chore: add regression test for scala#4528

Author: hamzaremmal

tests/run/i4528.scala

@@ -0,0 +1,9 @@
+import scala.reflect.Selectable.reflectiveSelectable
+
+type T = {val a: Int; def a_=(x: Int): Unit}
+
+@main def Test() =
+  val x: T = (new { var a = 10 }).asInstanceOf[T]
+  assert(x.a == 10)
+  x.a = 11
+  assert(x.a == 11)