[GHC] #11964: Without TypeInType, inconsistently accepts Data.Kind.Type but not type synonym

20 Apr 2016

      #11964: Without TypeInType, inconsistently accepts Data.Kind.Type but not type
synonym
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           Reporter:  ezyang         |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 For convenience, I'll use GHCi to demonstrate flag behavior. First, we
 define a file:

 ```
 {-# LANGUAGE TypeInType #-}
 import Data.Kind
 type Star = Type
 newtype T k (t :: k) = T ()
 ```

 Next, we load it up in GHCi, WITHOUT `-XTypeInType`. Now we observe
 strange behavior:

 ```
 ezyang@sabre:~$ ghc-8.0 --interactive C.hs
 GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( C.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> :k T Type Int
 T Type Int :: *
 *Main> :k T Star Int

 <interactive>:1:3: error:
     • Data constructor ‘Star’ cannot be used here
         (Perhaps you intended to use DataKinds)
     • In the first argument of ‘T’, namely ‘Star’
       In the type ‘T Star Int’
 ```

 Of course, if we pass `-TypeInType` to GHCi that fixes the problem (BTW,
 `DataKinds` does NOT solve the problem.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11964
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