#11762: GHC 8 superclass chain constraint regression -------------------------------------+------------------------------------- Reporter: _deepfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc2 (Type checker) | 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: -------------------------------------+------------------------------------- It seems that GHC 8 regresses with regard to GHC 7.10, when it tries to satisfy the constraints, implied by instance context. The following does not build on GHC 8.0.1 RC2, but does on 7.10.3: {{{#!hs {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} --{-# LANGUAGE UndecidableSuperClasses #-} -- this doesn't matter {-# LANGUAGE UnicodeSyntax #-} module Foo where class Super a class Super a ⇒ Left a class Super a ⇒ Right a instance (Left a) ⇒ Right a }}} the error being:
• Could not deduce (Super a) arising from the superclasses of an instance declaration from the context: Left a bound by the instance declaration at repro.hs:9:10-27 Possible fix: add (Super a) to the context of the instance declaration • In the instance declaration for ‘Right a’
It also /roughly/ seems that https://phabricator.haskell.org/D1594 could possibly be related. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11762 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler