[GHC] #12957: In a record-update construct:ghc-stage2: panic! (the 'impossible' happened)

[GHC]

#12957: In a record-update construct:ghc-stage2: panic! (the 'impossible' happened)
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        Reporter:  niteria           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      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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Comment (by RyanGlScott):

 I think it very well might be the same bug, considering the form of the
 panic is identical to how niteria originally reported it:

 {{{
 $ ghci -fdefer-type-errors Bug.hsGHCi, version 8.0.2:
 http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:3:16: warning: [-Woverlapping-patterns]
     Pattern match is redundant
     In a case alternative: (_ : _) -> ...

 Bug.hs:3:25: warning: [-Wdeferred-out-of-scope-variables]
     Variable not in scope: a :: A

 Bug.hs:3:25: warning: [-Woverlapping-patterns]
     Pattern match is redundant
     In a record-update construct:ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-linux):
         unused
 }}}

 Also reproducible on GHC HEAD.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12957#comment:4>
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