[GHC] #12428: Allow pattern synonyms to optionally carry coerceability

[GHC]

#12428: Allow pattern synonyms to optionally carry coerceability
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      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 dfeuer):

 Replying to [comment:1 nomeata]:
 > Can you give a concrete examples? I’m not quite sure what you mean by
 “carry coerceability”, and how that is tied to pattern synonyms.

 Having a newtype constructor in scope in a module brings with it certain
 `Coercible` instances. There is no other way to bring those into scope
 globally in a module, although they can be used locally by matching on a
 `Coercion`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12428#comment:2>
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