[GHC] #12928: Too easy to trigger CUSK condition using TH

[GHC]

#12928: Too easy to trigger CUSK condition using TH
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        Reporter:  int-index         |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             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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Comment (by int-index):

 Here's a complete GHCi session to demonstrate the asymmetry:

 {{{
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 Prelude> :set -XTypeInType -XScopedTypeVariables
 Prelude> import Data.Proxy
 Prelude Data.Proxy> import Data.Kind
 Prelude Data.Proxy Data.Kind> data TyFun :: * -> * -> *
 Prelude Data.Proxy Data.Kind> fffSym0 (x :: TyFun (Proxy a) ()) =
 undefined :: Type
 Prelude Data.Proxy Data.Kind> :type fffSym0
 fffSym0 :: forall k (a :: k). TyFun (Proxy a) () -> Type
 Prelude Data.Proxy Data.Kind> data FffSym0 (x :: TyFun (Proxy a) ()) ::
 Type

 <interactive>:7:1: error:
     You have written a *complete user-suppled kind signature*,
     but the following variable is undetermined: k0 :: *
     Perhaps add a kind signature.
     Inferred kinds of user-written variables:
       a :: k0
       x :: TyFun (Proxy a) ()
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12928#comment:8>
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