[GHC] #9195: ImpredicativeTypes and ConstraintKinds don't interact as expected

[GHC]

#9195: ImpredicativeTypes and ConstraintKinds don't interact as expected
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       Reporter:            |             Owner:
  MikeIzbicki               |            Status:  new
           Type:  bug       |         Milestone:
       Priority:  normal    |           Version:  7.8.2
      Component:  Compiler  |  Operating System:  Unknown/Multiple
       Keywords:            |   Type of failure:  GHC rejects valid program
   Architecture:            |         Test Case:
  Unknown/Multiple          |          Blocking:
     Difficulty:  Unknown   |
     Blocked By:            |
Related Tickets:            |
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 Given this code:

 {{{

 {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ImpredicativeTypes,
 ConstraintKinds #-}

 class P a b
 instance P a b

 type Test a = forall b. P a b
 }}}

 I would expect to be able to actually use the Test Constraint in a
 function, like so:

 {{{
 foo :: Test a => a -> a
 foo = id
 }}}

 But GHC complains that:

 {{{
     Illegal polymorphic or qualified type: Test a
     In the type signature for ‘foo’: foo :: Test a => a -> a
 }}}

 I get the exact same error message if I replace ImpredicativeTypes with
 RankNTypes.  If this is intended behavior under ImpredicativeTypes, how
 difficult would it be to add another extension that allows this type of
 polymorphism?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9195>
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