[GHC] #13365: Kind-inference for poly-kinded GADTs

[GHC]

#13365: Kind-inference for poly-kinded GADTs
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        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Ryan nailed it. This is a CUSK issue. When you have a
 [https://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html
 #complete-user-supplied-kind-signatures-and-polymorphic-recursion CUSK],
 inference runs quite differently from when you don't. The error message is
 actually correct, given that we're doing non-CUSK inference here. What
 would be better is an `NB: Adding a complete user-specified kind signature
 might help`. Not clear how easy or hard it would be to know when to add
 the `NB`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13365#comment:2>
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