#15621: Error message involving type families points to wrong location -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.4.3 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I agree that the fix for #14185 will fix this precise example, but will it fix all such examples? What if there is a `Refl :: F Int :~: F Int` and a `Refl :: F Int :~: F Bool` in the same function? Then we'd be right back here. So I think #14185 is a bit of a red herring here. (I also think that fix might not fix all cases of #14185 itself.) With the way deriveds currently work, the "wanteds don't rewrite wanteds" story applies really only to skolems (which can't be "improved" through deriveds). Here, we want it also to apply to unification variables. Yet we also want to keep the work that deriveds do in other cases. I'm out of time now, but I don't think this will be so easy, somehow... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15621#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler