[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):

 > So, if this all works, there has to be some way of dealing with the
 overlap and a small amount of "search".

 Well, the way is „do whatever comes first“. It works because the rewrite
 system is confluent (I hope that’s the right term), i.e. we don’t run into
 dead ends if we do on or the other first.

 > Is the case in this ticket somehow different/worse?

 Yes, as `Coercible Foo1 Foo2` would be a dead end.

 It ''might'' work to try `Coercible f g => Coercion (f a) (g a)` only if
 no other instance matches. The code is able to do so, but I don’t
 particularly like that. We can add it if we don’t have a better idea
 ''and'' there there is a use case for it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117#comment:5>
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