[GHC] #10715: Possible regression in Coercible a (X a) between 7.8 and 7.10

[GHC]

#10715: Possible regression in Coercible a (X a) between 7.8 and 7.10
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        Reporter:  inaki             |                   Owner:  goldfire
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by inaki):

 Replying to [comment:10 inaki]:
 > but it see no way of writing doCoerce in a way that makes ghc 7.10
 happy.
 Well, the "surprising"
 {{{#!hs
 doCoerce :: Coercible a (X b) => a -> X a
 doCoerce = coerce
 }}}
 definition works in 7.10, but this does not work in 7.8, which I also want
 to support in my code if at all possible.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10715#comment:12>
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