the code from http://joyoftypes.blogspot.com/2012/08/generalizednewtypederiving-is.html type checks in the release candidate

also see discussion here http://www.reddit.com/r/haskell/comments/y8kca/generalizednewtypederiving_is_very_very_unsafe/

(punch line: the generalized newtype deriving + type families or gadts allows users to write unsafeCoerce etc)

perhaps a simple near term solution is to have any module using generalized new type deriving be inferred to be unsafe by default?