[GHC] #12187: Clarify the scoping of existentials for pattern synonym signatures

[GHC]

#12187: Clarify the scoping of existentials for pattern synonym signatures
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12108            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 And how do we reject

 {{{
 pattern Bad :: () => forall a. T a
 }}}

 Do we follow your proposed rules and then check to make sure that there
 are no existentials in the (expanded) return type?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12187#comment:6>
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