regression: crasher with higher kinded types and scope escape checking

[scabug]

This appears specific to higher kinded types.

class Bop[A[_]]

object Foo {
  private val stToSK = new Bop[ScopedTaskKey]
  private type ScopedTaskKey[T] = String
}

error: java.lang.Error: unexpected alias type: type ScopedTaskKey
	at scala.tools.nsc.symtab.SymbolTable.abort(SymbolTable.scala:35)
	at scala.tools.nsc.symtab.Symbols$$Symbol.existentialBound(Symbols.scala:970)
	at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$39.apply(Typers.scala:2743)
	at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$39.apply(Typers.scala:2738)
	at scala.collection.TraversableLike$$$$anonfun$$map$$1.apply(TraversableLike.scala:241)
	at scala.collection.TraversableLike$$$$anonfun$$map$$1.apply(TraversableLike.scala:241)
	at scala.collection.LinearSeqOptimized$$class.foreach(LinearSeqOptimized.scala:61)  
	at scala.collection.immutable.List.foreach(List.scala:45)
	at scala.collection.TraversableLike$$class.map(TraversableLike.scala:241)
	at scala.collection.immutable.List.map(List.scala:45)
	at scala.tools.nsc.typechecker.Typers$$Typer.existentialTransform(Typers.scala:2738)
	at scala.tools.nsc.typechecker.Typers$$Typer.packSymbols(Typers.scala:2763)
	at scala.tools.nsc.typechecker.Typers$$Typer$$checkNoEscaping$$.check(Typers.scala:354)
	at scala.tools.nsc.typechecker.Typers$$Typer$$checkNoEscaping$$.locals(Typers.scala:341)
	at scala.tools.nsc.typechecker.Typers$$Typer.typedDefDef(Typers.scala:1690)
	at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:3807)

If stToSK is not private, we can see the potential issue it is investigating when it crashes.

0304.scala:9: error: private type ScopedTaskKey escapes its defining scope as part of type Bop[Foo.ScopedTaskKey]
  val stToSK = new Bop[ScopedTaskKey]
      ^
one error found  

It compiles in 2.8.1, fails with r24371.

[scabug]

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

[scabug]

@ingoem said:
Could you have a look whether that's for you Adriaan? Feel free to reassign.

[scabug]

@adriaanm said:
as in [#SI-4559], this requires clarification of expected behaviour for avoiding escaping symbols

I'd say checkNoEscape(sym) should not have to be called when sym.isAliasType, since normalisation could be used to chase away type aliases (private or otherwise) -- in principle, we only need to check the normalisation. The problem here arises due to a private type constructor alias, which requires eta-expansion to normalise. Eta-expansion yields a polytype, which is not directly expressible in the surface syntax (unless the alias was applied to its type arguments).

This issue shows that we currently use dealias instead of normalize, hence the need for a type constructor to trigger the problem: these have to be eta-expanded. The argument from [#SI-4559] remains the same, in principle, but it depends on the spec...

For now, only fixing the crasher.