[GHC] #9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas

[GHC]

#9242: Implement {-# OVERLAPPABLE #-} and {-# INCOHERENT #-} pragmas
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              Reporter:  simonpj     |            Owner:  diatchki
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.2
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by diatchki):

 We can already mark individual instances as INCOHERENT.  For example, in
 `Data.Typeable.Internal` we have an instance like this:
 {{{
 instance {-# INCOHERENT #-} (Typeable f, Typeable a) => Typeable (f a)
 where ...
 }}}

 In the fourth bullet of the commit message (comment:16 above), I just
 meant that instances annotated with INCOHERENT work in the same way they
 did before.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:19>
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