[GHC] #8423: contraint solver doesn't reduce reducible closed type family expressions (even with undecidable instances!)

[GHC]

#8423: contraint solver doesn't  reduce reducible closed type family expressions
(even with undecidable instances!)
--------------------------------------------+------------------------------
        Reporter:  carter                   |            Owner:
            Type:  feature request          |           Status:  new
        Priority:  normal                   |        Milestone:  7.10.1
       Component:  Compiler (Type checker)  |          Version:  7.7
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:  #4259
--------------------------------------------+------------------------------

Comment (by carter):

 relatedly: Ranjit Jhala took the motiviating code and demo'd checking it
 with liquid haskell! Which is pretty darn awesome

 http://goto.ucsd.edu:8090/index.html#?demo=permalink%2F1381284690.hs

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8423#comment:5>
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