[GHC] #11319: ImpredicativeTypes even more broken than usual (was: ImpredicativeTypes cause trouble (affects deriving of Traversable))

[GHC]

#11319: ImpredicativeTypes even more broken than usual
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.11
  checker)                           |             Keywords:
      Resolution:                    |  ImpredicativeTypes
Operating System:  Linux             |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by rwbarton):

 * cc: goldfire (added)


Comment:

 Nothing special about deriving here, try:
 {{{
 {-# LANGUAGE ImpredicativeTypes #-}

 f :: Applicative f => f (Maybe a)
 f = pure Nothing

 main = return ()

 {-
 Error.hs:4:5: error:
     • Couldn't match type ‘forall a1. Maybe a1’ with ‘Maybe a’
       Expected type: f (Maybe a)
         Actual type: f (forall a. Maybe a)
     • In the expression: pure Nothing
       In an equation for ‘f’: f = pure Nothing
     • Relevant bindings include
         f :: f (Maybe a) (bound at Error.hs:4:1)
 -}
 }}}

 This is with an up-to-the-minute version of HEAD, that contains the
 relevant-looking Phab:1715.

 As `ImpredicativeTypes` is unsupported anyways, perhaps we should just
 take the opportunity to kill it?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11319#comment:2>
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