[GHC] #14110: GHC Panic on over-saturated associated type family

[GHC]

#14110: GHC Panic on over-saturated associated type family
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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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 If we over-saturate a type family

 {{{#!hs
 {-# Language TypeFamilies, ScopedTypeVariables, PolyKinds, DataKinds #-}

 import Data.Kind

 class R (c :: k -> Constraint) where
   type R_ (c :: k -> Constraint) :: k -> Type

 instance R Eq where
   type R_ Eq a = a -> a -> Bool
 }}}

 we get

 {{{
 $ ghci -ignore-dot-ghci test.hs
 GHCi, version 8.3.20170605: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( test.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.3.20170605 for x86_64-unknown-linux):
         repSplitAppTys
   a_a2qu[sk:1]
   a_a2qu[sk:1] -> Bool
   []
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:809:9 in ghc:Type

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

 >
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14110>
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