[GHC] #10782: Existential variables not scoped?

[GHC]

#10782: Existential variables not scoped?
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              Reporter:  goldfire    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.11
  (Type checker)                     |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 I was looking into scoping of type variables, and it looks to me that
 type-checking the introduction of existentials (that is, checking a GADT
 pattern-match) doesn't extend the type environment. I doubt there's any
 way to tickle this bug. But the existential skolems don't seem to be added
 to the `tcl_tyvars` field of the `TcLclEnv`, which means that inner
 quantification, should uniques collide(!), might do the wrong thing in
 `quantifyTyVars`.

 To be clear, I don't wish the variables actually to scope in Haskell
 source, just for GHC to remember that the internal variables are in scope,
 to return from `tcGetGlobalTyVars`.

 Is my analysis wrong somewhere? Do we just assume that uniques being
 unique will take care of this?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10782>
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