js.compile.ignore=true
=====
>>> main.whiley
public export method test():
   int[] xs = [1,2,3]
   (int[])|(bool[]) ys = xs[5:=4]
   assert ys == [1,4,3]
---
E708 main.whiley 3,28
E725 main.whiley 3,28
=====
>>> main.whiley 3:4
   int[]|bool[] ys = xs[1:=4]
---