Handle OrType in local class checking

Author: dwijnand

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

@@ -154,7 +154,7 @@ object TypeTestsCasts {
       case tp2: RefinedType     => recur(X, tp2.parent) && TypeComparer.hasMatchingMember(tp2.refinedName, X, tp2)
       case tp2: RecType         => recur(X, tp2.parent)
       case _
-      if P.classSymbol.isLocal && P.classSymbol.isInaccessibleChildOf(X.classSymbol) => // 8
+      if P.classSymbol.isLocal && foundClasses(X, Nil).exists(P.classSymbol.isInaccessibleChildOf) => // 8
         false
       case _                    => true
     })
@@ -178,15 +178,6 @@ object TypeTestsCasts {
         def derivedTree(expr1: Tree, sym: Symbol, tp: Type) =
           cpy.TypeApply(tree)(expr1.select(sym).withSpan(expr.span), List(TypeTree(tp)))
 
-        def effectiveClass(tp: Type): Symbol =
-          if tp.isRef(defn.PairClass) then effectiveClass(erasure(tp))
-          else if tp.isRef(defn.AnyValClass) then defn.AnyClass
-          else tp.classSymbol
-
-        def foundClasses(tp: Type, acc: List[Symbol]): List[Symbol] = tp.dealias match
-          case OrType(tp1, tp2) => foundClasses(tp2, foundClasses(tp1, acc))
-          case _ => effectiveClass(tp) :: acc
-
         def inMatch =
           tree.fun.symbol == defn.Any_typeTest ||  // new scheme
           expr.symbol.is(Case)                // old scheme
@@ -376,4 +367,13 @@ object TypeTestsCasts {
     }
     interceptWith(expr)
   }
+
+  private def effectiveClass(tp: Type)(using Context): Symbol =
+    if tp.isRef(defn.PairClass) then effectiveClass(erasure(tp))
+    else if tp.isRef(defn.AnyValClass) then defn.AnyClass
+    else tp.classSymbol
+
+  private def foundClasses(tp: Type, acc: List[Symbol])(using Context): List[Symbol] = tp.dealias match
+    case OrType(tp1, tp2) => foundClasses(tp2, foundClasses(tp1, acc))
+    case _ => effectiveClass(tp) :: acc
 }

tests/neg/i4812.scala

@@ -108,6 +108,12 @@ object Test:
     x match
       case x: B => x
 
+  def test12 =
+    class Foo
+    class Bar
+    val x: Foo | Bar = new Foo
+    x.isInstanceOf[Foo]
+
   def main(args: Array[String]): Unit =
     test(1)
     val x: String = test("") // was: ClassCastException: java.lang.Integer cannot be cast to java.lang.String