#12144: GHC panic when using DeriveAnyClass with functor-like class and datatype with a type variable in a contravariant position -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Keywords: Generics | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code {{{#!hs {-# LANGUAGE DeriveAnyClass #-} {-# LANGUAGE KindSignatures #-} module Example where class C (a :: * -> *) data T a = MkT (a -> Int) deriving C }}} fails to compile with the following GHC panic: {{{ $ /opt/ghc/8.0.1/bin/ghc Example.hs [1 of 1] Compiling Example ( Example.hs, Example.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-linux): contravariant Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} The culprit is that when `DeriveAnyClass` comes up with the constraints for typeclass instances whose argument is of kind `* -> *`, it calls [http://git.haskell.org/ghc.git/blob/0676e68cf5fe8696f1f760fef0f35dba14db1104... deepSubtypesContaining], which immediately bails out when it sees a type variable in a contravariant or "bad" position. I'm less sure of what the fix should be. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12144 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler