You can't do that. You have to think about how Ruby would be able to determine what belongs to *ary and what belongs to the optional hash. Since Ruby can't read your mind, the above argument combination (splat + optional) is impossible for it to solve logically.
You either have to rearrange your arguments:
def test(id, h, *a)
In which case h will not be optional. Or then program it manually:
def test(id, *a)
h = a.last.is_a?(Hash) ? a.pop : nil
# ^^ Or whatever rule you see as appropriate to determine if h
# should be assigned a value or not.
