QuantifiedConstraints: Can't print type of quantified constraint

{-# Language QuantifiedConstraints, FlexibleInstances, UndecidableInstances, MonoLocalBinds #-}

class    (forall xx. Functor (f xx)) => Functor' f
instance (forall xx. Functor (f xx)) => Functor' f

fmap' :: Functor' f => (b -> b') -> (f a b -> f a b')
fmap' = fmap

load in ghci and check the type of fmap'

$ ... -ignore-dot-ghci Bug2.hs 
GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( Bug2.hs, interpreted )
Ok, one module loaded.
*Main> :t fmap'

<interactive>:1:1: error:
    No instance for (Functor (f xx)) arising from a use of ‘fmap'’
*Main> 

Simpler example

GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
Prelude> :set -XRankNTypes -XQuantifiedConstraints 
Prelude> let a :: (forall xx. Monoid (f xx)) => f a; a = mempty
Prelude> :t a

<interactive>:1:1: error:
    No instance for (Monoid (f xx)) arising from a use of ‘a’
Prelude>

I expected the same output as :t +v

Prelude> :t +v a
a :: (forall xx. Monoid (f xx)) => f a

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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