[GHC] #11554: Self quantification in GADT data declarations

[GHC]

#11554: Self quantification in GADT data declarations
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        Reporter:  Rafbill           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.2
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by alexvieth):

 I have a patch which will reject the example in comment:7 and give this
 error

 {{{
     • Type constructor ‘A’ cannot be used here
         (it is defined and used in the same recursive group)
     • In the kind ‘A’
       In the definition of data constructor ‘B’
       In the data declaration for ‘A’
 }}}

 The details:

 - `ATcTyThing` has a new variant `ATcTyConUnpromoted TyCon` meaning the
 `TyCon` can't be used as a kind.
 - `tcTyVar` will give a promotion error (`TyConPE`) whenever an
 `ATcTyConUnpromoted` is encountered and the `TcTyMode` is kind level.
 - When checking the constructors of a `DataDecl`, the name of the type is
 associated with an `ATcTyConUnpromoted`, rather than `ATcTyCon` as it is
 now.

 Think this is a sane thing to do? If so I'll upload it to phab.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11554#comment:8>
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