#8390: regression in handling of type variables in constraints on instances which do not appear in the instance head ----------------------------------------------+---------------------------- Reporter: aavogt | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by goldfire): No, not quite. I think that, given that uses of the `Compose` instance work just fine without extra annotations, then that means that the instance is not ambiguous, tautologically by my operating definition of "ambiguous". So, I think we need to choose between these two design alternatives: 1. Permit the instance for `Compose` without `-XAllowAmbiguousTypes`. 2. Admit that the ambiguity checker is over-eager and sometimes rejects code that is unambiguous. Or, I suppose 3. Define ambiguous differently. It's possible that I'm very confused about something here, so if that's the case, please try to enlighten me! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8390#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler