crashy interplay between value class, existentials, inference

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class Observable1[+T](val asJava: JObservable[_ <: T]) extends AnyVal {
  private def foo[X](a: JObservable[X]): JObservable[X] = ???
  def synchronize: Observable1[T] = new Observable1(foo(asJava))
}

class JObservable[T]

scalac-hash v2.11.0-M4 sandbox/value-class-existential.scala
[info] v2.11.0-M4 => /Users/jason/usr/scala-v2.11.0-M4-0-g8b41240
error: scala.reflect.internal.Types$TypeError: type mismatch;
 found   : Observable1[_$1] where type _$1 <: T
 required: Observable1[T]
	at scala.tools.nsc.typechecker.Contexts$Context.issue(Contexts.scala:549)
	at scala.tools.nsc.typechecker.ContextErrors$ErrorUtils$.issueTypeError(ContextErrors.scala:97)
	at scala.tools.nsc.typechecker.ContextErrors$ErrorUtils$.issueNormalTypeError(ContextErrors.scala:86)
	at scala.tools.nsc.typechecker.ContextErrors$TyperContextErrors$TyperErrorGen$.AdaptTypeError(ContextErrors.scala:184)
	at scala.tools.nsc.typechecker.Typers$Typer.adaptMismatchedSkolems$1(Typers.scala:1126)
	at scala.tools.nsc.typechecker.Typers$Typer.fallbackAfterVanillaAdapt$1(Typers.scala:1195)
	at scala.tools.nsc.typechecker.Typers$Typer.vanillaAdapt$1(Typers.scala:1232)
	at scala.tools.nsc.typechecker.Typers$Typer.adapt(Typers.scala:1275)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:5370)
	at scala.tools.nsc.typechecker.Typers$Typer.transformedOrTyped(Typers.scala:5554)
	at scala.tools.nsc.typechecker.Typers$Typer.typedDefDef(Typers.scala:2307)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:5281)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:5352)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:5411)
	at scala.tools.nsc.typechecker.Typers$Typer.typedPos(Typers.scala:5418)
	at scala.tools.nsc.transform.ExtensionMethods$Extender$$anonfun$transformStats$1$$anonfun$10$$anonfun$apply$9.apply(ExtensionMethods.scala:260)
	at scala.tools.nsc.transform.ExtensionMethods$Extender$$a

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Imported From: https://issues.scala-lang.org/browse/SI-7818?orig=1
Reporter: @retronym
Affected Versions: 2.10.0

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@retronym said:
Workaround is the explicitly add type argument:

def synchronize: Observable1[T] = new Observable1[T](foo(asJava))

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@retronym said:
This problem adds to the list of lamentations that comment on adaptMismatchedSkolems.

In this case our expected type contains a new type parameter (of the extension method), whereas the type of the tree contains an existential skolem still pinned to the corresponding class type parameter.

tree.tpe         = Observable1#7037[_$1#12344]
<_$1#12344>.info =  <: T#7040

pt               = Observable1#7037[T#15644]

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@retronym said:
scala/scala#2916

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Samuel Grütter (samuelgruetter) said:
I've run into a second problem related to this:

object Test {
  class Observable[+T](val asJava: JObservable[_ <: T]) extends AnyVal {
    private def foo[X](a: JObservable[X]): JObservable[X] = ???
    def window(count: Int): Observable[Observable[T]] = {
      // this line makes the compiler crash (if it's the last line of this method):
      // convert(asJava.window())
      
      // this line gives an error:
      // "type mismatch; found : Test.JObservable[Test.JObservable[_$1]] required: Test.JObservable[Test.JObservable[T]]"
      // convert(asJava.window(): JObservable[JObservable[T]])
      
      // workaround:
      convert(asJava.window().asInstanceOf[JObservable[JObservable[T]]])
    }
  }
  
  class JObservable[T] {
    def window(): JObservable[JObservable[T]] = ???
  }
  
  def convert[T](jObs: JObservable[JObservable[T]]): Observable[Observable[T]] = ???
}

The only workaround I found was a typecast. Is there a safer workaround?

[scabug]

@retronym said:
Here's a workaround that uses a type ascription, rather than a cast:

cat  sandbox/test.scala
object Test {
  class Observable[+T](val asJava: JObservable[_ <: T]) extends AnyVal {
    private def foo[X](a: JObservable[X]): JObservable[X] = ???
    def window(count: Int): Observable[Observable[T]] = {
      convert(asJava.window()): Observable[Observable[T]]
    }
  }

  class JObservable[T] {
    def window(): JObservable[JObservable[T]] = ???
  }

  def convert[T](jObs: JObservable[JObservable[T]]): Observable[Observable[T]] = ???
}
topic/2894-rebase ~/code/scala scalac-hash v2.10.2 sandbox/test.scala
[info] v2.10.2 => /Users/jason/usr/scala-v2.10.2-0-g60d462e
topic/2894-rebase ~/code/scala

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Samuel Grütter (samuelgruetter) said:
Great thanks! I did not even try this because I thought the return type of the method would have the same effect as this ascription...