Yes, because these are superclasses of Unbox. So can't you simply say this:
newtype Foo = Foo Int deriving (Eq, Show, Unbox, M.MVector MVector, G.Vector Vector)
I don't think this should work. This is the basic set up:
[...]
I don't understand what GeneralizedNewtypeDeriving does with deriving(G.Vector Vector) in this situation but it can't possibly do anything sensible without a data instance Vector Foo. The fact that it doesn't fail is just a bug, IMO.
I've actually started wondering the same after I posted this. I tried to use -ddump-deriv to get at the code, but GHC seems to be cheating and doesn't show what it generates. I guess I understand newtype-deriving less than I thought ... Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com