[GHC] #15434: DerivingVia (and perhaps even GND) works badly with DeriveGeneric

[GHC]

#15434: DerivingVia (and perhaps even GND) works badly with DeriveGeneric
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        Reporter:  konn              |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:  deriving
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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Changes (by RyanGlScott):

 * keywords:   => deriving


Comment:

 I'm not sure what the bug is here. You're explicitly requesting that you
 reuse the `Generic` instance for `Bad Foo`, and lo and behold, GHC gives
 you it. If you don't like this behavior, don't combine `DerivingVia` with
 `Generic`—the former wasn't meant to be combined with the latter (indeed,
 this code will give an error if combined with the `Safe` extension).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15434#comment:1>
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