GeneralizedNewtypeDeriving + TypeFamilies doesn't work

It would be nice if we could do stuff like this:

{-# LANGUAGE GeneralizedNewtypeDeriving, TypeFamilies, FlexibleContexts, FlexibleInstances #-}
class K a where
  bar :: a -> a

class K (B a) => M a where
  data B a :: *
  foo :: B a -> B a

instance M Bool where
  data B Bool = B1Bool Bool | B2Bool Bool
  foo = id

instance K (B Bool) where
  bar = id

instance M Int where
  newtype B Int = BInt (B Bool) deriving K
  foo = id

which currently gives the error

foo.hs:17:41:
    Can't make a derived instance of `K (B Int)'
      (even with cunning newtype deriving:
       the newtype may be recursive)
    In the newtype instance declaration for `B'

However, the newtype is not recursive, it is just an associated datatype from another class, so it seems like this ought to work.

Trac metadata

Trac field	Value
Version	6.10.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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