From e5b18ebb804453941ce3f8816d3f04a8b880fa7e Mon Sep 17 00:00:00 2001
From: liu fengyun <liu@fengy.me>
Date: Wed, 11 Apr 2018 23:06:41 +0200
Subject: [PATCH] use ApproximatingTypeMap to simplify exhaustivity check code

---
 .../tools/dotc/transform/patmat/Space.scala   | 24 ++++---------------
 1 file changed, 4 insertions(+), 20 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index 20057a560016..2cb690805a9c 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
@@ -605,32 +605,16 @@ 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
+    // expose type param references to their bounds according to variance
+    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)