The following code:

import GHC.Generics

data family F typ :: * -> *
data A
data instance F A a = AData a deriving Generic1

produces an error when run with ghc 7.8.2 or 7.8.3:

Couldn't match type ‘Rep1 (F A)’
              with ‘M1 t0 t1 (M1 t2 t3 (M1 t4 t5 Par1))’
The type variables ‘t0’, ‘t1’, ‘t2’, ‘t3’, ‘t4’, ‘t5’ are ambiguous
Expected type: Rep1 (F A) a
  Actual type: M1 t0 t1 (M1 t2 t3 (M1 t4 t5 Par1)) a
In the pattern: M1 (M1 (M1 g1))
In an equation for ‘to1’: to1 (M1 (M1 (M1 g1))) = AData (unPar1 g1)
In the instance declaration for ‘Generic1 (F A)’

whereas ghc 7.6.3 simply refuses to go ahead with the message:

generic1.hs:14:40:
    Derived instance `Generic1 (F A)'
      requires illegal partial application of data type family F
    In the data instance declaration for `F'

Either a check has gone missing in 7.8.x, or a bug has crept up in the Generic1 support. I have gone through past tickets and #5936 (closed) suggests that it is the latter.