[Haskell-cafe] Constraints on data-types, mis-feature?

Hello In the archives of haskell-cafe I found a mention of constraints on datatypes as a mis-feature of Haskell. In particular, that they're not propagated well. Can someone elaborate on that? Also, are they still considered a mis-feature with the emergence of GADTs ? If I have data GADT a where ... Alt :: (a -> b -> c) -> GADT a -> GADT b -> GADT c ... and I want to constrain all a, b, c. Would it be better to expose all of them as type vars, rather than constrain in-place? It would lead to a rather verbose code.