"Iface type variable out of scope" when compiling with -c

Take these two files:

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE TypeFamilies #-}
module Foo where

import GHC.Generics

type FakeOut a = Int

data family   TyFamily y z
data instance TyFamily a b = TyFamily Int (FakeOut b)
  deriving Generic1

module Bar where

import Foo
import GHC.Generics

main :: IO ()
main = print $ from1 $ TyFamily 1 2

And compile them like so:

$ ghc -c Foo.hs
$ ghc -c Bar.hs
./Foo.hi
Declaration for Rep1_R:TyFamilyab
Axiom branches Rep1_R:TyFamilyab:
  Iface type variable out of scope:  b
Cannot continue after interface file error

I can reproduce this with every version of GHC from 7.8.4 onward, so this is quite an old bug!

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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