[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, we actually already have an ordering dependence on the cases, in
 order to avoid dead ends -- but we didn't realize it!

 Well spotted. The fix for this should be simple, just re-order the cases
 in `getCoercibleInst`. I also believe that unwrapping `newtype`s should be
 strictly more powerful than the other.

 I’m still with Simon’s worries about changing situation. It is not
 coherence that we need to worry about, but, as you have just demonstrated,
 solvability. Adding the eta-contraction might give new  possibilities to
 run into dead ends. But I’m still not able to give a good example of what
 I mean, so I guess I could just implement this and we’ll see what happens.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117#comment:8>
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