[GHC] #15281: GHC 8.6.1 can't be bootstrapped with GHC 8.2.1

[GHC]

#15281: GHC 8.6.1 can't be bootstrapped with GHC 8.2.1
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           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:
-------------------------------------+-------------------------------------
 60e4bb4d305bc1a65457ee79b1e69c11b9ed747d (#9136) triggers a bug in GHC
 8.2.1 which causes the build to fail with,
 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.2.1 for x86_64-unknown-linux):
         runtimeRepPrimRep
   typePrimRep (r_aqbE :: TYPE rep_aqbD)
   rep_aqbD
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler/simplStg/RepType.hs:360:5 in
 ghc:RepType

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

 This is a manifestation of #14393, which is fixed in 8.2.2 and later.

 We should probably add a `configure` check warning about this brokenness
 when the user tries to bootstrap with GHC 8.2.1.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15281>
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