GHC 8 superclass chain constraint regression

[Kosyrev Serge]

Good day!

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’

A look through the GHC 8 status page, produced no bugs that seemed
related to my uneducated guess.

It also /roughly/ seems that https://phabricator.haskell.org/D1594
could possibly be related.

Thanks to lyxia on #haskell for pointing that this actually works on 7.10..

-- 
с уважениeм / respectfully,
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