GHC panic with TypeInType and type family

This rather simple type family,

{-# LANGUAGE TypeFamilies, TypeOperators, TypeInType #-}

module GenWhoops where
import GHC.Generics

type family GenComp k (x :: k) (y :: k) :: Ordering where
  GenComp ((x :+: y) p) ('L1 x) ('L1 y) = GenComp (x p) x y

produces the following panic:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.3.20170828 for x86_64-unknown-linux):
        piResultTy
  k_a1LK[tau:1]
  p_a1Lz[sk:1]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1138:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:949:35 in ghc:Type

This happens with both GHC 8.2.1 and something very close to HEAD.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	goldfire
Operating system
Architecture