[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):

 Replying to [comment:5 AntC]:
 > If the module has `IncoherentInstances`, do both those instances get
 accepted even without the pragmas? (Or perhaps a more devious scenario
 where one of the instances is imported.)
 >

 Yes, here we go:
 {{{
 {-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving,
 FlexibleInstances,
                           IncoherentInstances  #-}

 main = print $ MkN (7 :: Int) + MkN 5

 newtype N a = MkN a   deriving (Show)

 instance Num a => Num (N a) where
   MkN x + MkN y = MkN (x - y)

 deriving instance Num (N Int)
 }}}

 Without `IncoherentInstances` the `MkN 7 + MkN 5` is rejected: overlapping
 instances [good! that's what I want].

 With `IncoherentInstances`, this is accepted and prints `12`. If I switch
 round the instance decls such that the hand-crafted one is for `Num (N
 Int)` and the derived is for `Num (N a)`, this prints `2`.

 Ugh!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14292#comment:6>
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