[GHC] #14386: GHC doesn't allow Coercion between partly-saturated type constructors

[GHC]

#14386: GHC doesn't allow Coercion between partly-saturated type constructors
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:  roles
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 nomeata):

 Maybe not quite intentional, but certainly expected. I think the coercion
 you want is not even expressible at the level of Core, because the newtype
 gives you the axiom

 {{{
 forall cat a b. Op cat a b ~R cat b a
 }}}

 And as you can see you can only use this axiom once `Op` has all three
 arguments.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14386#comment:1>
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