[GHC] #8757: Disallow local pattern synonym declarations

[GHC]

#8757: Disallow local pattern synonym declarations
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       Reporter:  cactus             |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler (Type     |           Version:  7.8.1-rc1
  checker)                           |  Operating System:  Unknown/Multiple
       Keywords:                     |   Type of failure:  GHC accepts
   Architecture:  Unknown/Multiple   |  invalid program
     Difficulty:  Easy (less than 1  |         Test Case:
  hour)                              |          Blocking:
     Blocked By:                     |
Related Tickets:                     |
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 Locally-defined pattern synonyms work by accident in GHC and crash GHCi
 (see #8749), but certainly not by design. It can be confusing because it
 introduces shadowing to constructor-like names.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8757>
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