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

[GHC]

#12928: Too easy to trigger CUSK condition using TH
-------------------------------------+-------------------------------------
        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):

 The offending definition has this shape:

 {{{
 data FffSym0 (x :: TyFun (Proxy a) ())
 }}}

 and the term-level equivalent is, I believe, this:

 {{{
 fffSym0 :: TyFun (Proxy a) () -> Type;
 fffSym0 = undefined
 }}}

 However, one is accepted and the other is not. The inferred kind of `a` is
 a polymorphic `k`:

 {{{
 > :type fffSym0
 fffSym0 :: forall k (a :: k). TyFun (Proxy k a) () -> Type
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12928#comment:5>
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