[GHC] #14121: ghc master requires -XTypeInType where 8.2.1 does not

[GHC]

#14121: ghc master requires -XTypeInType where 8.2.1 does not
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        Reporter:  duog              |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.3
  checker)                           |
      Resolution:  invalid           |             Keywords:
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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Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 This program should indeed require `TypeInType`, and the fact that it is
 accepted without `TypeInType` in 8.2.1 is a fluke. The ability to bind a
 kind variable and use it as the kind of a different type variable in the
 same telescope is something that wasn't possible in a pre-`TypeInType`
 GHC, as this error message in GHC 7.10.3 reveals:

 {{{
 GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
 λ> :set -XKindSignatures -XRankNTypes -XPolyKinds
 λ> import Data.Proxy
 λ> let f :: forall (k :: *). forall (a :: k). Proxy a; f = Proxy

 <interactive>:4:10:
     Kind variable also used as type variable: ‘k’
     In the type signature for ‘f’
 }}}

 `RuntimeRep` wasn't introduced until 8.0, so I can't test your exact
 example in 7.10.3. But the principle is the same: if you use something as
 both type and kind, then you need `TypeInType`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14121#comment:2>
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