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:

{-# 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.