Fix isInstanceOfEvaluator

We had:

scala> null.isInstanceOf[String]
-- Warning: <console>:5:17 -----------------------------------------------------
5 |null.isInstanceOf[String]
  |^^^^^^^^^^^^^^^^^^^^^^^^^
  |this will always yield true if the scrutinee is non-null, since `Null` is a subclass of `String` (will be optimized away)
val res0: Boolean = true
scala> val x: AnyRef = null
val x: AnyRef = null
scala> x.isInstanceOf[String]
val res1: Boolean = false

Fixed by using `isNullable`.

Author: odersky

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -777,7 +777,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
       else Erasure.Boxing.adaptToType(tree, tp)
 
     /** `tree ne null` (might need a cast to be type correct) */
-     def testNotNull(implicit ctx: Context): Tree =
+    def testNotNull(implicit ctx: Context): Tree =
       tree.ensureConforms(defn.ObjectType)
         .select(defn.Object_ne).appliedTo(Literal(Constant(null)))
 

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

@@ -90,15 +90,15 @@ class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
      *  If `to` is set to true and the qualifier is not a primitive, the
      *  instanceOf is replaced by a null check, since:
      *
-     *  `scrutinee.isInstanceOf[Selector]` if `scrutinee eq null`
+     *  `scutinee == null` implies `!scrutinee.isInstanceOf[Selector]`
      */
     def rewrite(qualifier: Tree, to: Boolean): Tree =
-      if (!to || !qualifier.tpe.widen.derivesFrom(defn.AnyRefAlias)) {
+      if (to && !qualifier.tpe.isNotNull) qualifier.testNotNull
+      else {
         val literal = Literal(Constant(to))
         if (!isPureExpr(qualifier)) Block(List(qualifier), literal)
         else literal
-      } else
-        Apply(qualifier.select(defn.Object_ne), List(Literal(Constant(null))))
+      }
 
     /** Attempts to rewrite type test to either `scrutinee ne null` or a
      *  constant. Any_typeTest nodes have been rewritten to Any_isInstanceOf at this point.

tests/run/nullInstanceEval.scala

@@ -0,0 +1,5 @@
+object Test extends App {
+  val x = null
+  assert(!x.isInstanceOf[String])
+  assert(!(x: AnyRef).isInstanceOf[String])
+}