[GHC] #15806: Impredicativity behavior in `:k` command in GHCi

[GHC]

#15806: Impredicativity behavior in `:k` command in GHCi
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        Reporter:  ningning          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14859            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I think the `:k` should be rejected to -- maybe we are missing a call to
 `checkValidType` somewhere?

 Yes if we had explicit impredicativity we should be ok

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15806#comment:1>
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