[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:  ningning
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.6.2
       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):  Phab:D5265
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"c4a876d5f99554e87400946dd26ca11819d11673/ghc"
 c4a876d5/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c4a876d5f99554e87400946dd26ca11819d11673"
 Fix `:k` command: add validity checking

 Summary:
 This patch fixes #15806, where we found that the `:k` command in GHCi
 misses a validity checking for the type.

 Missing validity checking causes `:k` to accept types that are not
 validated.
 For example, `:k (Maybe (forall a. a -> a))` (incorrectly) returns `*`,
 while
 impredictivity of type instantiation shouldn't be allowed.

 Test Plan: ./validate

 Reviewers: simonpj, goldfire, bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, carter

 GHC Trac Issues: #15806

 Differential Revision: https://phabricator.haskell.org/D5265
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15806#comment:4>
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