new BuiltList<A>(builtListOfB) should be efficient

[dskloetg]

B extends A.
If we have a BuiltList<B> but we need it as a BuiltList<A>, we currently need to create a copy of the list.
new BuiltList<A>() should be able to reuse the underlying list and without checking the type of each element.

[davidmorgan]

Now that we get rid of explicit type checks and just rely on the checks of the underlying collection, I think this is unfortunately infeasible.