[GHC] #12652: Type checker no longer accepting code using function composition and rank-n types

[GHC]

#12652: Type checker no longer accepting code using function composition and rank-n
types
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        Reporter:  ezyang            |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by bgamari):

 * owner:   => simonpj


@@ -4,1 +4,1 @@
- {{{
+ {{{#!hs

New description:

 The following program (reduced from Cabal code that uses HasCallStack)
 typechecks in GHC 8.0, but not on HEAD:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ImplicitParams #-}
 module Foo where

 type T a = (?x :: Int) => a
 type M a = T (IO a)

 f :: T (T a -> a)
 f x = x

 g :: Int -> M ()
 g = undefined

 h :: Int -> M ()
 -- h x = f (g x) -- works on HEAD
 h = f . g -- fails on HEAD, works on GHC 8.0
 }}}

 It's possible that this is just fall out from the recent impredicativity
 changes but I just wanted to make sure that this was on purpose.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12652#comment:2>
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