[GHC] #9456: Weird behavior with polymorphic function involving existential quantification and GADTs

[GHC]

#9456: Weird behavior with polymorphic function involving existential
quantification and GADTs
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              Reporter:  haasn       |            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:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by haasn):

 It would be great if the error message could somehow mention that this
 type signature was less polymorphic than it could be, and that perhaps the
 user wants to add NoMonoLocalBinds or an explicit type signature.

 Is that a case that can be detected?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9456#comment:5>
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