Compiler crash when using package private modifiers and abstract types.

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The following code compiles under Scala-2.8.1.final but causes the compiler to crash under Scala-2.9.0.RC4.

package mypkg {

  trait M { type C <: M }
  trait F extends M { type C = F }

  // requires private[mypkg] to reproduce
  private[mypkg] abstract class Private[E <: M] {
    type Sub <: Private[E#C]
    final val sub: Sub = null.asInstanceOf[Sub]

    def foo: E
  }

  trait Public[E <: M] extends Private[E] {
    type Sub = Public[E#C]
  }
}

import mypkg._

object Test {
  val seq = null.asInstanceOf[Public[F]]
  seq.sub.foo
}

The only workaround I can think of is to strip all the package private modifiers. Of course that would expose internal implementation as public API.

=== Versions ===

• Scala: 2.9.0.RC4
• Java: 1.6.0_u24
• Operating system: GNU/Linux 2.6.38

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-4559?orig=1
Reporter: @lexn82
See #4312

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@paulp said:
I think the bug is that it ever compiled. If you make Private just plain private, it gives the message I would expect.

./a.scala:15: error: private class Private escapes its defining scope as part of type mypkg.Private[E]
   trait Public[E <: M] extends Private[E] {
                                ^
one error found

That is just as true a statement if Private is private[mypkg] as if it's private.

I've never heard of public classes with private parents. Is this supported in the spec?

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@lexn82 said:
The class should probably be public. However the compiler should give a proper warning instead of crashing with an assertion error.

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@dragos said:
Adriaan, you added this assert in r22508. Can you please have a look?

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@dragos said:
Sorry, I noticed now that you only added a proper message.. however, you may know more about erasure.

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@adriaanm said (edited on Jun 2, 2011 12:44:04 AM UTC):
I have a fix for the crash and an improvement of a situation that was clearly wrong.
However, I can't find where the specs defines when it's ok for a symbol to escape its defining scope, and how we avoid such problematic types.

Martin, re-assigning to you for clarification. Feel free to bounce back for the actual implementation.

[som-snytt]

Compiles without crashing in 2.13. Also compiles in dotc after tweaking. No escaping defining scope message.