Behaviour of autogenerated setter function with multiple constructors

[pietervdvn]

Given type A = {X f:Int, Y}

What should the default implementation of setF be?

Either:

f : Int -> A -> A
i (X _) = X i
i Y      = Y

Or

f : Int -> A -> Maybe A
i (X _) = Just (X i)
_ _     = Nothing