GHC panics on "print_equality ~"

{-# Language PolyKinds, TypeFamilies #-}

import Data.Kind

class Category (p :: i -> i -> Type) where
  type Ob p :: i -> Constraint

data I a b 
instance Category I where
  type Ob I = (~) Int

fails with

$ ghci -ignore-dot-ghci /tmp/Error.hs 
GHCi, version 8.1.20160503: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( /tmp/Error.hs, interpreted )

/tmp/Error.hs:10:15: error:ghc: panic! (the 'impossible' happened)
  (GHC version 8.1.20160503 for x86_64-unknown-linux):
        print_equality ~

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

> 

Trac metadata

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