> This has the code smell of trying to use typeclasses for OOP.  That won't work.  (Yes, really.)

I am not trying to use OOP, I am just writing some typecasting at all.

> This would be correct.  Constraints on an instance are applied *after* the instance is selected, so when Haskell is looking for an instance, these two are identical.

I didn't understand why these two instances are identical ? The constraints are different and OverlappingInstances should permit overlapping typeclasses in constraints and select more specific instance clause.