#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: patch Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Generics Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash | Test Case: Blocked By: | Blocking: Related Tickets: #5462, #7346, | Differential Rev(s): Phab:D2961 #12423 | Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott <ryan.gl.scott@…>): In [changeset:"639e702b6129f501c539b158b982ed8489e3d09c/ghc" 639e702/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="639e702b6129f501c539b158b982ed8489e3d09c" Refactor DeriveAnyClass's instance context inference Summary: Currently, `DeriveAnyClass` has two glaring flaws: * It only works on classes whose argument is of kind `*` or `* -> *` (#9821). * The way it infers constraints makes no sense. It basically co-opts the algorithms used to infer contexts for `Eq` (for `*`-kinded arguments) or `Functor` (for `(* -> *)`-kinded arguments). This tends to produce overly constrained instances, which in extreme cases can lead to legitimate things failing to typecheck (#12594). Or even worse, it can trigger GHC panics (#12144 and #12423). This completely reworks the way `DeriveAnyClass` infers constraints to fix these two issues. It now uses the type signatures of the derived class's methods to infer constraints (and to simplify them). A high-level description of how this works is included in the GHC users' guide, and more technical notes on what is going on can be found as comments (and a Note) in `TcDerivInfer`. Fixes #9821, #12144, #12423, #12594. Test Plan: ./validate Reviewers: dfeuer, goldfire, simonpj, austin, bgamari Subscribers: dfeuer, thomie Differential Revision: https://phabricator.haskell.org/D2961 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12144#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler