#10285: Bug in Coerciible -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: highest | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Richard and I uncovered this {{{ {-# LANGUAGE RoleAnnotations #-} module Bar( N ) where -- Data constructor not exported type role N representational newtype N a = MkN Int }}} and {{{ {-# LANGUAGE FlexibleContexts #-} module Foo where import Data.Coerce import Data.Type.Coercion import Bar f :: Coercible (N a) (N b) => Coercion a b f = Coercion }}} This compiles but it should not! It wrongly compiles by using `nth (N a ~R N b)`; but (as Fig 4 of [http://research.microsoft.com/en- us/um/people/simonpj/papers/ext-f/ the Corcible paper] says), we can't decompose a newtype at representational role. Easy to fix. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10285 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler