[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 simonpj):

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

 No, it would not.  Under the `Proxy` we switch off the arbitrary-rank
 thing.  Try it today!  (The proposals doesn't change the behaviour of type
 signatures.

 > Would that count as impredicative?

 I think we have decided that behaviour never changes when type synonyms
 are expanded; and have already gone to some trouble to make this so (in a
 recent ticket).  So yes it would be impredicative, and would (or should)
 be caught.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:5>
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