use ApproximatingTypeMap to simplify exhaustivity check code

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -606,31 +606,15 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
    */
   def instantiate(tp1: NamedType, tp2: Type)(implicit ctx: Context): Type = {
     // expose abstract type references to their bounds or tvars according to variance
-    class AbstractTypeMap(maximize: Boolean)(implicit ctx: Context) extends TypeMap {
-      def expose(lo: Type, hi: Type): Type =
-        if (variance == 0)
-          newTypeVar(TypeBounds(lo, hi))
-        else if (variance == 1)
-          if (maximize) hi else lo
-        else
-          if (maximize) lo else hi
+    class AbstractTypeMap(maximize: Boolean)(implicit ctx: Context) extends ApproximatingTypeMap {
+      variance = if (maximize) 1 else -1
 
       def apply(tp: Type): Type = tp match {
         case tp: TypeRef if tp.underlying.isInstanceOf[TypeBounds] =>
           val lo = this(tp.info.loBound)
           val hi = this(tp.info.hiBound)
           // See tests/patmat/gadt.scala  tests/patmat/exhausting.scala  tests/patmat/t9657.scala
-          val exposed = expose(lo, hi)
-          debug.println(s"$tp exposed to =====> $exposed")
-          exposed
-
-        case AppliedType(tycon: TypeRef, args) if tycon.underlying.isInstanceOf[TypeBounds] =>
-          val args2 = args.map(this)
-          val lo = this(tycon.info.loBound).applyIfParameterized(args2)
-          val hi = this(tycon.info.hiBound).applyIfParameterized(args2)
-          val exposed = expose(lo, hi)
-          debug.println(s"$tp exposed to =====> $exposed")
-          exposed
+          range(lo, hi)
 
         case _ =>
           mapOver(tp)