The following code should fail (since we don't promote data families), but it should do so gracefully:

{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE TypeFamilies               #-}
{-# LANGUAGE PolyKinds                  #-}

module Bug1 where


data family DF a 
data instance DF Int = DFInt

data U = U1 (DF Int)

data I :: U -> * where I1 :: I (U1 DFInt)

{-
    GHC internal error: `DFInt' is not in scope during type checking, but it passed the renamer
    tcl_env of environment: [(r9P, AThing U -> *), (r9Q, ANothing)]
    In the type `I (U1 DFInt)'
    In the definition of data constructor `I1'
    In the data type declaration for `I'
-}