[GHC] #12788: While using Data.Aeson.TH, "Irrefutable pattern failed for pattern sel_id : _"

[GHC]

#12788: While using Data.Aeson.TH, "Irrefutable pattern failed for pattern sel_id :
_"
-------------------------------------+-------------------------------------
        Reporter:  jchia             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
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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@@ -3,1 +3,1 @@
-
+ {{{
@@ -15,0 +15,1 @@
+ }}}

New description:

 I got the following error compiling simple source file Bad.hs. The bug is
 likely related to TH as the crash vanishes without the TH line.
 {{{
 $ stack ghc -- -c Bad.hs
 Run from outside a project, using implicit global project config
 Using resolver: lts-7.5 from implicit global project's config file:
 /home/jchia/.stack/global-project/stack.yaml
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-linux):
         compiler/typecheck/TcExpr.hs:798:15-35: Irrefutable pattern failed
 for pattern sel_id : _


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

--

Comment (by simonpj):

 Thank you Ryan!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12788#comment:3>
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