[GHC] #10928: Refine pattern synonym signatures

[GHC]

#10928: Refine pattern synonym signatures
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        Reporter:  mpickering        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
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Comment (by simonpj):

 * I do agree about switching the order of provided and required
 constraints.  Let's just do that for GHC 8.0.  This isn't yet a widely-
 used feature, so lets' fix it asap.

 * We should definitely complain if the existentials are used by required
 constraints.  That's a bug to fix.

 * I'm much less certain about Richard's proposed syntax.  Not dead set
 against but we could get on with the first two while debating the third.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10928#comment:6>
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