[GHC] #13109: CUSK improvements

[GHC]

#13109: CUSK improvements
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 Inspired by looking at `RnTypes.bindLHsTyVarBndr`, and driven by #11592,
 Richard and I decided to make some improvements to the treatment of CUSKs.

 Here's a scrappy Skype dump as a memory-jogger; it is a memory-jogger, not
 a full description.
 Richard has notes too
 {{{
 Richard Eisenberg: data F (x :: B a)
                    data G (x :: C b)
   You have written a complete user-suppled kind signature,
     but the following variable is undetermined: a0 :: A
     Perhaps add a kind signature.
     Inferred kinds of user-written variables:
       b :: B a0
       x :: C b

 SPJ claim: CUSKs should be handled INDIVIDUALLY and in isoloation, just
 like term-level type signatures
 SPJ claim: get rid of the "after the ::" side condition for data types
 (last bullet of HsDecls Note about CUSKs
 All of this should mean (SPJ claims) no need for dep_vars returns by
 bindHsQTyVar, bindLHsTyVarBnrds etc
 ToDo: clarify comments etc in bindLHStyVarBndr

 Related to D2914
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13109>
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