[GHC] #13990: Core Lint error on empty case

[GHC]

#13990: Core Lint error on empty case
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        Reporter:  mbieleck          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.2
       Component:  Compiler          |              Version:  8.2.1-rc3
      Resolution:                    |             Keywords:  core-lint
                                     |  case
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 >  Or find out why exprIsHNF scrut is True here.

 It's because in a case alternative
 {{{
    K x -> ...x...
 }}}
 where `K` is strict, we know that when we unpack `K x`, the field `x` must
 be evaluated.  So it is marked as such, and hence responds True to
 `exprIsHNF`.

 Joachim's solition is right; but it's very much a corner case and a bit
 hard to get right.  E.g.
 {{{
 data T a = T1 !(F a) Int | ...
 }}}
 When we pattern match on `T1 x y :: T X`, is that pattern impossible?
 WEll it depends on whether `(F X)` evaluates to an empty type.  Hard to be
 sure it is fully nailed.

 Better, I think, to say that
 {{{
 case x of {}
 }}}
 is fine if `x :: T` where `T` has no data constructors.   That would
 silence the warning and is obviously correct.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13990#comment:7>
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