[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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Comment (by simonpj):

 * A `CHoleCan` ''is'' a `Ct`!  (`CHoleCan` is a data constructor of `Ct`.)

 * How exactly do I build a CHoleCan?  Currently they are born in one
 place, in `TcExpr.tcUnboundId`:
 {{{
 tcUnboundId rn_expr unbound res_ty
  = do { ty <- newOpenFlexiTyVarTy  -- Allow Int# etc (Trac #12531)
       ; let occ = unboundVarOcc unbound
       ; name <- newSysName occ
       ; let ev = mkLocalId name ty
       ; loc <- getCtLocM HoleOrigin Nothing
       ; let can = CHoleCan { cc_ev = CtWanted { ctev_pred = ty
                                               , ctev_dest = EvVarDest ev
                                               , ctev_nosh = WDeriv
                                               , ctev_loc  = loc}
                            , cc_hole = ExprHole unbound }
 ...
 }}}
   Perhaps you can copy that?

 * Also, where exactly should I be calling tcNormalise?  Call it in
 `pmTopNormaliseType`. (It might need to become monadic to access the
 ambient constraints.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14813#comment:8>
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