[GHC] #15220: ScopedTypeVariables binds a non-existent variable

[GHC]

#15220: ScopedTypeVariables binds a non-existent variable
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.5
      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 nomeata):

 As a side node, there is need for `undefined` to trigger this:
 {{{
 f :: (forall a. (a -> (), a)) -> ()
 f (p :: (b -> (), b)) = fst p (snd p)
 }}}
 also has `p` instantiated with `Any`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15220#comment:13>
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