[GHC] #11961: PatBind test suite failure with `-DDEBUG`

[GHC]

#11961: PatBind test suite failure with `-DDEBUG`
-------------------------------------+-------------------------------------
           Reporter:  nomeata        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.1
           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:
-------------------------------------+-------------------------------------
 I get
 {{{
 Compile failed (status 256) errors were:
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.1.20160420 for x86_64-unknown-linux):
         ASSERT failed!
   CallStack (from HasCallStack):
   assertPprPanic, called at compiler/types/TyCoRep.hs:2082:56 in
 ghc:TyCoRep
   checkValidSubst, called at compiler/types/TyCoRep.hs:2118:17 in
 ghc:TyCoRep
   substTy, called at compiler/types/TyCoRep.hs:2060:3 in ghc:TyCoRep
   in_scope InScope {a_apN t_apW a_apX}
   tenv [apC :-> t_apW[tau:3], apN :-> a_apX[tau:3]]
   tenvFVs [apD :-> t_apD[tau:3], apW :-> t_apW[tau:3],
            apX :-> a_apX[tau:3]]
   cenv []
   cenvFVs []
   tys [a_apN[sk] -> a_apN[sk]]
   cos []

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


 *** unexpected failure for PatBind(normal)
 }}}
 with a `-DDEBUG` build, e.g. on travis: https://s3.amazonaws.com/archive
 .travis-ci.org/jobs/124426776/log.txt

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11961>
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