Make TypeTestsCasts an Object

No need to use inheritance here.

Author: odersky

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

@@ -214,10 +214,7 @@ object Types {
       case _ => NoType
     }
 
-    /** Is this type guaranteed not to have `null` as a value?
-     *  For the moment this is only true for modules, but it could
-     *  be refined later.
-     */
+    /** Is this type guaranteed not to have `null` as a value? */
     final def isNotNull(implicit ctx: Context): Boolean = this match {
       case tp: ConstantType => tp.value.value != null
       case tp: ClassInfo => !tp.cls.isNullableClass && tp.cls != defn.NothingClass

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -139,9 +139,9 @@ class Erasure extends Phase with DenotTransformer { thisTransformer =>
         i"The type $tp - ${tp.toString} of class ${tp.getClass} of tree $tree : ${tree.tpe} / ${tree.getClass} is illegal after erasure, phase = ${ctx.phase.prev}")
 }
 
-object Erasure extends TypeTestsCasts{
-
+object Erasure {
   import tpd._
+  import TypeTestsCasts._
 
   object Boxing {
 

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -7,6 +7,7 @@ import ast.Trees._
 import Erasure.Boxing._
 import TypeErasure._
 import ValueClasses._
+import SymUtils._
 import core.Flags._
 import util.Positions._
 
@@ -22,7 +23,7 @@ import util.Positions._
  * Unfortunately this phase ended up being not Y-checkable unless types are erased. A cast to an ConstantType(3) or x.type
  * cannot be rewritten before erasure.
  */
-trait TypeTestsCasts {
+object TypeTestsCasts {
   import ast.tpd._
 
   def interceptTypeApply(tree: TypeApply)(implicit ctx: Context): Tree = ctx.traceIndented(s"transforming ${tree.show}", show = true) {
@@ -157,7 +158,7 @@ trait TypeTestsCasts {
             transformIsInstanceOf(expr, erasure(testType), flagUnrelated)
         }
 
-        if ((sym eq defn.Any_isInstanceOf) || (sym eq defn.Any_typeTest))
+        if (sym.isTypeTest)
           transformTypeTest(expr, tree.args.head.tpe, flagUnrelated = true)
         else if (sym eq defn.Any_asInstanceOf)
           transformAsInstanceOf(erasure(tree.args.head.tpe))