[GHC] #11514: Impredicativity is still sneaking in

[GHC]

#11514: Impredicativity is still sneaking in
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           Reporter:  goldfire       |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         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:
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 This is erroneously accepted:

 {{{
 {-# LANGUAGE RankNTypes #-}

 module Bug where

 foo :: forall a. (Show a => a -> a) -> ()
 foo = undefined
 }}}

 `undefined` is being instantiated at `(Show a => a -> a) -> ()`, which
 requires impredicativity. GHC does the right thing (reject!) if we try to
 instantiate with a type involving proper foralls.

 I imagine this is a simple missing check.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11514>
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