Treat private as "effectively sealed"

src/compiler/scala/tools/nsc/transform/patmat/MatchAnalysis.scala

@@ -121,7 +121,7 @@ trait TreeAndTypeAnalysis extends Debugging {
             def enumerateChildren(sym: Symbol) = {
               sym.sealedChildren.toList
                 .sortBy(_.sealedSortName)
-                .filterNot(x => x.isSealed && x.isAbstractClass && !isPrimitiveValueClass(x))
+                .filterNot(x => (x.isSealed || x.isPrivate) && x.isAbstractClass && !isPrimitiveValueClass(x))
             }
 
             // enumerate only direct subclasses,
@@ -147,7 +147,7 @@ trait TreeAndTypeAnalysis extends Debugging {
               // all of their children must be and they cannot otherwise be created.
               sym.sealedDescendants.toList
                 sortBy (_.sealedSortName)
-                filterNot (x => x.isSealed && x.isAbstractClass && !isPrimitiveValueClass(x))
+                filterNot (x => (x.isSealed || x.isPrivate) && x.isAbstractClass && !isPrimitiveValueClass(x))
             )
 
             List(debug.patmatResult(s"enum sealed tp=$tp, tpApprox=$tpApprox as") {

test/files/pos/t6159.scala

@@ -0,0 +1,11 @@
+// scalac: -Werror
+trait A {
+  sealed abstract class X
+  private class X1 extends X with X2 { }
+  private trait X2 extends X
+  sealed trait X3 extends X
+
+  def f(x: X) = x match {
+    case _: X1 => 0
+  }
+}