[GHC] #10746: No non-exhaustive pattern match warning given for empty case analysis

[GHC]

#10746: No non-exhaustive pattern match warning given for empty case analysis
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7669, #11806     |  Differential Rev(s):  Phab:D2105
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"b10353216f5ff5d5e410334e4c118b6695b628d0/ghc" b103532/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="b10353216f5ff5d5e410334e4c118b6695b628d0"
 Exhaustiveness check for EmptyCase (Trac #10746)

 Empty case expressions have strict semantics so the problem boils down
 to inhabitation checking for the type of the scrutinee. 3 main functions
 added:

 - pmTopNormaliseType_maybe for the normalisation of the scrutinee type

 - inhabitationCandidates for generating the possible patterns of the
   appropriate type

 - checkEmptyCase' to filter out the candidates that give rise to
   unsatisfiable constraints.

 See Note [Checking EmptyCase Expressions] in Check
 and Note [Type normalisation for EmptyCase] in FamInstEnv

 Test Plan: validate

 Reviewers: simonpj, goldfire, dfeuer, austin, bgamari

 Reviewed By: bgamari

 Subscribers: mpickering, thomie

 Differential Revision: https://phabricator.haskell.org/D2105

 GHC Trac Issues: #10746
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10746#comment:28>
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