[GHC] #16311: Suggest -XExistentialQuantification for 'forall' in data declarations

[GHC]

#16311: Suggest -XExistentialQuantification for 'forall' in data declarations
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        Reporter:  int-index         |                Owner:  (none)
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.7
  (Parser)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/363
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Changes (by int-index):

 * differential:   => https://gitlab.haskell.org/ghc/ghc/merge_requests/363
 * version:  8.6.3 => 8.7


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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16311#comment:5>
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