[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:  (none)
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.1
      Resolution:                    |             Keywords:  deriving,
                                     |  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10577, #13117    |  Differential Rev(s):  Phab:D4047
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"1317ba625d40fbd51cb0538b3fde28f412f30c01/ghc"
 1317ba62/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1317ba625d40fbd51cb0538b3fde28f412f30c01"
 Implement the EmptyDataDeriving proposal

 This implements the `EmptyDataDeriving` proposal put forth in
 https://github.com/ghc-proposals/ghc-
 proposals/blob/dbf51608/proposals/0006-deriving-empty.rst.
 This has two major changes:

 * The introduction of an `EmptyDataDeriving` extension, which
   permits directly deriving `Eq`, `Ord`, `Read`, and `Show` instances
   for empty data types.
 * An overhaul in the code that is emitted in derived instances for
   empty data types. To see an overview of the changes brought forth,
   refer to the changes to the 8.4.1 release notes.

 Test Plan: ./validate

 Reviewers: bgamari, dfeuer, austin, hvr, goldfire

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #7401, #10577, #13117

 Differential Revision: https://phabricator.haskell.org/D4047
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7401#comment:53>
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