#12814: Should GND infer an instance context when deriving method-free classes? -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11369, #12810 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): It's really not simple... one instance may need other instances, so we need to solved simultaneous equations; see `Note [Simplifying the instance context]` in `TcDerivInfer`. Also we aren't really special-casing the 0-method case. For each method we need 1. `C rep-ty (same for each method) 2. A coercible constraint (different for each method) Instead of generating N copies of (1) we generate just 1. But if N=0 we can generate zero of them. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12814#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler