[GHC] #8581: Pattern synonym used in an expression context could have different constraints to pattern used in a pattern context

[GHC]

#8581: Pattern synonym used in an expression context could have different
constraints to pattern used in a pattern context
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        Reporter:  cactus            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
                                     |  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 mpickering):

 I've looked a bit now. It seems like the correct thing to do is
 disassemble the `PatSyns` so that the builder/matcher can be handled
 separately. The way to do this seems to be to modify `ValBindsOut` to use
 a new datatype instead of `HsBinds`, split apart the pat syn when
 calculating sccs and then stitch it back together after typechecking if
 that is still necessary.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:32>
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