[GHC] #10746: No non-exhaustive pattern match warning given for empty case analysis

[GHC]

#10746: No non-exhaustive pattern match warning given for empty case analysis
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7669, #11806     |  Differential Rev(s):  Phab:D2105
       Wiki Page:                    |
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Comment (by simonpj):

 This all seems a bit more complicated than it need be.  My guiding light
 is that it should all be very similar to `case x of { C y -> blah }`.  For
 this we enumerate all the data constructors other than `C y` as non-
 matching (modulo inaccessible GADT constructors).

 We can do the same here.  The only real difference is that we don't have a
 data constructor `C` to start from.  Instead we start from the type of `x
 :: tyx`.

 So it looks simple:

 * Normalise `x`'s type, to get it to the form `T ty1 .. tyn`. (I don't
 understand the "bounded" bit.)  For this, we must reduce type families,
 but NOT newtypes.  For pattern matching purposes, newtypes behave just
 like data types.  So use `FamInstEnv.normaliseType`.

 * If `T` has no data constructors, we are done.  For example, empty data
 types, which don't produce an error message here.

 * If none of `T`'s data constructors are GADTs, then just produce an error
 of form
 {{{
 Blah.hs:4:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In a case alternative: Patterns not matched: _ :: Bool
 }}}
   Here I've added the "`:: Bool`" part so the reader understands the type
 of the pattern.  No need to enumerate `True` and `False`.

 * If some of `T`'s data constructor are GADTs, then enumerate them all and
 recurse.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10746#comment:25>
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