[GHC] #14385: Clarify error message when missing GADTs extension

[GHC]

#14385: Clarify error message when missing GADTs extension
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           Reporter:  lyxia          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If we write a GADT declaration and forget to enable `GADTs` we get the
 following error message:

 {{{
 ww.hs:1:1: error:
     • Illegal generalised algebraic data declaration for ‘T’
         (Use GADTs to allow GADTs)
     • In the data declaration for ‘T’
 }}}

 It could be made more explicit for new users that the suggestion is to use
 the '''extension''' `GADTs`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14385>
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