#13297: Panic when deriving Applicative instance through transformer -------------------------------------+------------------------------------- Reporter: blaze | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I'm not sure if code below is valid, but panic is wrong anyway. {{{#!hs {-# Language TypeFamilies, StandaloneDeriving, GeneralizedNewtypeDeriving, UndecidableInstances #-} module Example where newtype N p m a = N (((CT p) m) a) deriving instance (CT p ~ f, Functor (f m)) => Functor (N p m) deriving instance (CT p ~ f, Applicative (f m)) => Applicative (N p m) -- panic when this line added class C p where type CT p :: (* -> *) -> * -> * }}} Compiler output for 8.0.1 and 8.0.2 is slightly different: {{{ test.hs:6:1: error:ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-linux): No skolem info: f1_auk[sk] }}} {{{ test.hs:6:1: error:ghc: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-linux): No skolem info: f1_aun[sk] }}} If I try to derive only Functor instance, code compiles just fine. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13297 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler