[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 osa1):

 > but I assume the same code is used to actually generate the instances in
 those two cases, right?

 Right, the whole point is to make those work the same. Either way(e.g.
 empty case or error) works for me.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7401#comment:37>
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