compiler fails with assertion when trait defines a closure over private[this] member

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There was strange assertion fail while compiling a project. I know scala very basic and don't sure if used construction is valid in language reference, but compiler should generate more sensible error output in my opinion.

I've attached example file with several sections. Clear ones are ok, commented make compiler fail

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Imported From: https://issues.scala-lang.org/browse/SI-5508?orig=1
Reporter: ayvango
Affected Versions: 2.9.1, 2.10.0
Attachments:

• private.scala (created on Feb 20, 2012 12:44:24 AM UTC, 1253 bytes)

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@paulp said:
Let me just say WHAT A GREAT TEST CASE. If everyone came up with such great test cases by now you'd be able to use the scala compiler to operate nuclear reactors in low earth orbit.

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@adriaanm said:
simplified:

object Test {
  trait NestedTrait { // must be nested and a trait
    private[this] val _st : Int = 0 // must be private[this]
    val escape = { () => _st }
  }
}

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@adriaanm said:
Paul, I can only assume your all-caps enthusiasm means you'd like to go nuclear on this one? If not, please reassign.

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@adriaanm said:
Un-assigning to foster work stealing, as announced in https://groups.google.com/forum/?fromgroups=#!topic/scala-internals/o8WG4plpNkw

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@adriaanm said:
Unassigning and rescheduling to M6 as previous deadline was missed.

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@retronym said:
Here's an analysis and one solution:

https://github.com/retronym/scala/tree/ticket/5508

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@retronym said:
scala/scala#3199