[GHC] #12943: Adjust AST to capture additional parens around a context

[GHC]

#12943: Adjust AST to capture additional parens around a context
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           Reporter:  alanz          |             Owner:  alanz
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #3384
Differential Rev(s):                 |         Wiki Page:
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 At the moment the parser strips redundant parens around a context when
 constructing the AST.  So the following two definitions would end up with
 the same AST.

 {{{#!hs
 foo  :: (Show a) => a -> String
 foo' :: ((Show a)) => a -> String
 }}}

 This causes roundtripping of an AST via `ppr` to fail, and adds complexity
 to the API Annotations.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12943>
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