#12957: In a record-update construct:ghc-stage2: panic! (the 'impossible' happened) -------------------------------------+------------------------------------- Reporter: niteria | Owner: mpickering Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3065, Wiki Page: | Phab:D3064 -------------------------------------+------------------------------------- Comment (by mpickering): In the original ticket, the record update gets desugared to something like {{{#!hs case emptyA of BFields _ -> BFields [a] _ -> error "Non-exhaustive patterns..." }}} GHC then warns that the first branch is inaccessible, which is true as the type of `emptyA` means that the value has to be either `EmptyFields` or `AFields`. At run-time, evaluating `f ()` causes a run-time error. So, is there anything here to fix apart from the panic? Usually warnings which affect incomplete pattern match warnings are only turned on by `-Wincomplete-record-updates`. This is however, a warning about inaccessibility/redundancy rather than incompleteness. What do you expect to happen niteria? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12957#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler