Fix matching on top types

Author: felixmulder

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

@@ -136,13 +136,14 @@ class IsInstanceOfEvaluator extends MiniPhaseTransform { thisTransformer =>
           /** Check if the selector's potential type parameters will be erased, and if so warn */
           val selTypeParam = tree.args.head.tpe.widen match {
             case tp @ AppliedType(_, arg :: _) =>
-              // If the type is `Array[X]` where `X` extends AnyVal or `X =:=
-              // Any`, this shouldn't yield a warning:
-              val isArray = tp.isRef(defn.ArrayClass)
-              val unerased = arg.derivesFrom(defn.AnyValClass) || arg.isRef(defn.AnyClass)
-              val hasAnnot = arg.hasAnnotation(defn.UncheckedAnnot)
-
-              if (!hasAnnot && !(isArray && unerased)) ctx.uncheckedWarning(
+              // If the type is `Array[X]` where `X` extends AnyVal
+              val anyValArray = tp.isRef(defn.ArrayClass) && arg.derivesFrom(defn.AnyValClass)
+              // param is: Any | AnyRef | java.lang.Object
+              val topType = defn.ObjectType <:< arg
+              // has @unchecked annotation to suppress warnings
+              val hasUncheckedAnnot = arg.hasAnnotation(defn.UncheckedAnnot)
+
+              if (!topType && !hasUncheckedAnnot && !anyValArray) ctx.uncheckedWarning(
                 ErasedType(hl"""|Since type parameters are erased, you should not match on them in
                                 |${"match"} expressions."""),
                 tree.pos

tests/repl/erasure.check

@@ -35,12 +35,6 @@ scala> def matchArray2[A](xs: Array[Any]) = xs match { case xs: Array[Int] => xs
 longer explanation available when compiling with `-explain`
 def matchArray2[A](xs: Array[Any]): [A] => (xs: Array[Any])Array[Int]
 scala> def matchArray3[A](xs: Array[A]) = xs match { case xs: Array[Int] => xs; case xs: Array[AnyRef] => ???; case xs: Array[Any] => ??? }
--- [E035] Erased Type Unchecked Warning: <console> -----------------------------
-5 |def matchArray3[A](xs: Array[A]) = xs match { case xs: Array[Int] => xs; case xs: Array[AnyRef] => ???; case xs: Array[Any] => ??? }
-  |                                                                                                ^
-  |      abstract type pattern is unchecked since it is eliminated by erasure
-
-longer explanation available when compiling with `-explain`
 def matchArray3[A](xs: Array[A]): [A] => (xs: Array[A])Array[Int]
 scala> def matchArray4(xs: Array[Any]) = xs match { case xs: Array[Int] => xs; case xs: Array[A] => ???; case xs: Array[Any] => ??? }
 -- [E035] Erased Type Unchecked Warning: <console> -----------------------------
@@ -56,4 +50,6 @@ scala> def matchList1(xs: List[Any]) = xs match { case xs: List[Int @unchecked]
 def matchList1(xs: List[Any]): Nothing
 scala> def matchList2(xs: List[Any]) = xs match { case List() => ???; case _ => ??? }
 def matchList2(xs: List[Any]): Nothing
+scala> def matchList3(xs: Seq[_]) = xs match { case List() => ???; case _ => ??? }
+def matchList3(xs: Seq[_]): Nothing
 scala> :quit