[GHC] #10194: Shouldn't this require ImpredicativeTypes?

[GHC]

#10194: Shouldn't this require ImpredicativeTypes?
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        Reporter:  kosmikus          |                   Owner:
            Type:  bug               |                  Status:  merge
        Priority:  normal            |               Milestone:  7.10.2
       Component:  Compiler (Type    |                 Version:  7.8.4
  checker)                           |                Keywords:
      Resolution:  fixed             |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC accepts       |               Test Case:
  invalid program                    |  typecheck/should_fail/T10194
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by refold):

 Replying to [comment:10 adamgundry]:

 > One would expect some programs that were previously (erroneously)
 accepted to now require `-XImpredicativeTypes`.

 Yes, that's what seems to happen in the `xmonad-contrib` case:

 {{{

 XMonad/Layout/Groups/Helpers.hs:181:8:
     Cannot instantiate unification variable `a0'
     with a type involving foralls: G.ModifySpec
     Perhaps you want ImpredicativeTypes
     In the expression: sendMessage . G.Modify
     In an equation for `wrap': wrap = sendMessage . G.Modify

 XMonad/Layout/Groups/Helpers.hs:181:22:
     Cannot instantiate unification variable `a0'
     with a type involving foralls: G.ModifySpec
     Perhaps you want ImpredicativeTypes
     In the second argument of `(.)', namely `G.Modify'
     In the expression: sendMessage . G.Modify

 }}}

 > That's not necessarily a reason to revert this patch.

 The issue doesn't affect me personally, so I'm not going to argue. My
 comment was JFYI.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10194#comment:11>
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