[GHC] #8264: Canonical way to prevent manual instances

[GHC]

#8264: Canonical way to prevent manual instances
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       Reporter:  nomeata                  |             Owner:
           Type:  task                     |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.6.3
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 For some classes (Typeable) the user must not provide hand-written
 instances, for other no hand-written instances in Safe mode (old Typeable)
 and for others no instances at all (Coercible). Currently the Typeable
 checks are in tcInstDecls1 in [source:compiler/typecheck/TcInstDcls.lhs];
 but a more proper place would be in checkValidInstHead in
 [source:compiler/typecheck/TcValidity.lhs].

 Why file a ticket and not just do it? The latter source location does not
 know whether the instance is a (stand-alone) deriving instance or a hand-
 written one, so this needs a bit of refactoring.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8264>
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