SI-8244 Fix raw type regression under separate compilation

In #1901, handling of raw types encountered in signatures during class
file parsing was changed to work in the same manner as
`classExistentialType`, by using
`existentialAbstraction(cls.tparms, cls.tpe_*)`

But this never creates fresh existential symbols, and just sticks
the class type parameters it `quantified`:

	scala> trait T[A <: String]
	defined trait T

	scala> val cls = typeOf[T[_]].typeSymbol
	cls = trait T#101864

	scala> cls.typeParams
	res0 = List(type A#101865)

	scala> cls.tpe_*
	res1 = T#101864[A#101865]

	scala> classExistentialType(cls)
	res3 = T#101864[_ <: String#7209]

	scala> val ExistentialType(quantified, result) = res3
	List(type A#101865)

In the enclosed test case, this class type parameter was substituted
during `typeOf[X] memberType sym`, which led us unsoundly thinking
that `Raw[_]` was `Raw[X]`.

I've added a TODO comment to review the other usages of
`classExistentialType`.

Test variations include joint and separate compilation, and the
corresponding Scala-only code. All fail with type errors now,
as we expect. I've also added a distillation of a bootstrap
error that failed when I forgot to wrap the `existentialType`.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
index 664645e53e..2f9cc01c0b 100644
--- a/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
+++ b/src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala
@@ -665,8 +665,11 @@ abstract class ClassfileParser {
               // so have to check unsafeTypeParams.isEmpty before worrying about raw type case below,
               // or we'll create a boatload of needless existentials.
               else if (classSym.isMonomorphicType || classSym.unsafeTypeParams.isEmpty) tp
-              // raw type - existentially quantify all type parameters
-              else debuglogResult(s"raw type from $classSym")(unsafeClassExistentialType(classSym))
+              else debuglogResult(s"raw type from $classSym"){
+                // raw type - existentially quantify all type parameters
+                val eparams = typeParamsToExistentials(classSym, classSym.unsafeTypeParams)
+                newExistentialType(eparams, typeRef(pre, classSym, eparams.map(_.tpeHK)))
+              }
             case tp =>
               assert(sig.charAt(index) != '<', s"sig=$sig, index=$index, tp=$tp")
               tp