[GHC] #9090: Untouchable higher-kinded type variable in connection with RankNTypes, ConstraintKinds and TypeFamilies

[GHC]

#9090: Untouchable higher-kinded type variable in connection with RankNTypes,
ConstraintKinds and TypeFamilies
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              Reporter:  kosmikus    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Reason:
  * We have a given `D (Eq a)`
  * Flattened to `fsk` (a `CIrredCan`) plus work-list `D (Eq a) ~ fsk`.
  * The `CIrredCan` fsk is put in the inert set
  * ...and that flips `No-eqs` to `False`
  * We solve `D (Eq a) ~ fsk`, getting `fsk ~ Eq a`
  * That kicks out the `CIrredCan` so it can be substituted
  * But it is too late!  The no-eqs flag has been flipped.

 Solution: the no-eqs flag should be computed from the final inert set, not
 the process.

 Will be fixed by the same fix as #9211

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9090#comment:3>
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