[GHC] #11355: TypeApplications + RankNTypes permits "impredicativity"

[GHC]

#11355: TypeApplications + RankNTypes permits "impredicativity"
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           Reporter:  adamgundry     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           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:
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 I was slightly surprised to discover that the combination of
 `TypeApplications` and `RankNTypes` allows impredicative instantiations,
 for example:
 {{{#!hs
 map @(forall a . a) :: ((forall a. a) -> b) -> [forall a. a] -> [b]
 }}}
 Is this desirable, or should it require `ImpredicativeTypes`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11355>
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