[marcosdumay]

data A = A

data B = B

data X = XA A | XB B

data Y = YA A | YB B

f :: X -> () f = undefined

let r = f (undefined :: Y)

There's absolutely no way to write this so it compiles. In fact, there isn't even a way to define the composed types so that they only express a sum, you have to add extra baggage.