#13990: Core Lint error on empty case -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- Comment (by nomeata): I believe the mistake is in `CoreLint` here. The question is: How can `CoreLint` know that `x_ap6` is both void and evaluated, hence cannot actually exist? Probably we just need to change the code {{{ 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) } }}} to something that runs the first check only when `scrut_diverges` is false (if `scrut_diverges`, then it cannot be in normal form). Or find out why `exprIsHNF scrut` is `True` here. Or maybe the outer `case` of {{{ case ds_dUn of { Foo x_ap6 -> case x_ap6 of { } } }}} should be empty, as the pattern `Foo x` is already absurd (due to the bang). Many options :-) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13990#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler