[GHC] #12975: Suggested type signature for a pattern synonym causes program to fail to type check

[GHC]

#12975: Suggested type signature for a pattern synonym causes program to fail to
type check
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        Reporter:  ocharles          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
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 ocharles):

 If I only supply one context, which context am I supplying? The provided
 one, or the required one? That is, are you suggesting `pattern Cast :: ()
 => Typeable b => ...` or `pattern Cast :: Typeable b => () => ...`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12975#comment:3>
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