Abstract closed type families don't work with Backpack

This should typecheck:

{-# LANGUAGE TypeFamilies #-}
unit p where
    module A where
        type family ClosedFam a where
            ClosedFam a = Int

unit sig where
    signature A where
        type family ClosedFam a where ..

unit main where
    dependency sig[A=p:A]

but instead we confusingly report:

foo.bkp:9:9: error:
    Type constructor ‘ClosedFam’ has conflicting definitions in the module
    and its hsig file
    Main module: type family ClosedFam a :: *
    Hsig file:  type family ClosedFam a :: *

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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