QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)'

This works with the latest change to the -XQuantifiedConstraints branch.

{-# Language QuantifiedConstraints, FlexibleInstances, MultiParamTypeClasses, AllowAmbiguousTypes, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-}

class    (forall xx. (xx => a) => Implies xx b) => F a b
instance (forall xx. (xx => a) => Implies xx b) => F a b

class    (a => b) => Implies a b 
instance (a => b) => Implies a b

but replacing Implies xx b with (xx => b) causes it to fail. I don't know if the cause of this overlaps with an existing ticket.

class    (forall xx. (xx => a) => (xx => b)) => F a b
instance (forall xx. (xx => a) => (xx => b)) => F a b

-- $ ghci -ignore-dot-ghci /tmp/ASD.hs 
-- GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
-- [1 of 1] Compiling Main             ( /tmp/ASD.hs, interpreted )
-- 
-- /tmp/ASD.hs:4:10: error:
--     • Could not deduce: xx0
--         arising from the superclasses of an instance declaration
--       from the context: forall (xx :: Constraint). (xx => a, xx) => b
--         bound by the instance declaration at /tmp/ASD.hs:4:10-53
--       or from: (xx => a, xx)
--         bound by a quantified context at /tmp/ASD.hs:1:1
--     • In the instance declaration for ‘F a b’
--   |
-- 4 | instance (forall xx. (xx => a) => (xx => b)) => F a b
--   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- Failed, no modules loaded.
-- Prelude> 

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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