With the canonical Enum instance for the following data type

data T = A | B | C | D

I get:

> enumFromTo A A :: Array T
[A]
> enumFromTo A B :: Array T
[A,B]
> enumFromTo A C :: Array T
[A,B,C]
> enumFromTo A D :: Array T
[A,B,C]

The last one returns a wrong result with the current implementation for enumFromTo because the unfoldr-function calls succ D, which short-circuits in the Maybe monad and breaks the unfoldr 'loop'.

Note: this only fails for bounded Enums