GHC 8 superclass chain constraint regression

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.

Trac metadata

Trac field	Value
Version	8.0.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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