I'm studying this extension method in JavaUtil.kt

inline fun <T> java.util.Collection<T>.toArray() : Array<T> {
  val answer = Array<T>(this.size)
  var idx = 0
  for (elem in this)
    answer[idx++] = elem
  return answer as Array<T>
}     

because I am finding that these two ArrayList<String> are not equal:

val a = arrayList("a", "b", "aa")
val b = arrayList(a.toArray())

In the IDE, the "as" in the extension function return statement is browned-out, with a tooltip that says "This cast can never succeed".  I don't understand why the "as Array<T>" is needed at all.  When I remove it, the "answer" in "return answer" goes red, with a error that says "inferred type was Array<T?> but expected Array<T>".

Why is the "as Array<T>" needed, and why is the inferred type Array<T?> if I remove it?