[GHC] #9840: Permit empty closed type families

[GHC]

#9840: Permit empty closed type families
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        Reporter:  adamgundry        |                   Owner:
            Type:  feature request   |                  Status:  patch
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.8.3
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #8028             |  Differential Revisions:  Phab:D841
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Comment (by Adam Gundry <adam@…>):

 In [changeset:"4efa421327cf127ebefde59b2eece693e37dc3c6/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="4efa421327cf127ebefde59b2eece693e37dc3c6"
 Permit empty closed type families

 Fixes #9840 and #10306, and includes an alternative resolution to #8028.
 This permits empty closed type families, and documents them in the user
 guide. It updates the Haddock submodule to support the API change.

 Test Plan: Added `indexed-types/should_compile/T9840` and updated
 `indexed-types/should_fail/ClosedFam4` and `th/T8028`.

 Reviewers: austin, simonpj, goldfire

 Reviewed By: goldfire

 Subscribers: bgamari, jstolarek, thomie, goldfire

 Differential Revision: https://phabricator.haskell.org/D841

 GHC Trac Issues: #9840, #10306
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9840#comment:9>
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