[GHC] #12411: GHC panic on TypeApplications + TemplateHaskell

[GHC]

#12411: GHC panic on TypeApplications + TemplateHaskell
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications, TemplateHaskell
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Shayan-Najd):

 As RyanGlScott pointed out in #12584, essentially the problem is with
 patterns (here "as" pattern) being accepted (temporarily in the renamer)
 as expressions. Here is a minimal way to trigger the same error:
 {{{
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -XTemplateHaskell
 Prelude> pure @Q [];return ()
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-linux):
         tcMonoExpr _

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12411#comment:4>
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