[GHC] #15675: Type operators in existential context cannot be parsed

[GHC]

#15675: Type operators in existential context cannot be parsed
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           Reporter:  int-index      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.4.3
  (Parser)                           |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!haskell
 {-# LANGUAGE TypeOperators, MultiParamTypeClasses,
 ExistentialQuantification #-}

 class a + b

 data D1 = forall a b. (a + b) => D1 a b
 data D2 = forall a b.  a + b  => D2 a b
 }}}

 The declaration `D1` is accepted, while `D2` is rejected. There is no
 reason to reject `D2` except for shortcomings of the current grammar.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15675>
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