 2.11.8 and 2.12.x.

PR adding test here: scala/scala#5300

@scabug scabug closed this as completed Aug 2, 2016
@scabug scabug added the quickfix label Apr 7, 2017
@scabug scabug added this to the 2.11.8 milestone Apr 7, 2017
@scabug scabug mentioned this issue Apr 7, 2017
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