[GHC] #15788: Core Lint error, from visible kind applications

[GHC]

#15788: Core Lint error, from visible kind applications
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  #12045
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):

 This is really #15743 in deep disguise.

 GHC correctly infers `A :: forall {k1} (k :: k1). Type`.

 We have `MkA :: A @p` (I've changed the variable name). Currently, the
 kind application branch has a bug that means we don't skip Inferred
 variables when instantiating. So, the `p` instantiates the `k1`. But, the
 kind of a data constructor has kind `Type`, while `A @p :: forall (k ::
 p). Type`. So we instantiate with a metavariable, getting `A @p @alpha`,
 where `alpha :: p`. Then, we try to generalize, skolemizing `alpha := a`..
 Generalization tries putting Inferred things before Specified things, and
 so we get `forall (a :: p) p. A p a`, which is clearly hogwash.

 I hope that once my Phab:D5208 patch lands, this will be fixed.

 Meanwhile, My will fix the forgetting-to-skip-Inferred variables bug. (See
 point (4) in https://phabricator.haskell.org/D5229#144361 .)

 Keeping this open, as we should test that it all works in the end.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15788#comment:3>
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