[GHC] #9420: Impredicative type instantiation without -XImpredicativeTypes

[GHC]

#9420: Impredicative type instantiation without -XImpredicativeTypes
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       Reporter:  goldfire           |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 Consider this module:

 {{{
 {-# LANGUAGE RankNTypes #-}

 module Bug where

 rank2 :: ((forall a. a -> a) -> b) -> ()
 rank2 _ = ()

 foo :: () -> ()
 foo x = x

 quux :: (forall a. a -> a) -> Int
 quux _ = 5

 bar = foo . rank2 $ quux
 }}}

 The `(.)` in the definition of `bar` requires an impredicative
 instantiation -- that is, one of its type variables is instantiated to a
 forall-type. Yet, the module compiles without `-XImpredicativeTypes`. To
 confirm this behavior, the following is an excerpt from `-ddump-simpl`:

 {{{
 Bug.bar =
   break<7>()
   (break<6>()
    GHC.Base..
      @ ()
      @ ()
      @ ((forall a_a1Tj. a_a1Tj -> a_a1Tj) -> GHC.Types.Int)
      Bug.foo
      (Bug.rank2 @ GHC.Types.Int))
     Bug.quux
 }}}

 Note the third type argument to `(.)`.

 Fixing this would be a breaking change that could affect users.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9420>
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