[GHC] #15787: GHC panic using kind application

[GHC]

#15787: GHC panic using kind application
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5275
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Richard Eisenberg <rae@…>):

 In [changeset:"4427315a65b25db22e1754d41b43dd4b782b022f/ghc" 4427315/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="4427315a65b25db22e1754d41b43dd4b782b022f"
 Fix #15787 by squashing a coercion hole.

 In type-incorrect code, we can sometimes let a coercion
 hole make it through the zonker. If this coercion hole then
 ends up in the environment (e.g., in the type of a data
 constructor), then it causes trouble later.

 This patch avoids trouble by substituting the coercion hole
 for its representative CoVar. Really, any coercion would do,
 but the CoVar was very handy.

 test case: polykinds/T15787
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15787#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler