[GHC] #14490: TTG Snags

[GHC]

#14490: TTG Snags
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        Reporter:  alanz             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14482            |  Differential Rev(s):
       Wiki Page:                    |
  ImplementingTreesThatGrow          |
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Comment (by simonpj):

 Why does Plan B not work in 8.0.2? This works:
 {{{
 {-# LANGUAGE TypeFamilies, GADTs, FlexibleInstances, StandaloneDeriving,
 DeriveDataTypeable #-}

 module Foo where

 import Data.Data

 data T a where
   MkT :: XT a -> a -> T a

 type family XT a
 type instance XT Bool = Int

 deriving instance Data (T Bool)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14490#comment:29>
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