[GHC] #9900: Support pattern synonyms in GHCi

[GHC]

#9900: Support pattern synonyms in GHCi
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              Reporter:  cactus      |            Owner:  cactus
                  Type:  feature     |           Status:  new
  request                            |        Milestone:  7.10.1
              Priority:  normal      |          Version:  7.8.3
             Component:  GHCi        |         Keywords:  PatternSynonyms
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #8749, #7253
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by cactus):

 As part of this, I would need to add `pattern ` as a declaration-
 introducing prefix to `InteractiveUI.declPrefixes`, but it has no way of
 being conditional on whether `PatternSynonyms` is turned on (see #9915).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9900#comment:3>
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