[GHC] #9575: -XAutoDeriveTypeable fails to generate instances

[GHC]

#9575: -XAutoDeriveTypeable fails to generate instances
-------------------------------------+-------------------------------------
       Reporter:  hvr                |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  high               |               Milestone:  7.8.4
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 The following doesn't compile with GHC 7.8.3, but works with GHC HEAD. I
 couldn't find a matching ticket, so I don't know if this was fixed
 knowingly or not...

 {{{#!hs
 {-# LANGUAGE AutoDeriveTypeable #-}

 import Data.Typeable (Typeable)

 data T1 = C1 Int
      deriving (Eq,Ord)

 tvoid :: Typeable a => a -> IO ()
 tvoid _ = return ()

 main :: IO ()
 main = tvoid (C1 0)
 }}}

 ...fails for GHC 7.8.3 with

 {{{
     No instance for (Typeable T1) arising from a use of ‘tvoid’
     In the expression: tvoid (C1 0)
     In an equation for ‘main’: main = tvoid (C1 0)
 }}}

 I'm marking this with high priority, as it makes `-XAutoDeriveTypeable`
 unusable on GHC 7.8.3 as it stands.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9575>
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