SI-8611 Avoid accidental patmat unification with refinement types

In the enclosed test, t8611a.scala, the patterns `O.{A, B}` were
incorrect treated as equivelent by the combination of
`uniqueTpForTree` and `Const.uniqueTpForTree`.

`uniqueTpForTree` used `Type#narrow` to try to create a distinct
type for each new pattern tree it encountered. However, narrowing a
`RefinedType` does not create a distinct type as we are used to
when narrowing, e.g. a class type.

    // Type
    def narrow: Type =
      if (phase.erasedTypes) this
      else {
        val cowner = commonOwner(this)
        refinedType(this :: Nil, cowner, EmptyScope, cowner.pos).narrow
      }

    // CompoundType
    override def narrow: Type = typeSymbol.thisType

This commit creates a fresh existential type symbol rather than
trying to use `narrow`.

I've included a unit test to show the sublteties of narrowing
refinment types.

1 files changed, 54 insertions, 0 deletions

diff --git a/test/files/run/t8611b.scala b/test/files/run/t8611b.scala
new file mode 100644
index 0000000000..2df17c9ca0
--- /dev/null
+++ b/test/files/run/t8611b.scala
@@ -0,0 +1,54 @@
+sealed trait KrafsDescription
+
+abstract class NotWorkingEnum extends Enumeration {
+
+  type ExtendedValue = Value with KrafsDescription
+
+  def Enum(inDescription: String): ExtendedValue = {
+    new Val(nextId) with KrafsDescription {
+    }
+  }
+}
+
+abstract class WorkingEnum extends Enumeration {
+
+  type ExtendedValue = Value
+
+  def Enum(inDescription: String): ExtendedValue = {
+    new Val(nextId) {
+    }
+  }
+}
+
+object NotWorkingTab extends NotWorkingEnum {
+  val a = Enum("A")
+  val b = Enum("B")
+}
+
+object WorkingTab extends WorkingEnum {
+  val a = Enum("A")
+  val b = Enum("B")
+}
+
+object Test extends App {
+  testGris()
+  testWorking()
+
+  def testGris() {
+    val pipp = NotWorkingTab.b
+    pipp match {
+      case NotWorkingTab.a => ???
+      case NotWorkingTab.b =>
+      case _ => ???
+    }
+  }
+
+  def testWorking() {
+    val stuff = WorkingTab.a
+    stuff match {
+      case WorkingTab.a =>
+      case WorkingTab.b => ???
+      case _ => ???
+    }
+  }
+}