[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:  patch
        Priority:  highest           |            Milestone:  8.2.3
       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):  Phab:D4160
       Wiki Page:                    |
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Comment (by simonpj):

 > But should we leave the scrut_diverges warning in place?

 You are referring to this code in the `Case` equation for `lintCoreExpr`:
 {{{
     -- See Note [No alternatives lint check]
      ; when (null alts) $
      do { checkL (not (exprIsHNF scrut))
           (text "No alternatives for a case scrutinee in head-normal
 form:" <+> ppr scrut)
         ; checkWarnL scrut_diverges
           (text "No alternatives for a case scrutinee not known to diverge
 for sure:" <+> ppr scrut)
         }
 }}}
 Well, suppose `x :: T Bool`, where `T` is in comment:8. Then if I say
 something like
 {{{
    case x of {}
 }}}
 then `x` is not sure to diverge but the case branches are legitimately
 empty.  Maybe you can add an example like this in a `Note` explaining why
 there is no null-alts check here?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13990#comment:16>
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