#15639: Surprising failure combining QuantifiedConstraints with Coercible -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints 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 simonpj):
So I don't think the current behavior matches the user's guide.
That's probably true. The built-in rules for `Coercible` apply first. So we reduce `Coercible (Yeah a) (Yeah b)` to `Coercible a (Yeah b)` and thence to `Coercible a b`. All this before we even start to consider instances. It's a bit like type-class overlap. GHC doesn't have backtracking, and just picks one path. If you have overlap (as here, in this case between a quantified constraint and a built-in equality-decomposition rule) then GHC just picks one (in this case the built-in rule). The best thing is to avoid overlap. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15639#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler