[GHC] #9117: Coercible constraint solver misses one

[GHC]

#9117: Coercible constraint solver misses one
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              Reporter:  goldfire    |            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:  Phab:D546   |
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Changes (by goldfire):

 * differential:   => Phab:D546


Comment:

 I've written a new solver for `Coercible`. It is able to solve for all the
 cases considered in this ticket. However, this comes at the cost of
 essentially ditching support for pathologically recursive newtypes (like
 `Fix`, or `newtype Loop = Loop Loop`). I think this exchange is a solid
 win.

 Still working out the final bugs; will set to "patch" when those are done.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117#comment:32>
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