#13674: GHC doesn't discharge heterogeneous equality constraint when it ought to -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): The original program is wrong, but the error message also is wrong. Because `Lcm` is a nullary type family, the constraint `m ~ Lcm m m` is morally equivalent to `m ~ Either m m`. This constraint is an occurs-check failure and is surely going to lead to an error. It seems that the error- reporting code is mischaracterizing it, though. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13674#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler