[GHC] #13476: Coercion maching in RULES is fragile

[GHC]

#13476: Coercion maching in RULES is fragile
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 Your Plan B means to take the `kco` parameter to `unify_ty` and do the
 same thing at the term level. Perhaps the precise code in `unify_ty` could
 be improved, but the idea is a good one, and you're propose to replicate
 this action on coercions. I think it's all the right direction.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13476#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler