[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
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           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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 The following program (reduced from Cabal code that uses HasCallStack)
 typechecks in GHC 8.0, but not on HEAD:

 {{{
 {-# 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.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12652>
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