[GHC] #14943: Make (=>) polykinded (:: k -> k -> Constraint)

[GHC]

#14943: Make (=>) polykinded (:: k -> k -> Constraint)
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints, wipT2893    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Would it be a good idea to treat `=>` in `-XQuantifiedConstraints` as

 {{{#!hs
 type family
   (=>) :: k -> k -> Constraint where
   (=>) = Implies0
   (=>) = Implies1
   (=>) = Implies2 ..
 }}}

 {{{#!hs
 class    (a => b) => Implies a b
 instance (a => b) => Implies a b

 class    (forall x. f x => g x) => Implies1 f g
 instance (forall x. f x => g x) => Implies1 f g

 class    (forall x y. f x y => g x y) => Implies2 f g
 instance (forall x y. f x y => g x y) => Implies2 f g
 ..
 }}}

 or will this get too confusing? This means type signatures like the ones
 from #14942

 {{{#!hs
 oneTwo   :: (forall x. semi x => Semigroup x) => Free semi Int

 nil      :: (forall x. mon  x => Monoid x)    => Free mon Int

 together :: (forall x. mon  x => Monoid x)    => [Free mon Int]
 }}}

 could equivalently be written

 {{{#!hs
 oneTwo   :: (semi => Semigroup) => Free semi Int

 nil      :: (mon  => Monoid)    => Free mon Int

 together :: (mon  => Monoid)    => [Free mon Int]
 }}}

 I'm not sold on this idea myself. It's quite possible this would screw
 with the parser.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14943>
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