[GHC] #14010: UndecidableSuperClasses - Could not deduce (Category d)

[GHC]

#14010: UndecidableSuperClasses - Could not deduce (Category d)
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        Reporter:  int-index         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  UndecidableSuperClasses
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by int-index):

 > Can we prove Category d given Category (c,d)? No we cannot.

 Yes, we can. The declaration of `Category` is `Super cat => Category cat`,
 so from `Category (c, d)` we can prove `Super (c, d)`, which reduces to
 `(Category c, Category d)`.


 Furthermore, if you study the original code that compiles, you can see
 that GHC manages to go via a similar route just fine.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14010#comment:3>
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