[GHC] #8204: Impossible when deriving Read for empty data declaration

30 Aug 2013

      #8204: Impossible when deriving Read for empty data declaration
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       Reporter:  tinctorius        |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Remember #4302?

 Me:

 {{{
 data X deriving Read
 main :: IO ()
 main = return ()
 }}}

 GHC:

 {{{
 [1 of 1] Compiling Main             ( deriving.hs, deriving.o )

 deriving.hs:2:17:
     Can't make a derived instance of `Read X':
       `X' must have at least one data constructor
       Possible fix: use a standalone deriving declaration instead
     In the data declaration for `X'
 }}}

 Me:

 {{{
 -- Ugh, fine...
 {-# LANGUAGE StandaloneDeriving #-}
 deriving instance Read X
 main :: IO ()
 main = return ()
 }}}

 GHC:

 {{{
 [1 of 1] Compiling Main             ( deriving.hs, deriving.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.3 for x86_64-unknown-linux):
         Prelude.foldr1: empty list

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

 }}}

 And so I did.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8204
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