[GHC] #9211: Untouchable type variable (regression from 7.6)

[GHC]

#9211: Untouchable type variable (regression from 7.6)
-------------------------------------+-------------------------------------
              Reporter:  simonpj     |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Notes from S and D
  * Level on skols
  * Orient skol eqs a~b
  * Level on EqCtList
  * getUnsolvedInerts also looks for given equalities FROM THIS LEVEL
  * …and finds non-let-bound ones
 Let-bound ones
  * a~ty, where a is a skol from this level
  * F tys ~ b, where b is a skol from this level, and b does not appear in
 any remaining ones
 (iterate)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9211#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler