Fix double evaluation of scrutinee with side-effects, add test

Author: felixmulder

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

@@ -91,16 +91,13 @@ class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
      *
      *  `scrutinee.isInstanceOf[Selector]` if `scrutinee eq null`
      */
-    def rewrite(tree: Select, to: Boolean): Tree = {
-      val expr =
-        if (!to || !tree.qualifier.tpe.widen.derivesFrom(defn.AnyRefAlias))
-          Literal(Constant(to))
-        else
-          Apply(tree.qualifier.select(defn.Object_ne), List(Literal(Constant(null))))
-
-      if (!isPureExpr(tree.qualifier)) Block(List(tree.qualifier), expr)
-      else expr
-    }
+    def rewrite(tree: Select, to: Boolean): Tree =
+      if (!to || !tree.qualifier.tpe.widen.derivesFrom(defn.AnyRefAlias)) {
+        val literal = Literal(Constant(to))
+        if (!isPureExpr(tree.qualifier)) Block(List(tree.qualifier), literal)
+        else literal
+      } else
+        Apply(tree.qualifier.select(defn.Object_ne), List(Literal(Constant(null))))
 
     /** Attempts to rewrite TypeApply to either `scrutinee ne null` or a
      *  constant

tests/run/isInstanceOf-eval.check

@@ -0,0 +1,2 @@
+1
+2

tests/run/isInstanceOf-eval.scala

@@ -0,0 +1,15 @@
+object Test extends dotty.runtime.LegacyApp {
+  lazy val any = {
+    println(1)
+    1: Any
+  }
+
+  any.isInstanceOf[Int]
+
+  lazy val int = {
+    println(2)
+    2
+  }
+
+  int.isInstanceOf[Int]
+}