[GHC] #9223: Type equality makes type variable untouchable

[GHC]

#9223: Type equality makes type variable untouchable
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              Reporter:  Feuerbach   |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by porges):

 I ran into something similar to this with a type like:

     forall t. (forall m. (C m, m ~ t) => m) -> t

 The solution was to write instead:

     forall t. ((C t) => t) -> t

 Are there any cases in which these aren't equivalent? If not, why is GHC
 not able to reduce the first to the second?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9223#comment:11>
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