[GHC] #11524: Something is amiss with quantification in pattern synonym type signatures

[GHC]

#11524: Something is amiss with quantification in pattern synonym type signatures
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by niteria):

 Better trace with `ghc-stage2`:

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.1.20160128 for x86_64-unknown-linux):
         ASSERT failed!
   CallStack (from ImplicitParams):
   assertPprPanic, called at compiler/types/TyCoRep.hs:1932:51 in
 ghc:TyCoRep
   checkValidSubst, called at compiler/types/TyCoRep.hs:1981:17 in
 ghc:TyCoRep
   substTys, called at compiler/types/TyCoRep.hs:2002:14 in ghc:TyCoRep
   substTheta, called at compiler/typecheck/TcPatSyn.hs:254:57 in
 ghc:TcPatSyn
   in_scope InScope {k_aB3 k_aB8 a_aB9 b_aBa}
   tenv [auD :-> a_aB9[tau:5], auE :-> b_aBa[tau:5],
         aB2 :-> k_aB8[tau:5]]
   cenv []
   tys [fun_auC[sk] ~ a_auD[sk] b_auE[sk]]
   cos []
   needInScope [auC :-> fun_auC[sk], aB3 :-> k_aB3[tau:1]]
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11524#comment:3>
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