[GHC] #14694: Can't coerce given assumptions

[GHC]

#14694: Can't coerce given assumptions
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      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 simonpj):

 Just to be clear about what Richard means when he says "incompleteness",
 it's this: there is a solution, we don't find it.

 Specifically we have
 {{{
 [G] (cat a b) ~R (a -> f b)
 [W]  cat a b  ~R (a -> WF f b)
 }}}
 Applying the `[G]` transitively with the `[W]` wanted, we'd get
 {{{
 [W] (a -> f b) ~R (a -> WF f b)
 }}}
 which we can certainly solve easily.  It's annoying that GHC's solver
 can't spot this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14694#comment:2>
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