[GHC] #8132: Warning for Typeable instances misplaced

[GHC]

#8132: Warning for Typeable instances misplaced
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        Reporter:  scottgw                       |            Owner:
            Type:  bug                           |  dreixel
        Priority:  normal                        |           Status:  new
       Component:  Compiler                      |        Milestone:
      Resolution:                                |          Version:  7.7
Operating System:  Unknown/Multiple              |         Keywords:
 Type of failure:  Incorrect warning at          |     Architecture:
  compile-time                                   |  Unknown/Multiple
       Test Case:                                |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Comment (by scottgw):

 Maybe I am misunderstanding but it seems like backwards compatibility is
 already broken, twice:

 1. Backwards compatibility is already broken by changing `typeOf` to
 `typeRep` in Typeable. So anyone coming from 7.6 is already going to have
 a compile error on any manually defined instance.
 2. Ignoring that, backwards compatibility is already broken by ignoring
 manually defined instances.

 Making this an error just turns a 3 step fixing process (typeOf ->
 typeRep, discover complaints about missing instance scratch head, use
 deriving Typeable) into a 2 step one (typeOf -> typeRep, use deriving
 Typeable).

 The other solution is to not have it ignore the hand written instances,
 which would also bring it back down to even 1 step (typeOf -> typeRep).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8132#comment:6>
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