[GHC] #15647: ghc: panic! (the 'impossible' happened)

[GHC]

#15647: ghc: panic! (the 'impossible' happened)
-------------------------------------+-------------------------------------
           Reporter:  monomon        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-linux):
         initTc: unsolved constraints
   WC {wc_insol =
         [W] ask_an9K :: t_an9J[tau:1] (CHoleCan: ask)
         [W] put_an9Q :: t_an9P[tau:1] (CHoleCan: put)}
 }}}

 So this code derives an instance of the Decimal type incorrectly, but not
 sure if this is causing the error.

 I have some nested records, which contain a few Decimals. I am trying to
 make them acidic.

 {{{#!hs
 instance SafeCopy (DecimalRaw a) where
   putCopy (Decimal d) = contain $ safePut d
   getCopy = contain $ Decimal <$> safeGet

 deriveSafeCopy 0 'base ''Client
 deriveSafeCopy 0 'base ''Article


 deriveSafeCopy 0 'base ''ServerData
 makeLenses ''ServerData

 writeState :: String -> Update ServerData ()
 writeState newValue
   = put (ServerData newValue)
 queryState :: Update ServerData String
 queryState = do ServerData string <- ask
                 return string

 makeAcidic ''ServerData ['writeState 'queryState]
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15647>
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