[GHC] #12583: Deriving standalone Ix instance for GADT leads to GHC panic

[GHC]

#12583: Deriving standalone Ix instance for GADT leads to GHC panic
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2521
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"0e7ccf6d233c66b23a60de4e35e039f78ea3e162/ghc" 0e7ccf6/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="0e7ccf6d233c66b23a60de4e35e039f78ea3e162"
 Fix TH ppr output for list comprehensions with only one Stmt

 A folow-up to D2521 (which addressed #12583), where the `Outputable` `ppr`
 output was tweaked to display a list comprehension with only one `Stmt` as
 `[Foo]` instead of `[Foo|]` (which isn't valid Haskell). I forgot to
 update
 the corresponding code in `Language.Haskell.TH.Ppr` which pretty-prints
 `CompE`, however, so this commit takes care of that.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12583#comment:3>
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