[GHC] #14292: Coercing between constraints of newtypes

[GHC]

#14292: Coercing between constraints of newtypes
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by AntC):

 `IncoherentInstances` is a module-wide setting; and nowadays we're
 supposed to use the more surgical `{-# INCOHERENT #-}` pragma on the
 instance.

 Does that also apply for the role mungling?

 How does the `{-# INCOHERENT #-}` pragma go for `StandaloneDeriving` or
 `GeneralizedNewtypeDeriving`? It seems to be accepted by GHC OK, does it
 have the right effect? (It's hard to put together a test case.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14292#comment:3>
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