[GHC] #14734: QuantifiedConstraints conflated with impredicative polymorphism?

[GHC]

#14734: QuantifiedConstraints conflated with impredicative polymorphism?
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:  invalid           |             Keywords:
                                     |  QuantifiedConstraints wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 It feels different, there is no way to get around regular impredicativity
 with type synonyms but in this case we can get around it with the `Limit`
 class synonym.

 Hypothetically if GHC were to generate an internal class synonym for
 impredicative constraints what would go wrong? I'm not aware of
 differences between `forall xx. f xx` and `Limit f`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14734#comment:3>
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