Dealias nestly for checkability of array

som-snytt · som-snytt · commit db98f295c426 · 2026-04-03T22:42:22.000-07:00
diff --git a/src/compiler/scala/tools/nsc/typechecker/Checkable.scala b/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
@@ -312,11 +312,24 @@ trait Checkable {
       if (P0.typeSymbol == SingletonClass)
         context.warning(tree.pos, s"fruitless type test: every non-null value will be a Singleton dynamically", WarningCategory.Other)
       else {
+        def deepDealias(tp: Type): Type =
+          tp.dealiasWiden match {
+            case TypeRef(pre, ArrayClass, args) => TypeRef(pre, ArrayClass, args.map(deepDealias))
+            case tp => tp
+          }
         // singleton types not considered here, dealias the pattern
-        val P = P0.dealiasWiden
+        val P = deepDealias(P0)
         val X = X0.widen
 
-        def PString = if (P eq P0) P.toString else s"$P (the underlying of $P0)"
+        def PString = {
+          val pstr = P.toString
+          if (P eq P0) pstr
+          else {
+            val ostr = P0.toString
+            if (pstr == ostr) pstr
+            else s"$pstr (the underlying of $ostr)"
+          }
+        }
 
         P match {
           // Prohibit top-level type tests for these, but they are ok nested (e.g. case Foldable[Nothing] => ... )
diff --git a/test/files/neg/t11843.check b/test/files/neg/t11843.check
@@ -14,7 +14,7 @@ t11843.scala:11: error: value $asInstanceOf is not a member of Symbol
 did you mean asInstanceOf? or perhaps isInstanceOf?
   ss.$asInstanceOf[String]
      ^
-t11843.scala:8: warning: fruitless type test: a value of type Symbol cannot also be a String (the underlying of String)
+t11843.scala:8: warning: fruitless type test: a value of type Symbol cannot also be a String
   ss.isInstanceOf[String]
                  ^
 1 warning
diff --git a/test/files/neg/t12408.check b/test/files/neg/t12408.check
@@ -4,7 +4,7 @@ t12408.scala:6: warning: abstract type pattern B is unchecked since it is elimin
 t12408.scala:7: warning: abstract type B in type pattern t12408.Renderer[B] is unchecked since it is eliminated by erasure
     def f2[B] = a match { case _: Renderer[B] => }              // warn
                                   ^
-t12408.scala:8: warning: non-variable type argument Int in type pattern List[Int] (the underlying of List[Int]) is unchecked since it is eliminated by erasure
+t12408.scala:8: warning: non-variable type argument Int in type pattern List[Int] is unchecked since it is eliminated by erasure
     def f3[B](xs: List[A]) = xs match { case _: List[Int] => }  // warn
                                                 ^
 t12408.scala:9: warning: abstract type A in type pattern t12408.Renderer[A] is unchecked since it is eliminated by erasure
diff --git a/test/files/neg/unchecked.check b/test/files/neg/unchecked.check
@@ -1,4 +1,4 @@
-unchecked.scala:20: warning: non-variable type argument String in type pattern Iterable[String] (the underlying of Iterable[String]) is unchecked since it is eliminated by erasure
+unchecked.scala:20: warning: non-variable type argument String in type pattern Iterable[String] is unchecked since it is eliminated by erasure
     case xs: Iterable[String] => xs.head // unchecked
              ^
 unchecked.scala:24: warning: non-variable type argument Any in type pattern scala.collection.immutable.Set[Any] (the underlying of Set[Any]) is unchecked since it is eliminated by erasure
diff --git a/test/files/pos/t13167.scala b/test/files/pos/t13167.scala
@@ -0,0 +1,15 @@
+//> using options -Werror
+
+object Test extends App {
+  type A[X] = scala.Array[X]
+  def test(x: Any) =
+    x match {
+      case _: A[A[Int]]     => println("int") // nowarn
+      case _: A[A[String]]  => println("str")
+      case _: A[A[AnyRef]]  => println("yes")
+      case _: Array[String] => println("arr")
+      case _                => println("no")
+    }
+  test(new A[A[String]](0))
+  test(new A[String](0))
+}

Original file line number	Diff line number	Diff line change
`@@ -14,7 +14,7 @@ t11843.scala:11: error: value $asInstanceOf is not a member of Symbol`
`14`	`14`	`did you mean asInstanceOf? or perhaps isInstanceOf?`
`15`	`15`	`ss.$asInstanceOf[String]`
`16`	`16`	`^`
`17`		`-t11843.scala:8: warning: fruitless type test: a value of type Symbol cannot also be a String (the underlying of String)`
	`17`	`+t11843.scala:8: warning: fruitless type test: a value of type Symbol cannot also be a String`
`18`	`18`	`ss.isInstanceOf[String]`
`19`	`19`	`^`
`20`	`20`	`1 warning`
Original file line number	Diff line number	Diff line change
`@@ -4,7 +4,7 @@ t12408.scala:6: warning: abstract type pattern B is unchecked since it is elimin`
`4`	`4`	`t12408.scala:7: warning: abstract type B in type pattern t12408.Renderer[B] is unchecked since it is eliminated by erasure`
`5`	`5`	`def f2[B] = a match { case _: Renderer[B] => } // warn`
`6`	`6`	`^`
`7`		`-t12408.scala:8: warning: non-variable type argument Int in type pattern List[Int] (the underlying of List[Int]) is unchecked since it is eliminated by erasure`
	`7`	`+t12408.scala:8: warning: non-variable type argument Int in type pattern List[Int] is unchecked since it is eliminated by erasure`
`8`	`8`	`def f3[B](xs: List[A]) = xs match { case _: List[Int] => } // warn`
`9`	`9`	`^`
`10`	`10`	`t12408.scala:9: warning: abstract type A in type pattern t12408.Renderer[A] is unchecked since it is eliminated by erasure`
Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-unchecked.scala:20: warning: non-variable type argument String in type pattern Iterable[String] (the underlying of Iterable[String]) is unchecked since it is eliminated by erasure`
	`1`	`+unchecked.scala:20: warning: non-variable type argument String in type pattern Iterable[String] is unchecked since it is eliminated by erasure`
`2`	`2`	`case xs: Iterable[String] => xs.head // unchecked`
`3`	`3`	`^`
`4`	`4`	`unchecked.scala:24: warning: non-variable type argument Any in type pattern scala.collection.immutable.Set[Any] (the underlying of Set[Any]) is unchecked since it is eliminated by erasure`