[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):

 Is this subsumed by #8555?

 Do we want a general “instance” {{{Coercible f g => Coercion (f a) (g
 a)}}}? Wouldn’t that run into a dead end in situations like
 {{{
 newtype Foo1 a = Foo1 (a,())
 newtype Foo2 a = Foo2 (a,())

 coerce :: Foo1 () -> Foo2 ()
 }}}
 which might simplify this to {{{Coercible Foo1 Foo2}}} if that happens to
 happen before the newtype-unwrapping instance?

 So while it surely is possible to add that to the solver somehow, e.g. as
 a final thing to try, I’m not sure about all the implications.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117#comment:2>
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