[GHC] #11350: Allow visible type application in patterns

[GHC]

#11350: Allow visible type application in patterns
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Notes, similar to [https://hackage.haskell.org/package/base-4.9.0.0/docs
 /GHC-OverloadedLabels.html GHC.OverloadedLabels.IsLabel]:

 {{{#!hs
 class IsLeibel (str::Symbol) a where
   fromLeibel :: a
 }}}

 would the following work

 {{{#!hs
 instance forall x r a. HasField x r a => IsLeibel x (r -> a) where
   fromLeibel :: r -> a
   fromLeibel @x = getField @x
 }}}

 Does `fromLeibel @x` bring `x` into scope, shadowing `instance forall x r
 a.`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11350#comment:8>
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