[GHC] #13871: GHC panic in 8.2 only: typeIsTypeable(Coercion)

[GHC]

#13871: GHC panic in 8.2 only: typeIsTypeable(Coercion)
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.2.1-rc2
  (Type checker)                     |
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  Typeable                           |
       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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 This code works fine in GHC 8.0.1 and 8.0.2:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Foo where

 import Data.Kind

 data Foo (a :: Type) (b :: Type) where
   MkFoo :: (a ~ Int, b ~ Char) => Foo a b

 data family Sing (a :: k)
 data SFoo (z :: Foo a b) where
   SMkFoo :: SFoo MkFoo
 }}}

 But in GHC 8.2 and HEAD, it panics:

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs
 GHCi, version 8.2.0.20170622: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Foo              ( Bug.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.0.20170622 for x86_64-unknown-linux):
         typeIsTypeable(Coercion)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13871>
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