[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
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        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):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * milestone:   => 8.8.1


Comment:

 Thanks, Simon! I'm afraid I'm not familiar enough with the solver
 interface in order to implement all of `tcNormalise` on my own, so I'm
 going to ask for more advice here. In particular:

 * `solveSimpleGivens` takes `[Ct]` as an arguments. How do I turn a
 `CHoleCan` into a `Ct`?
 * Moreover, how exactly do I build a `CHoleCan`? It takes a `CtEvidence`
 and a `Hole` as arguments. I have no idea what the `Hole` should be.
 Presumably, the `CtEvidence` should be a `CtWanted`, but that takes three
 other arguments in addition to `ctev_pred`, and I have no idea what any of
 them should be.
 * Also, where exactly should I be calling `tcNormalise`? In the pattern-
 match coverage checker? Somewhere else? (I wonder if it's possible to fix
 the buglet in the first program in comment:2 as well, which would seem to
 suggest we should be calling it somewhere //besides// the coverage
 checker.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14813#comment:7>
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