[GHC] #15878: Unused data type with a "deriving" clause is falsely considered used

[GHC]

#15878: Unused data type with a "deriving" clause is falsely considered used
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           Reporter:  EyalLotem      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 module M () where data Foo = Bar deriving (Eq)

 If there are no explicit references to Foo and none to Bar, the data-type
 is necessarily unused, so I expect a warning about an unused data-type.

 Even if it is referenced, but only from instance declarations, it is
 necessarily unused in the program.

 One potential exception is functional dependencies, where the instance
 declaration itself has an effect. But even then, perhaps it is worth
 warning that the *type* is unused.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15878>
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