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

[GHC]

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

 The interaction between a hypothetical `-XNoOrphanInstances`, Safety, and
 overlapping instances is subtle -- the flags have to be put  potentially
 on different modules to get the Safety requirement to work out. For this
 reason and in order to avoid blunt instruments, I favor also deprecating
 !OverlappingInstances.

 For the bulk of you surprised by this mysterious `-XNoOrphanInstances`,
 it's an idea that Edward Kmett, Gershom Bazerman, and I cooked up a few
 months ago. More information in due course!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9242#comment:10>
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