[GHC] #15839: DerivingStrategies defaulting warning has no associated enable/suppress flag

[GHC]

#15839: DerivingStrategies defaulting warning has no associated enable/suppress
flag
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           Reporter:  tejon          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When `DerivingStrategies`, `DeriveAnyClass` and
 `GeneralizedNewtypeDeriving` are enabled together, an instance that could
 be derived with either `newtype` or `anyclass` throws a warning when no
 strategy is specified. It appears to be impossible to suppress this
 warning. Even `-w` doesn't work!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15839>
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