[GHC] #9117: Coercible constraint solver misses one

[GHC]

#9117: Coercible constraint solver misses one
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        Reporter:  goldfire          |            Owner:  nomeata
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by nomeata):

 I implemented the changed order (code is running a validate right now).
 There are still weird corner-cases. Consider:

 {{{
 newtype Foo a = Foo (Foo a)
 newtype Age = MkAge Int

 ex1 :: (Foo Age) -> (Foo Int)
 ex1 = coerce
 }}}

 Previously, this would be solvable; now it starts unrolling the infinite
 newtype. (But we don’t have a good story for newtypes anyways).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117#comment:19>
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