[GHC] #14921: Type inference breaks on constraint-kinded parameter used by a Rank-2 callback

[GHC]

#14921: Type inference breaks on constraint-kinded parameter used by a Rank-2
callback
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        Reporter:  glittershark      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10651            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * related:   => #10651


Comment:

 This is more or less expected behavior. See #10651 for a similar scenario.
 In general, GHC doesn't unify underneath equalities or something that
 might turn into an equality, such as `cls k` (this is why `a` is
 untouchable in your error message, as you'd have to unify it underneath
 `cls k`).

 As SPJ notes in https://ghc.haskell.org/trac/ghc/ticket/10651#comment:2,
 this limitation is explained in Section 5 of the
 [http://research.microsoft.com/~simonpj/papers/constraints/jfp-
 outsidein.pdf OutsideIn paper], and it's unknown how to improve this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14921#comment:1>
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