Add a test case representing the fallout seen in bootstrapping

Author: dwijnand

tests/patmat/i12805-fallout.scala

@@ -0,0 +1,26 @@
+import scala.annotation.unchecked.uncheckedVariance
+
+type Untyped = Null
+
+class Type
+
+abstract class Tree[-T >: Untyped] {
+  type ThisTree[T >: Untyped] <: Tree[T]
+
+  protected var myTpe: T @uncheckedVariance = _
+
+  def withType(tpe: Type): ThisTree[Type] = {
+    val tree = this.asInstanceOf[ThisTree[Type]]
+    tree.myTpe = tpe
+    tree
+  }
+}
+
+case class  Ident[-T >: Untyped]() extends Tree[T]
+case class DefDef[-T >: Untyped]() extends Tree[T]
+
+def test[T >: Untyped](tree: Tree[T], tp: Type) = tree.withType(tp) match {
+  case Ident()  => 1
+  case DefDef() => 2
+  case _        => 3
+}