[GHC] #12788: While using Data.Aeson.TH, "Irrefutable pattern failed for pattern sel_id : _"

[GHC]

#12788: While using Data.Aeson.TH, "Irrefutable pattern failed for pattern sel_id :
_"
-------------------------------------+-------------------------------------
        Reporter:  jchia             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Here's a version which doesn't require installing `aeson`:

 {{{#!hs
 -- Bug2.hs
 module Bug2 where

 import Language.Haskell.TH

 data Options = Options
     { fieldLabelModifier :: String -> String
     , constructorTagModifier :: String -> String
     , allNullaryToStringTag :: Bool
     , omitNothingFields :: Bool
     , sumEncoding :: SumEncoding
     , unwrapUnaryRecords :: Bool
     }

 data SumEncoding =
     TaggedObject { tagFieldName      :: String
                  , contentsFieldName :: String
                  }
   | ObjectWithSingleField
   | TwoElemArray

 deriveJSON :: Options -> Name -> Q [Dec]
 deriveJSON _ _ = return []
 }}}

 {{{#!hs
 -- Bug.hs
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where

 import Bug2
 import Language.Haskell.TH

 data Bad = Bad { _bad :: String } deriving (Eq, Ord, Show)

 $(deriveJSON defaultOptions{} ''Bad)
 }}}

 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs
 [1 of 2] Compiling Bug2             ( Bug2.hs, Bug2.o )
 [2 of 2] Compiling Bug              ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.1.20161010 for x86_64-unknown-linux):
         compiler/typecheck/TcExpr.hs:820:15-35: Irrefutable pattern failed
 for pattern sel_id : _


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

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12788#comment:2>
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