[GHC] #13766: Confusing "redundant pattern match" in 8.0, no warning at all in 8.2

[GHC]

#13766: Confusing "redundant pattern match" in 8.0, no warning at all in 8.2
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        Reporter:  edsko             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 This is the expected behaviour, the commit message explains the reasoning.

 {{{
 Previously when the checker encountered an unsatisfiable term of type
 context it would return an empty initial uncovered set. This caused all
 pattern matches in the context to be reported as redudant.

 This is arguably correct behaviour as they will never be reached but it
 is better to recover and provide accurate warnings for these cases to
 avoid error cascades. It would perhaps be better to report an error to
 the user about an inacessible branch but this is certainly better than
 many confusing redundant match warnings.
 }}}

 Warning that the RHS is inaccessible would be valuable but I don't think
 it's the domain of the pattern match checker to warn indirectly of this by
 warning of "redundant" clauses. So it looks like the tickets that Edward
 linked are more relevant.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13766#comment:5>
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