[GHC] #7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so.

[GHC]

#7401: Can't derive instance for Eq when datatype has no constructor, while it is
trivial do do so.
-------------------------------------+-------------------------------------
        Reporter:  jpbernardy        |                   Owner:  osa1
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.6.1
      Resolution:                    |                Keywords:  deriving,
Operating System:  Unknown/Multiple  |  newcomer
 Type of failure:  GHC rejects       |            Architecture:
  valid program                      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D978
-------------------------------------+-------------------------------------

Comment (by rwbarton):

 I agree with jpbernardy that generating an empty case on (or equivalently
 forcing) the first argument would be more correct than producing a new
 error. However, it seems that in this case the "blame" could be assigned
 to jpbernardy, who wrote the original report with `instance Eq D where
 (==) = undefined` :)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7401#comment:34>
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