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can't existentially abstract over parameterized type F #9281

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scabug opened this issue Apr 24, 2015 · 1 comment
Open

can't existentially abstract over parameterized type F #9281

scabug opened this issue Apr 24, 2015 · 1 comment

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@scabug
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@scabug scabug commented Apr 24, 2015

Consider the following snippet. Casting to existential causing the error "can't existentially abstract over parameterized type F".

test1 fails for both scala 2.10 and 2.11
test2 works for 2.10 but fails for 2.11
test3 works for 2.10 and 2.11

  // define container trait, type alias and value
  trait Container[F[_]]
  type CC[F[_]] = Container[F]
  val c = new CC[Option] {}

  // fails for 2.10 and 2.11
  // error: can't existentially abstract over parameterized type F
  type SomeContainer = Container[F] forSome { type F[_] }
  val test1 = c.asInstanceOf[SomeContainer]

  // works for 2.10 but fails for 2.11
  type SomeCC = CC[F] forSome { type F[_] }
  implicitly[SomeContainer =:= SomeCC]
  val test2 = c.asInstanceOf[SomeCC]

  // workaround for 2.11 but isn't it the same as test2?
  type SomeF[X] = F[X] forSome { type F[_] }
  implicitly[SomeCC =:= CC[SomeF]]
  val test3 = c.asInstanceOf[CC[SomeF]]

from my point of view test1 and test2 should work either

@scabug
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@scabug scabug commented Apr 24, 2015

Imported From: https://issues.scala-lang.org/browse/SI-9281?orig=1
Reporter: Dmitry Bushev (4e6)
Affected Versions: 2.10.5, 2.11.6

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