[GHC] #8773: Require -XIncoherentInstances to write role annotations on class definitions

[GHC]

#8773: Require -XIncoherentInstances to write role annotations on class
definitions
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       Reporter:  goldfire          |             Owner:  goldfire
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.1-rc1
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:  #8745             |
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 With a role annotation declaring a representational role for a class
 parameter, it is very easy to create instance incoherence. See
 [https://ghc.haskell.org/trac/ghc/ticket/8745#comment:5 this comment]. The
 solution: require `-XIncoherentInstances` when writing such a role
 annotation. This also has the knock-on effect (happily) of preventing such
 shenanigans in Safe Haskell.

 Class role annotations asking GHC to infer roles or with nominal roles,
 although rather useless, will still be accepted without
 `-XIncoherentInstances`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8773>
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