[GHC] #15343: Implicitly quantifying a kind variable causes GHC 8.6 to panic (coercionKind)

[GHC]

#15343: Implicitly quantifying a kind variable causes GHC 8.6 to panic
(coercionKind)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Here's a smaller example:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Data.Kind

 data HEq :: forall j k. j -> k -> Type where
   HRefl :: HEq a a

 data Sing :: forall j k (a :: j) (b :: k).
              HEq a b -> Type where
   SHRefl :: Sing HRefl

 elimSing :: forall (j :: Type) (k :: Type) (a :: j) (b :: k)
                    (p :: forall (z :: Type) (y :: z). HEq a y -> Type)
                    (r :: HEq a b).
             Sing r
          -> p HRefl
          -> p r
 elimSing SHRefl pHRefl = pHRefl

 data    WhySym (a :: j) :: forall   (y :: z). HEq a y -> Type
 -- data WhySym (a :: j) :: forall z (y :: z). (a :~~: y) ~> Type
 -- The version above does NOT panic

 hsym :: forall (j :: Type) (k :: Type) (a :: j) (b :: k) (r :: HEq a b).
         Sing r -> WhySym a r
 hsym singEq = elimSing @j @k @a @b @(WhySym a) @r singEq undefined
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15343#comment:1>
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