#14323: Occurs check regressions in GHC 8.2.1 (and HEAD) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Some reactions to all this: * comment:3 talks about parking `Coercible` constraints in the insolubles. I hope we don't -- they're not insoluble. I imagine you mean that they're parked in the "irreducibles", which is more sensible. * The representational equality solver is known to be incomplete. I believe the problem is simply undecidable. Perhaps this could be better documented, but its incompleteness is precisely in recursive situations. * I agree that the reason for occurs-checks isn't about solubility, but rather about keeping the solver from going into gratuitous loops. Bottom line: I don't have any better ideas. I doubt we can make the `Coercible` solver more complete without gross hacks. And, absent a Real Use Case, I'm not keen on trying. I have not digested all of #12466 about nominal equalities, which I tend to think are more worrisome. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14323#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler