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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`.
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