[GHC] #13761: Can't create poly-kinded GADT with TypeInType enabled, but can without

[GHC]

#13761: Can't create poly-kinded GADT with TypeInType enabled, but can without
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  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 goldfire):

 I mean something like

 {{{#!hs
 type T :: k -> Type   -- this is the kind signature
 data T _ where
   MkT :: T Int
 }}}

 Note that there is no `=` in the kind signature, distinguishing it from a
 type synonym declaration. The `_` in the `data` line is a recognition that
 a datatype with a kind signature does not need anything after the `T`. Not
 sure what the best concrete syntax around that is. (In my thought about
 this feature, `data T a where` and `data T :: ... where` would also both
 be acceptable.)

 Types with kind signatures will be treated just like types with CUSKs are
 today. Eventually, I would expect types without kind signatures to be
 treated just like types without CUSKs today. We would then cease to have a
 notion of CUSK.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13761#comment:4>
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