Iface type variable out of scope with default associated types and polykinds

I have

{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies #-}

module A where

import Data.Proxy

class kproxy ~ 'KProxy => PEq (kproxy :: KProxy a) where
  type F (x :: a) :: Bool
  type F (x :: a) = False

{-# LANGUAGE DataKinds, KindSignatures, TypeFamilies #-}

module B where

import A
import Data.Proxy

data Void

instance PEq ('KProxy :: KProxy Void)

module C where

import B

Now, this happens in a terminal window:

rae:21:38:44 ~/temp> ghc A.hs
[1 of 1] Compiling A                ( A.hs, A.o )
rae:21:38:46 ~/temp> ghc B.hs
[2 of 2] Compiling B                ( B.hs, B.o )
rae:21:38:47 ~/temp> ghc C.hs
[3 of 3] Compiling C                ( C.hs, C.o )
The interface for ‘B’
Declaration for R:FVoidx:
  Iface type variable out of scope:  a
Cannot continue after interface file error

If any two of the files are compiled together, the error does not occur. The emptiness of the Void type is irrelevant -- the error initially occurred with an inhabited type.

This is a regression from 7.8.2.

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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