[GHC] #12834: GHC panic: while printing Non type-variable argument

[GHC]

#12834: GHC panic: while printing Non type-variable argument
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           Reporter:  phadej         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 {-# LANGUAGE GADTs, TypeFamilies, DataKinds, TypeOperators,
 MultiParamTypeClasses, UndecidableInstances, UndecidableSuperClasses,
 FlexibleInstances, PolyKinds, KindSignatures #-}

 import GHC.Exts (Constraint)

 newtype I a = I a

 data NP :: (k -> *) -> [k] -> * where
   Nil  :: NP f '[]
   (:*) :: f x -> NP f xs -> NP f (x ': xs)

 infixr 5 :*

 class (AllF f xs, SListI xs) => All (f :: k -> Constraint) (xs :: [k])
 instance (AllF f xs, SListI xs) => All f xs

 data SList :: [k] -> * where
   SNil  :: SList '[]
   SCons :: SListI xs => SList (x ': xs)

 class SListI (xs :: [k]) where
   -- | Get hold of the explicit singleton (that one can then
   -- pattern match on).
   sList :: SList xs

 instance SListI '[] where
   sList = SNil

 instance SListI xs => SListI (x ': xs) where
   sList = SCons

 -- | Type family used to implement 'All'.
 --
 type family AllF (c :: k -> Constraint) (xs :: [k]) :: Constraint
 type instance AllF _c '[]       = ()
 type instance AllF  c (x ': xs) = (c x, All c xs)

 semigroup :: All ((~) (Maybe Int)) xs => NP I xs -> NP I xs -> NP I xs
 semigroup = undefined
 }}}

 Causes

 {{{
 ghc-failure-all.hs:37:14: error:
     • Non type-variable argumentghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         print_equality ~
 }}}

 If `AllF` is used directly in the definition of `sappend`, there is no
 error whatsoever.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12834>
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