[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:  RyanGlScott
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 An alternative would be to require an instance declaration for DAD, thus
 {{{
 instance C a => C (T a)
 }}}
 That's all that is required, provided `C` has suitable generic default
 methods and `T` is an instance of `Generic`.  And if DAC always used this
 route, there'd be less ambiguity in the `deriving` clause of a data type
 declaration.

 I think (but I am not sure) that we don't allow standalone deriving for
 DAC; thus
 {{{
 deriving instance C a => C (T a)
 }}}
 would not work for DAC.  Is that right?  It seems reasonable to disallow
 it, because it's one word longer than the ordinary instance declaration.
 And if it is disallowed, then builtin and GND are already treated
 differently to DAC.  So we'd get
  * GND/builtin: `deriving` clauses and `deriving instance` declarations
  * DAC: always a plain `instance` decl

 I know that's not backward compatible..

 On Ryan's synonym thing, like oerjan I'm uncomfortable with pressing
 synonyms into service like this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10598#comment:27>
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