[GHC] #8749: Pattern synonyms crash GHCi

[GHC]

#8749: Pattern synonyms crash GHCi
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       Reporter:  monoidal          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 {{{
 ghci -XPatternSynonyms
 > let pattern P x = x
 }}}

 causes a panic. I understand pattern synonyms are currently unsupported in
 GHCi, but we should give an error message, not a panic.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8749>
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