[GHC] #14288: ScopedTypeVariables with nested foralls broken since 8.0.2

[GHC]

#14288: ScopedTypeVariables with nested foralls broken since 8.0.2
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        Reporter:  MikolajKonarski   |                Owner:  RyanGlScott
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  GHC rejects       |  (amd64)
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14498            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I had been pushing for this change since I was convinced it was necessary
 to bring into scope the existentially quantified type variables in pattern
 synonym type signatures. But the discussion in #14498 has marched in the
 opposite direction (in particular, see
 https://ghc.haskell.org/trac/ghc/ticket/14498#comment:16). There, the
 consensus seems to be that we should //not// bring into scope these
 existentially quantified variables.

 In light of this, my original motivation for pursuing this ticket has
 disappeared, so I think we should go with the plan in comment:5 and
 document the current behavior.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14288#comment:22>
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