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
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

assigned to @simonpj

I have a patch that fixes this, among other things. But it'll have to wait till I get back from holiday on 14 July.

mentioned in commit 9b8ba629

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	→ polykinds/T9063

mentioned in issue #9584 (closed)

added Pnormal label

mentioned in issue #23329