approximate existential types when checking pattern conformance

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trait Outer[T] {
  sealed trait Inner
  case class C1(val x: T) extends Inner
  case class C2(val x: T) extends Inner
}

object Test3 {
  type SomeInner = Outer[X]#Inner forSome { type X }

  // Clearly Test1.C1(5) is an instance of SomeInner, as seen here:
  var x: SomeInner = _
  x = Test1.C1(5)

  // And yet,
  //
  // def f0(x: SomeInner) = x match { case Test1.C1(_) => }
  //
  // ./a.scala:35: error: constructor cannot be instantiated to expected type;
  //  found   : Test1.C1
  //  required: Outer[X]#Inner where type X
  //   def f0(x: SomeInner) = x match { case Test1.C1(_) => }
  //
  // Whereas this is no problem:
  def f1(x: Any) = x match { case Test1.C1(_) => }
}

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Imported From: https://issues.scala-lang.org/browse/SI-6587?orig=1
Reporter: @adriaanm
See #9394, #6583

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

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@retronym said:
I'm assuming the test case above is missing:

object Test1 extends Outer[Int]

(I'm just sweeping through bugs about " cannot be instantiated to expected type" to see if my proposed fix for #9394 progresses any of them.)