Fix #6451: Add missing case to AppliedType#lowerBound

Author: odersky

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

@@ -3556,6 +3556,8 @@ object Types {
             else lo.applyIfParameterized(args)
           case _ => NoType
         }
+      case tycon: AppliedType =>
+        tycon.lowerBound.applyIfParameterized(args)
       case _ =>
         NoType
     }

tests/pos/i6451.scala

@@ -0,0 +1,10 @@
+object Test {
+
+  type YZ = [Y[_]] => [Z[_]] => [T] => Y[Z[T]]
+
+  val r1: List[List[Int]] = ???
+  def r2(): List[List[Int]] = ???
+
+  val l1: YZ[List][List][Int] = r1
+  val l2: YZ[List][List][Int] = r2()
+}