Enumerate subtypes by approximating type skolems to their upper bound

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

@@ -96,9 +96,16 @@ trait TreeAndTypeAnalysis extends Debugging {
           val parentSubtypes = tp.parents.flatMap(parent => enumerateSubtypes(parent, grouped))
           if (parentSubtypes exists (_.nonEmpty)) {
             // If any of the parents is enumerable, then the refinement type is enumerable.
-            // We must only include subtypes of the parents that conform to `tp`.
-            // See neg/virtpatmat_exhaust_compound.scala for an example.
-            parentSubtypes map (_.filter(_ <:< tp))
+            // We must only include subtypes of the parents that conform to `tpApprox`.
+            // See neg/virtpatmat_exhaust_compound.scala and pos/t9657.scala for examples.
+            val approximateTypeSkolemsToUpperBound = new TypeMap { // from approximateAbstracts
+              def apply(tp: Type): Type = tp.dealiasWiden match {
+                case TypeRef(_, sym, _) if sym.isTypeSkolem => tp.upperBound
+                case _                                      => mapOver(tp)
+              }
+            }
+            val tpApprox = approximateTypeSkolemsToUpperBound(tp)
+            parentSubtypes.map(_.filter(_ <:< tpApprox))
           }
           else Nil
         // make sure it's not a primitive, else (5: Byte) match { case 5 => ... } sees no Byte

test/files/pos/t9657.scala

@@ -0,0 +1,21 @@
+// scalac: -Werror
+sealed trait PowerSource
+case object Petrol extends PowerSource
+case object Pedal  extends PowerSource
+
+sealed abstract class Vehicle { type A <: PowerSource }
+case object Bicycle       extends Vehicle { type A = Pedal.type  }
+case class Bus(fuel: Int) extends Vehicle { type A = Petrol.type }
+case class Car(fuel: Int) extends Vehicle { type A = Petrol.type }
+
+object Test {
+  def refuel[P <: Petrol.type](vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_) => Car(100)
+    case Bus(_) => Bus(100) // was: "unreachable code" warning
+  }
+
+  def main(args: Array[String]): Unit = {
+    println(refuel(Car(100)))
+    println(refuel(Bus(5)))
+  }
+}