Define topLevelClass for NoDenotation

Fixes #3647

Author: odersky

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -925,11 +925,14 @@ object SymDenotations {
     /** The top-level class containing this denotation,
      *  except for a toplevel module, where its module class is returned.
      */
-    final def topLevelClass(implicit ctx: Context): Symbol = {
-      def topLevel(d: SymDenotation): Symbol = {
-        if (d.isEffectiveRoot || (d is PackageClass) || (d.owner is PackageClass)) d.symbol
-        else topLevel(d.owner)
-      }
+    def topLevelClass(implicit ctx: Context): Symbol = {
+
+      def topLevel(d: SymDenotation): Symbol =
+        if (!exists || d.isEffectiveRoot || (d is PackageClass) || (d.owner is PackageClass))
+          d.symbol
+        else
+          topLevel(d.owner)
+
       val sym = topLevel(this)
       if (sym.isClass) sym else sym.moduleClass
     }

tests/pos/i3647.scala

@@ -0,0 +1,21 @@
+object App {
+  def main(args: Array[String]): Unit = {
+    trait FooT {
+      type T
+      type Bar[A]
+
+      def get(k: Bar[T]): String
+    }
+    val test: FooT = new FooT {
+      type T = String
+      type Bar[A] = J[A]
+      sealed abstract class J[A]
+      final case object JName extends J[T]
+      final case object JInt extends J[Int]
+
+      def get(k: J[T]): String = k match {
+        case JName => "Age"
+      }
+    }
+  }
+}