[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):

 I wonder if (3) has anything to do with #9269? `KindOf2 A` is a bit weird
 in that it's an application of a type family that returns `forall a -> a
 -> Type`, which is a polymorphic type. Normally, GHC disallows that—you
 wouldn't be able to define `type family FAA where FAA = forall a -> a ->
 Type`, for instance. Yet `KindOf2` offers a way around this restriction,
 so perhaps (3) is a manifestation of GHC trying to deal with something it
 doesn't know how to reason about.

 Then again, `KindOf2 A` seems to work in other places, as comment:1
 demonstrates. It's just in type family equations where things go haywire.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15869#comment:9>
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