#14333: GHC doesn't use the fact that Coercible is symmetric -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# Language ScopedTypeVariables, GADTs, DeriveGeneric #-} import GHC.Generics import Data.Type.Coercion data AB = A | B deriving (Generic) data XY = X | Y deriving (Generic) data This ab xy where At :: This AB XY foo :: This ab xy -> Coercion (Rep ab a) (Rep xy a) foo At = Coercion bar :: forall ab xy a. This ab xy -> Coercion (Rep ab a) (Rep xy a) bar at | Coercion <- foo at :: Coercion (Rep ab a) (Rep xy a) = Coercion }}} This compiles on 8.2.1 and 8.3.20170920 but flipping the arguments to `Coercion` fails {{{#!hs -- ... -- • Could not deduce: Coercible (Rep xy a) (Rep ab a) -- arising from a use of ‘Coercion’ -- from the context: Coercible (Rep ab a) (Rep xy a) -- bound by a pattern with constructor: bar :: forall ab xy a. This ab xy -> Coercion (Rep xy a) (Rep ab a) bar at | Coercion <- foo at :: Coercion (Rep ab a) (Rep xy a) = Coercion }}} Conceptually there should be an (`Undecidable`) superclass constraint for `Coercible` {{{#!hs class Coercible b a => Coercible a b }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14333 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler