[GHC] #11681: ghc panic with TypeError

[GHC]

#11681: ghc panic with TypeError
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           Reporter:  inaki          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           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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 When playing around with `TypeError` I got a ghc panic with the following
 code:
 {{{#!hs
 {-# LANGUAGE DataKinds, FlexibleContexts, TypeOperators,
      FlexibleInstances, UndecidableInstances #-}

 import GHC.TypeLits

 class C t where

 instance
     (TypeError (Text "A" :<>: {- Text -} "B"))
     => C t where

 main :: IO ()
 main = return ()
 }}}

 Compiling this results in
 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.0.20160204 for x86_64-unknown-linux):
         fvProv falls into a hole {a1bh}
 }}}

 Commenting "in" the second `Text` removes the panic.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11681>
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