#15639: Surprising failure combining QuantifiedConstraints with Coercible -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 (Type checker) | Keywords: | 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: -------------------------------------+------------------------------------- I don't understand what's going wrong here. {{{#!hs -- Fishy.hs {-# language RankNTypes, QuantifiedConstraints, RoleAnnotations #-} module Fishy (Yeah, yeahCoercible) where import Data.Coerce data Yeah_ a = Yeah_ Int newtype Yeah a = Yeah (Yeah_ a) type role Yeah representational yeahCoercible :: ((forall a b. Coercible (Yeah a) (Yeah b)) => r) -> r yeahCoercible r = r -- Fishy2.hs module Fishy2 where import Fishy import Data.Type.Coercion import Data.Coerce yeah :: Coercion [Yeah a] [Yeah b] yeah = yeahCoercible Coercion }}} I get {{{ Fishy2.hs:8:22: error: • Couldn't match representation of type ‘a’ with that of ‘b’ arising from a use of ‘Coercion’ ‘a’ is a rigid type variable bound by the type signature for: yeah :: forall a b. Coercion [Yeah a] [Yeah b] at Fishy2.hs:7:1-34 ‘b’ is a rigid type variable bound by the type signature for: yeah :: forall a b. Coercion [Yeah a] [Yeah b] at Fishy2.hs:7:1-34 • In the first argument of ‘yeahCoercible’, namely ‘Coercion’ In the expression: yeahCoercible Coercion In an equation for ‘yeah’: yeah = yeahCoercible Coercion • Relevant bindings include yeah :: Coercion [Yeah a] [Yeah b] (bound at Fishy2.hs:8:1) | 8 | yeah = yeahCoercible Coercion | }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15639 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler