[GHC] #12949: Pattern coverage mistake

[GHC]

#12949: Pattern coverage mistake
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        Reporter:  dfeuer            |                Owner:  gkaracha
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      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):
       Wiki Page:                    |
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Comment (by gkaracha):

 Replying to [comment:1 dfeuer]:
 > Note that everything works as expected if I pass in the `Boolly` values
 explicitly. Perhaps the pattern coverage checker is confusing `knownBool`
 called at different types?

 Indeed you are right, this is not a bug in the type constraints that the
 checker generates but the term constraints. I expected the `knownBool`s to
 be already desugared to `(knownBool $dKnownBool_a)` and `(knownBool
 $dKnownBool_b)` since they are overloaded but I was mistaken. Hence, it
 now (wrongly) generates `{knownBool ~ Falsey, knownBool ~ Truey}` which is
 of course unsatisfiable.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12949#comment:6>
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