# Newtype deriving doesn't work with type families

This assumes `-XTypeFamiles -XGeneralizedNewtypeDeriving`

. Example:

```
class C a where
type T a
foo :: a -> T a
instance C Int where
type T Int = Int
foo = id
newtype N = N Int deriving(C)
```

This happily produces an `instance C N`

but no `type instance T N`

. It should either (preferably) generate

`type instance T N = Int`

or fail. The example also compiles if `T`

is a data family (the `Int`

instance needs to be change accordingly). It should probably fail in this case.

BTW, this also compiles fine, with rather dramatic consequences:

```
type family T a
class C a where
foo :: a -> T a
type instance T Int = Int
instance C Int where
foo = id
type instance T N = Double
newtype N = N Int deriving(C)
```

I guess this last example is the same bug as #1496 (closed). I wonder if the deriving clause could generate something like:

`instance T Int ~ T N => C Int`