Enumerate refinement type's subtypes by deSkolemizing

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

@@ -96,14 +96,31 @@ trait TreeAndTypeAnalysis extends Debugging {
       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 `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)
+        // See neg/virtpatmat_exhaust_compound.scala for a base example,
+        // and pos/t9657.scala & pos/t9657-less-fishy.scala for examples involving type skolems.
+        val deskolemizeAliases = new TypeMap { // from approximateAbstracts
+          def apply(tp: Type) = mapOver(tp)
+
+          // The default only allows mapping the types of the symbols
+          // but we need to map the symbols themselves
+          override def mapOver(syms: List[Symbol]) = syms.mapConserve(deskolemizeAlias)
+
+          /** Replace any alias type symbols which dealias to type skolems
+           *  with new abstract types, assigning the de-skolemized's type to it.
+           *
+           *  For example, in `def m[A <: B](x: X { type U = A }): X = x`
+           *  replace `type U = A` with `type U <: B`.
+           */
+          private def deskolemizeAlias(sym: Symbol) = {
+            val dealiasedSym = sym.info.typeSymbol
+            if (dealiasedSym.isTypeSkolem) {
+              sym.owner.newAbstractType(sym.name.toTypeName, sym.pos, sym.flags).setInfo {
+                dealiasedSym.deSkolemize.info
+              }
+            } else sym
           }
         }
-        val tpApprox = approximateTypeSkolemsToUpperBound(tp)
+        val tpApprox = deskolemizeAliases(tp)
         parentSubtypes.map(_.filter(_ <:< tpApprox))
       } else Nil
     }

test/files/pos/t9657-less-fishy.scala

@@ -0,0 +1,28 @@
+// scalac: -Werror
+
+// like pos/t9657
+// but with a more plausible reason for why to have a method type parameter
+
+sealed trait PowerSource
+sealed trait NonRenewablePowerSource extends PowerSource
+
+case object Petrol extends NonRenewablePowerSource
+case object Coal   extends NonRenewablePowerSource
+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 <: NonRenewablePowerSource](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)))
+  }
+}

test/pending/neg/t9657-less-fishy.check

@@ -0,0 +1,9 @@
+t9657-less-fishy.scala:27: warning: unreachable code
+    case Bicycle => ???      // expected unreachable 1
+                    ^
+t9657-less-fishy.scala:33: warning: unreachable code
+    case _: Bicycle.type => ???      // expected unreachable 2
+                            ^
+error: No warnings can be incurred under -Werror.
+2 warnings
+1 error

test/pending/neg/t9657-less-fishy.scala

@@ -0,0 +1,49 @@
+// scalac: -Werror
+
+// like {pos,neg}/t9657
+// but with a more plausible reason for why to have a method type parameter
+
+sealed trait PowerSource
+sealed trait NonRenewablePowerSource extends PowerSource
+
+case object Petrol extends NonRenewablePowerSource
+case object Coal   extends NonRenewablePowerSource
+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 refuel2[P <: NonRenewablePowerSource](vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)  => Car(100)
+    case Bus(_)  => Bus(100) // was: "unreachable code" warning 2
+    case Bicycle => ???      // expected unreachable 1
+  }
+
+  def refuel3[P <: NonRenewablePowerSource](vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)          => Car(100)
+    case Bus(_)          => Bus(100) // was: "unreachable code" warning 3
+    case _: Bicycle.type => ???      // expected unreachable 2
+  }
+}
+
+class Test[P <: NonRenewablePowerSource] {
+  def refuel(vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_) => Car(100)
+    case Bus(_) => Bus(100) // was: "unreachable code" warning 4
+  }
+
+  def refuel2(vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)  => Car(100)
+    case Bus(_)  => Bus(100) // was: "unreachable code" warning 5
+    case Bicycle => ???      // expected unreachable 3
+  }
+
+  def refuel3(vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)          => Car(100)
+    case Bus(_)          => Bus(100) // was: "unreachable code" warning 6
+    case _: Bicycle.type => ???      // expected unreachable 4
+  }
+}

test/pending/neg/t9657.check

@@ -0,0 +1,9 @@
+t9657.scala:15: warning: unreachable code
+    case Bicycle => ???      // expected unreachable 1
+                    ^
+t9657.scala:21: warning: unreachable code
+    case _: Bicycle.type => ???      // expected unreachable 2
+                            ^
+error: No warnings can be incurred under -Werror.
+2 warnings
+1 error

test/pending/neg/t9657.scala

@@ -0,0 +1,37 @@
+// 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 refuel1[P <: Petrol.type](vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)  => Car(100)
+    case Bus(_)  => Bus(100) // was: "unreachable code" warning 1
+    case Bicycle => ???      // expected unreachable 1
+  }
+
+  def refuel2[P <: Petrol.type](vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)          => Car(100)
+    case Bus(_)          => Bus(100) // was: "unreachable code" warning 2
+    case _: Bicycle.type => ???      // expected unreachable 2
+  }
+}
+
+class Test[P <: Petrol.type] {
+  def refuel1(vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)  => Car(100)
+    case Bus(_)  => Bus(100) // was: "unreachable code" warning 3
+    case Bicycle => ???      // expected unreachable 3
+  }
+
+  def refuel2(vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_)          => Car(100)
+    case Bus(_)          => Bus(100) // was: "unreachable code" warning 4
+    case _: Bicycle.type => ???      // expected unreachable 4
+  }
+}

test/pending/pos/t9657-less-fishy.scala

@@ -0,0 +1,23 @@
+// scalac: -Werror
+
+// like pos/t9657
+// but with a more plausible reason for why to have a method type parameter
+
+sealed trait PowerSource
+sealed trait NonRenewablePowerSource extends PowerSource
+
+case object Petrol extends NonRenewablePowerSource
+case object Coal   extends NonRenewablePowerSource
+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 }
+
+class Test[P <: NonRenewablePowerSource] {
+  def refuel(vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_) => Car(100)
+    case Bus(_) => Bus(100) // was: "unreachable code" warning 2
+  }
+}

test/pending/pos/t9657.scala

@@ -0,0 +1,16 @@
+// 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 }
+
+class Test[P <: Petrol.type] {
+  def refuel(vehicle: Vehicle { type A = P }): Vehicle = vehicle match {
+    case Car(_) => Car(100)
+    case Bus(_) => Bus(100) // was: "unreachable code" warning 2
+  }
+}