[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)
-------------------------------------+-------------------------------------
           Reporter:  niteria        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 While compiling `Repro.hs` with `HEAD` I get this:
 {{{
 $ ~/local/ghc-inaccessible-right-side/inplace/bin/ghc-stage2 Repro.hs
 [1 of 1] Compiling Repro            ( Repro.hs, Repro.o )

 Repro.hs:22:35: warning: [-Woverlapping-patterns]
     Pattern match has inaccessible right hand side
     In a record-update construct:ghc-stage2: panic! (the 'impossible'
 happened)
   (GHC version 8.1.20161209 for x86_64-unknown-linux):
         unused

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

 }}}

 It doesn't reproduce on 7.10.2.
 I'm 90% sure it reproduces on 8.0.2 (I have a branch with some unrelated
 patches).

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