[GHC] #15798: Flag to warn when deriving strategy is not explicitly specified

[GHC]

#15798: Flag to warn when deriving strategy is not explicitly specified
-------------------------------------+-------------------------------------
           Reporter:  andrewthad     |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           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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 In my code, I aim to always specify deriving strategies. I would like to
 add a flag `-fwarn-unspecified-deriving` that would cause a warning to be
 emitted when someone wrote this:

 {{{
 newtype Foo = Foo Int
   deriving (Show)
 }}}

 The user would be required to instead write:

 {{{
 newtype Foo = Foo Int
   deriving newtype (Show)
 }}}

 Or they could use `stock` if that was the behavior they wanted. This flag
 would be off by default.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15798>
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