[GHC] #4347: Bug in unification of polymorphic and not-yet-polymorphic type

[GHC]

#4347: Bug in unification of polymorphic and not-yet-polymorphic type
-------------------------------------+-------------------------------------
              Reporter:  dolio       |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.1
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  x86_64 (amd64)
      Operating System:  Linux       |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Changes (by thomie):

 * difficulty:   => Unknown


Comment:

 With HEAD, the example from the description works (fails with 7.8.3). I
 don't know if this is incidental, or if anything got really fixed?

 {{{
 $ ghci -XRankNTypes
 GHCi, version 7.9.20141125: ...
 Prelude> let f = id :: (forall a. a) -> (forall b. b)
 Prelude>
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4347#comment:14>
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