[GHC] #12563: Bad error message around lack of impredicativity

[GHC]

#12563: Bad error message around lack of impredicativity
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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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Comment (by RyanGlScott):

 This appears to be fixed, as of GHC 8.2.2:

 {{{
 $ /opt/ghc/8.2.2/bin/ghci Bug.hs -XRankNTypes
 GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:4:15: error:
     • Cannot instantiate unification variable ‘p0’
       with a type involving foralls: (forall a. f0 a) -> f0 r0
         GHC doesn't yet support impredicative polymorphism
     • In the first argument of ‘foo’, namely ‘g’
       In the expression: foo g
       In the expression: \ g -> foo g
     • Relevant bindings include
         g :: p0 (bound at Bug.hs:4:6)
         x :: p0 -> f0 r0 (bound at Bug.hs:4:1)
   |
 4 | x = \g -> foo g
   |               ^
 }}}

 I'll add a regression test.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12563#comment:1>
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