Break looping in checkable/hasPoison

Author: dwijnand

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

@@ -158,11 +158,13 @@ object TypeTestsCasts {
         val methodSymbol = sym.owner
         val tpSyms = typer.ErrorReporting.substitutableTypeSymbolsInScope(sym).toSet
         def isAccessible(sym: Symbol): Boolean = sym == methodSymbol || sym.isType && isAccessible(sym.owner)
+        val seen = scala.collection.mutable.Set.empty[Type]
         def hasPoison(tp: Type): Boolean =
+          seen += tp
           tp.baseClasses.filter(isAccessible).exists { sym =>
             sym.info.decls.exists { sym =>
               sym.info.existsPart(tp => tpSyms.contains(tp.typeSymbol))
-                || isAccessible(sym.info.typeSymbol.maybeOwner) && hasPoison(sym.info)
+                || !seen.contains(sym.info) && isAccessible(sym.info.typeSymbol.maybeOwner) && hasPoison(sym.info)
             }
           }
         !hasPoison(tp2)

tests/neg/i4812.scala

@@ -53,6 +53,20 @@ object Test:
           prev = new A(x)
           x
 
+  def test6[T](x: T): T =
+    class Foo { var bar: Bar = null }
+    class Bar { var foo: Foo = null; var elem: T = _ }
+    prev match
+      case prev: Foo => // error: the type test for A cannot be checked at runtime
+        prev.bar.elem
+      case _ =>
+        val foo = new Foo
+        val bar = new Bar
+        bar.elem = x
+        foo.bar = bar
+        prev = foo
+        x
+
   def main(args: Array[String]): Unit =
     test(1)
     val x: String = test("") // was: ClassCastException: java.lang.Integer cannot be cast to java.lang.String