[GHC] #16140: Cannot create type synonym for quantified constraint without ImpredicativeTypes

[GHC]

#16140: Cannot create type synonym for quantified constraint without
ImpredicativeTypes
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        Reporter:  Ashley Yakeley    |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints,
                                     |  ImpredicativeTypes
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 The spirit of this proposal sounds nice, but there's a couple things that
 I'm worried about:

 * This proposal suggests accepting polytypes in all types of kind
 `Constraint`. Would that mean that the following would be accepted?

   {{{#!hs
   f :: (?x :: Proxy (forall a. Eq a => Eq (f a) :: Constraint)) => Int; f
 = 42
   }}}

   Unlike in the original example, `Proxy (forall a. Eq a => Eq (f a) ::
 Constraint)` appears to be truly impredicative.
 * Let's suppose you define `type F2 f = (Functor f, forall a. Eq (f a))`
 (which would now be accepted under this proposal without needing
 `ImpredicativeTypes`) and later try to define `g :: Proxy (F2 Maybe); g =
 Proxy`. Would //that// count as impredicative?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:4>
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