[GHC] #13430: Can't scope type variables when pattern matching on GADTs

[GHC]

#13430: Can't scope type variables when pattern matching on GADTs
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        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
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 crockeea):

 > the type signature is matched before we get to the A part

 That may be the case, but it's quite strange that I wrote `A :: Foo Bool`
 in the GADT definition, but I can't write `A :: Foo Bool` in the pattern.
 `A` certainly does have that type. I think the notation is natural enough
 that it should work.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13430#comment:3>
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