derive Generic for phantom types

[zudov]

Compiling the following code

data Void
derive instance genericVoid :: GenericVoid

Throws

psc: Prelude.head: empty list

The derived instance should probably look like

toSignature _ = SigProd []
fromSpine _ = Nothing
toSpine = absurd

[zudov]

Defining Void as

newtype Void = Void Void

doesn't cause that problem (obviously because the constructor list isn't empty anymore :) )

[paf31]

@nicodelpiano Would you mind looking into this? I think it's in the exhaustivity checker.

[paf31]

You can get the length of the list of arguments from the Case statement and pass it as a function argument.

[nicodelpiano]

@paf31 Sure! I'm working on this.