#12787: Weird type constraint with undecidable instances -------------------------------------+------------------------------------- Reporter: nome | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: | UndecidableInstances Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): This looks like correct behavior to me. There is one, universal instance of `PartialOrd`. As soon as GHC needs the `PartialOrd` constraint to accept the use of `tryCompare`, it will find this instance. But the instance context requires an instance for `TotalOrd`, which does not exist. How would you expect this to be accepted? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12787#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler