[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:
       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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Comment (by simonpj):

 OK I undersatand now. My question in comment:3 was after reading
 comment:2.   But comment:1 makes it clear why we want to improve
 `inhabitationCandidates`.

 Essentially you want to normalise a type using both top-level instances
 ''and'' local equalities.  The constraint solver does this all the time,
 so the Right Thing is probably just to expose a new API to the constraint
 solver, alongside `tcCheckSatisfiability`.  Something like
 {{{
 tcNormalise :: Bag EvVar -> Type -> TcM Type
 }}}
 How would it work? A bit like `tcCheckSatisfiability`, except that after
 the `solveGivens` call `solveWanteds` passing a single `CHoleCan`
 constraint.  It is not "solved", but it ''is'' normalised.  So the
 `solveWanteds` will return a `CHoleCan` whose type is the desired
 normalised one.

 For efficiency, you might want to do a quick check before invoking
 `tcNormalise`, in case the type is ''already'' an algebraic data type (a
 common case), in which case normalisation will be a no-op.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14813#comment:6>
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