#8557: Panic "tcTyVarDetails" when :forcing a kind-polymorphic data family instance ------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- I have {{{ {-# LANGUAGE TypeFamilies, DataKinds, PolyKinds #-} data family Sing (a :: k) data instance Sing (a :: [k]) = SNil x :: Sing '[] x = SNil }}} When I load into GHCi and tried to force `x`, this happened: {{{ *Main> :force x ghc: panic! (the 'impossible' happened) (GHC version 7.7.20131031 for x86_64-apple-darwin): tcTyVarDetails k{tv a3Gv} [tv] }}} This was tested on the build from Oct 31 -- apologies if it has since been fixed. If I make the `[k]` in the instance declaration `k`, the file compiles fine. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8557 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler