[GHC] #14042: Datatypes cannot use a type family in their return kind

[GHC]

#14042: Datatypes cannot use a type family in their return kind
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
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 goldfire):

 I thought of the same problem (comment:10) afterward, too. But regardless,
 we could require that the ending kind is `Type` at all usage sites,
 instead of at the definition site. Indeed, I wonder if we can just omit
 the check entirely, as long as we check the types of data constructors.
 Those types end in `-> T blah blah blah`, so if `T blah blah blah`'s kind
 is other than `Type`, the data constructor type will fail to kind-check.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14042#comment:11>
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