[GHC] #6068: Panic in GHCi when using functional dependencies and promoted kinds

[GHC]

#6068: Panic in GHCi when using functional dependencies and promoted kinds
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  GHCi              |          Version:  7.5
      Resolution:                    |         Keywords:  PolyKinds
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  GHCi crash        |       Difficulty:  Unknown
       Test Case:  polykinds/T6068   |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"dff0e99d37f3529041bb2bb66ffda1ea22af14e0/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="dff0e99d37f3529041bb2bb66ffda1ea22af14e0"
 Fix a subtle bug in kind-mis-matched equalities (Trac #6068)

 When we have an equality constraint where the LHS and RHS
 have ill-matched kinds, it get turned into a CIrredEvCan
 because a CTyEqCan/CFunEqCan are guaranteed kind-compatible.

 But that in turn led to a bug because in the constraint
     c  =  (a:k1) ~ (b:k2)
 the kind variables k1 and k2 don't show up in tyVarsOfType c.
 Why not?  Because it looks like
     (~) k1 (a:k1) (b:k2)
 Maybe (~) should have two kind arguments?  That seemed
 like too big a change for not (we wait for NoKinds), so
 this patch fixes the bug for now.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6068#comment:7>
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