[GHC] #14813: EmptyCase thinks pattern match involving type family is not exhaustive, when it actually is

[GHC]

#14813: EmptyCase thinks pattern match involving type family is not exhaustive,
when it actually is
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5094
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Krzysztof Gogolewski <krz.gogolewski@…>):

 In [changeset:"e72d7880b940881d38b8c3db9a00d5d007b1458f/ghc" e72d788/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e72d7880b940881d38b8c3db9a00d5d007b1458f"
 Normalise EmptyCase types using the constraint solver

 Summary:
 Certain `EmptyCase` expressions were mistakently producing
 warnings since their types did not have as many type families reduced
 as they could have. The most direct way to fix this is to normalise
 these types initially using the constraint solver to solve for any
 local equalities that may be in scope.

 Test Plan: make test TEST=T14813

 Reviewers: simonpj, bgamari, goldfire

 Reviewed By: simonpj

 Subscribers: rwbarton, carter

 GHC Trac Issues: #14813

 Differential Revision: https://phabricator.haskell.org/D5094
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14813#comment:10>
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