Revert to keeping approx as TypeComparer state

Passing it as an additional parameter seemed to cause a slight (~2%) performance
degradation.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -107,9 +107,18 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         assert(isSatisfiable, constraint.show)
   }
 
+  private[this] var approx: ApproxState = NoApprox
+  protected def approxState = approx
+
+  protected def isSubType(tp1: Type, tp2: Type, a: ApproxState): Boolean = {
+    val saved = approx
+    this.approx = a
+    try recur(tp1, tp2) finally this.approx = saved
+  }
+
   protected def isSubType(tp1: Type, tp2: Type): Boolean = isSubType(tp1, tp2, NoApprox)
 
-  protected def isSubType(tp1: Type, tp2: Type, approx: ApproxState): Boolean = trace(s"isSubType ${traceInfo(tp1, tp2)} $approx", subtyping) {
+  protected def recur(tp1: Type, tp2: Type): Boolean = trace(s"isSubType ${traceInfo(tp1, tp2)} $approx", subtyping) {
 
     def monitoredIsSubType = {
       if (pendingSubTypes == null) {
@@ -817,9 +826,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         false
       } else isSubType(tp1, tp2, approx.addLow)
 
-    def recur(tp1: Type, tp2: Type) = isSubType(tp1, tp2, approx)
-
-    // begin isSubType
+    // begin recur
     if (tp2 eq NoType) false
     else if (tp1 eq tp2) true
     else {
@@ -1655,9 +1662,9 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
     case _ => String.valueOf(res)
   }
 
-  override def isSubType(tp1: Type, tp2: Type, approx: ApproxState) =
-    traceIndented(s"${show(tp1)} <:< ${show(tp2)}${if (Config.verboseExplainSubtype) s" ${tp1.getClass} ${tp2.getClass}" else ""} $approx ${if (frozenConstraint) " frozen" else ""}") {
-      super.isSubType(tp1, tp2, approx)
+  override def recur(tp1: Type, tp2: Type) =
+    traceIndented(s"${show(tp1)} <:< ${show(tp2)}${if (Config.verboseExplainSubtype) s" ${tp1.getClass} ${tp2.getClass}" else ""} $approxState ${if (frozenConstraint) " frozen" else ""}") {
+      super.isSubType(tp1, tp2, approxState)
     }
 
   override def hasMatchingMember(name: Name, tp1: Type, tp2: RefinedType): Boolean =