[GHC] #10598: DeriveAnyClass and GND don't work well together

[GHC]

#10598: DeriveAnyClass and GND don't work well together
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        Reporter:  osa1              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by oerjan):

 Hm so here is my "ideal" made-up-on-the-spot system.  It somewhat combines
 all of 2, 3 and 4 from
 https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/GenericDeriving#InteractionwithGeneralizedNewtypeDeriving.
 It is intended to be backwards-compatible, except for one added warning.

 * By 3, classes may be annotated (pragma?) to say they prefer GND or DAC
 deriving. Builtin-derived classes count as annotated for their own style
 of deriving. To actually derive a class in a module, any extension for the
 derivation style still needs to be enabled as well.
 * By 4, if `GeneralizedNewtypeDeriving` is enabled, the `newtype` keyword
 may be used to signify that an instance for a newtype should be GND
 derived, even if this is against the annotated behavior for the class.
 This might even include builtin-derived classes like `Show`. (Obviously
 not `Typeable`, though.)
 * Also by 4, if both `GeneralizedNewtypeDeriving` and `DeriveAnyClass` are
 enabled (or for least surprise, maybe even with just the latter), the
 `default` keyword may be used to signify that an instance for a newtype
 should ''not'' be GND-derived, even if this is the annotated behavior for
 the class.
 * By 2, if ''neither'' the newtype deriving nor the class is annotated,
 then the behavior depends on which of `GeneralizedNewtypeDeriving` and
 `DeriveAnyClass` is enabled. If ''both'' are enabled, then a ''warning''
 should be given. (This prevents surprises when a user adds both extensions
 for unrelated instances.) Then it defaults to DAC as today.
 * Although the proper extensions need to be enabled for whichever
 annotations/derivation styles end up being used, the ''only'' case where
 simply changing the extensions enabled will change code from one legal
 style of derivation to another should be the one in the previous point.
 (And thus the warning.)

 As of now, I don't remember any classes with builtin-derivations that also
 are useful with `DeriveAnyClass`. So I think there isn't much need to be
 able to distinguish those two cases. Which also means that none of this
 matters to `data` declarations, only `newtype`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10598#comment:19>
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