#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 dfeuer): Let's go through this [http://downloads.haskell.org/~ghc/8.6.2/docs/html/users_guide/glasgow_exts.h... #instance-lookup according to the user's guide]:
In the light of the overlap decision, instance lookup works like this when trying to solve a class constraint `C t`
1. First see if there is a given un-quantified constraint `C t`. If so, use it to solve the constraint. 2. If not, look at all the available given quantified constraints; if exactly one one matches `C t`, choose it; if more than one matches, report an error. 3. If no quantified constraints match, look up in the global instances, as described in ...
When type checking the use of the `Coercion` constructor in `yeah`, we want `Coercible [Yeah a] [Yeah b]`. There are no given constraints (quantified or otherwise) matching that, so we use the global instance `Coercible x y => Coercible [x] [y]`, producing the wanted `Coercible (Yeah a) (Yeah b)`. We have no matching unquantified givens, so we look for quantified givens. We (should) have a match: `forall a b. Coercible (Yeah a) (Yeah b)`, which should solve the wanted. However, it ''appears'' that GHC is instead choosing the global instance `forall a b. Coercible a b => Coercible (Yeah a) (Yeah b)`, leading to an unsatisfiable `Coercible a b`. So I don't ''think'' the current behavior matches the user's guide. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15639#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler