Kind incompatibility error

A.hs

{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE PolyKinds #-}
module A where

import Control.Lens
import Data.Vinyl
import GHC.TypeLits

class A f where
    a :: f a a

instance A (->) where
    a = id

type Foo = forall f a. A f => f a a

foo :: Foo
foo = a

bar :: forall f a. A f => f a a
bar = a

B.hs

module B where

import A

import Control.Lens
import Data.Vinyl
import GHC.TypeLits

baz x = bar x -- this is fine

baz' x = foo x

{-

B.hs:11:10:
    Kind incompatibility when matching types:
      a0 :: AnyK
      t1 :: *
    Expected type: t1 -> t
      Actual type: f0 a0 a0
    Relevant bindings include
      x :: t1 (bound at B.hs:11:6)
      baz' :: t1 -> t (bound at B.hs:11:1)
    The function ‘foo’ is applied to one argument,
    but its type ‘f0 a0 a0’ has none
    In the expression: foo x
    In an equation for ‘baz'’: baz' x = foo x
Failed, modules loaded: A.

-}