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, 6 insertions, 3 deletions

diff --git a/src/reflect/scala/reflect/internal/Definitions.scala b/src/reflect/scala/reflect/internal/Definitions.scala
index 78e639fdff..645d6aa4ff 100644
--- a/src/reflect/scala/reflect/internal/Definitions.scala
+++ b/src/reflect/scala/reflect/internal/Definitions.scala
@@ -898,12 +898,15 @@ trait Definitions extends api.StandardDefinitions {
      *
      *    C[E1, ..., En] forSome { E1 >: LB1 <: UB1 ... en >: LBn <: UBn }.
      */
+    // TODO Review the way this is used. I see two potential problems:
+    //  1. `existentialAbstraction` here doesn't create fresh existential type symbols, it just
+    //     uses the class type parameter symbols directly as the list of quantified symbols.
+    //     See SI-8244 for the trouble that this can cause.
+    //     Compare with callers of `typeParamsToExistentials` (used in Java raw type handling)
+    //  2. Why don't we require a prefix? Could its omission lead to wrong results in CheckabilityChecker?
     def classExistentialType(clazz: Symbol): Type =
       existentialAbstraction(clazz.typeParams, clazz.tpe_*)
 
-    def unsafeClassExistentialType(clazz: Symbol): Type =
-      existentialAbstraction(clazz.unsafeTypeParams, clazz.tpe_*)
-
     // members of class scala.Any
 
     // TODO these aren't final! They are now overriden in AnyRef/Object. Prior to the fix