[GHC] #12131: Can't solve constraints with UndecidableSuperClasses but can infer kind (+ undesired order of kinds)

[GHC]

#12131: Can't solve constraints with UndecidableSuperClasses but can infer kind (+
undesired order of kinds)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  UndecidableSuperClasses
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11480 #12025     |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 `Product` doesn't make ghc spin, but oddly `j` appears before `i` even
 though `Cat i` appears before `Cat j`, etc.

 {{{
 ghci> :t Product
 Product
   :: forall j i (p :: i -> i -> *) (q :: j -> j -> *) (a :: (i,
                                                              j)) (b :: (i,
 j)).
      p (Fst a) (Fst b) -> q (Snd a) (Snd b) -> Product p q a b
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12131#comment:2>
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