[GHC] #15869: Discrepancy between seemingly equivalent type synonym and type family

[GHC]

#15869: Discrepancy between seemingly equivalent type synonym and type family
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.6.2
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  TypeInType
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):

 To be clear, I am in agreement with you that the program in the second
 bullet point of comment:5 should be rejected if `ImpredicativeTypes` is
 not enabled. I am simply wondering if it being accepted //with//
 `ImpredicativeTypes` enabled should count as a bug in its own right. (It's
 such a squirrelly extension that I can never tell whether it's working as
 intended or not.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15869#comment:7>
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