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extension methods for cake members are very brittle #8300

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scabug opened this issue Feb 17, 2014 · 3 comments
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extension methods for cake members are very brittle #8300

scabug opened this issue Feb 17, 2014 · 3 comments
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@retronym
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2.11.0-RC1

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@scabug
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scabug commented Feb 17, 2014 

trait Universe {
  type Symbol >: Null <: AnyRef with SymbolApi
  trait SymbolApi

  type FreeTypeSymbol >: Null <: FreeTypeSymbolApi with Symbol
  trait FreeTypeSymbolApi extends SymbolApi

  implicit class CompatibleSymbol(sym: Symbol) {
    def asFreeType: FreeTypeSymbol = ???
  }
}

object Test extends App {
  val u: Universe = ???
  import u._

  val sym: Symbol = ???
  sym.asFreeType
}
21:43 ~/Projects/palladium0/sandbox (topic/palladium0)$ s
Test.scala:18: error: value asFreeType is not a member of Test.u.Symbol
  sym.asFreeType
      ^
one error found
@scabug
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scabug commented Feb 17, 2014

Imported From: https://issues.scala-lang.org/browse/SI-8300?orig=1
Reporter: @xeno-by
Affected Versions: 2.11.0-M8
See #8301

@scabug
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scabug commented Feb 17, 2014

@xeno-by said (edited on Feb 17, 2014 8:48:09 PM UTC):
This is a change in behavior in comparison with 2.10.x (regression?).

FreeTypeSymbol is now designated as volatile, which prevents: 1) pattern matching against vals of such type, 2) extension methods that return such type. If I change FreeTypeSymbolApi with Symbol to Symbol with FreeTypeSymbolApi, it makes everything work fine, but then erasures of Symbol and FreeTypeSymbol start clashing.

I have several questions actually:

  1. Is the change in volatility correct? FreeTypeSymbol transitively extends AnyRef, so why is it volatile?
  2. Why do we disregard extension methods that return volatile types?

@scabug
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scabug commented Feb 17, 2014

@retronym said:
I opened #8301 on this a few moments before I saw this one. Marking this as a duplicate.

@scabug scabug closed this as completed Feb 17, 2014
@scabug scabug added this to the 2.11.0-RC1 milestone Apr 7, 2017
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