[GHC] #11251: isInstance does not work on Typeable with base-4.8 anymore

[GHC]

#11251: isInstance does not work on Typeable with base-4.8 anymore
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        Reporter:  songzh            |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  7.10.1
      Resolution:                    |             Keywords:  Typeable,
                                     |  isInstance
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by songzh):

 I also find it is allowed to derive `Typeable` class several times for one
 data type.
 I am no with GHC 8.2
 {{{
 {-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
 import Data.Typeable

 data TT = TT

 deriving instance Typeable TT
 -- allowed to derive twice.
 deriving instance Typeable TT

 deriving instance Show TT
 -- Not allowed to derive twice
 -- deriving instance Show TT
 }}}

 Should this be allowed? Or they are the same problem with this ticket. I
 speculate these are the same problem since GHC does not know `TT` is
 already an instance of `Typeable` or not after the first time of deriving.

  Do I need to open another ticket for this problem?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11251#comment:3>
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