[GHC] #8132: Warning for Typeable instances misplaced

[GHC]

#8132: Warning for Typeable instances misplaced
-------------------------+-------------------------------------------------
       Reporter:         |             Owner:
  scottgw                |            Status:  new
           Type:  bug    |         Milestone:
       Priority:         |           Version:  7.7
  normal                 |  Operating System:  Unknown/Multiple
      Component:         |   Type of failure:  Incorrect warning at
  Compiler               |  compile-time
       Keywords:         |         Test Case:
   Architecture:         |          Blocking:
  Unknown/Multiple       |
     Difficulty:         |
  Unknown                |
     Blocked By:         |
Related Tickets:         |
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 There is currently a warning for hand-rolled instances of Typeable.
 However, this leads to some pretty surprising error messages: because the
 instance is ignored any code that relies on that instance in the same
 module just fails, which appears pretty weird to the user.

 {{{
 #!haskell
 import Data.Typeable

 data K = K

 instance Typeable K where
     typeRep _ = undefined

 ex :: TypeRep
 ex = typeRep (undefined :: Proxy K)
 }}}

 Gives as an error:

 {{{
     No instance for (Typeable * K) arising from a use of ‛typeRep’
     In the expression: typeRep (undefined :: Proxy K)
     In an equation for ‛ex’: ex = typeRep (undefined :: Proxy K)
 }}}

 Which is of course surprising because the user just defined the instance.

 After commenting out the code that uses the instance, one gets:

 {{{
 TypEx.hs:1:1: Warning:
     Typeable instances can only be derived; ignoring the following
 instance:
       instance Typeable * K -- Defined at TypEx.hs:5:10
 }}}

 This information should clearly be somehow presented before or with the
 previous error if the user is to have any way to figure out what is going
 on.

 I suppose this could be fixed by:

 1. Turning the Typeable instance warning into an error
 2. Checking the missing instance in the existing error for Typeable and
 suggest that the user should look for hand written instances as a source
 of the problem.

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