[GHC] #14363: :type hangs on coerce

[GHC]

#14363: :type hangs on coerce
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      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:                    |
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Comment (by simonpj):

 Gah!  How has this bug survived so long?  I see this
 {{{
 Inert set:   [G] co1:  a  ~R#  f b
 Work item:   [B] co2:  b  ~R#  f a
 }}}
 We can't rewrite the work item `co2` with the inert `co1`, because the
 role of f's argument is Nominal, sot`co1` can't rewrite it.

 So we add `co2` to the inert set.  Alas we then kick out `co1` becuase it
 has a free `b`, thinking that it might be rewritten by `co2`.  But of
 course it can't and we get an infinite loop instead.  Sigh!

 What's wrong?  I think that `anyRewritableTyVar` needs to beecome role-
 aware.

 Richard do you agree?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14363#comment:3>
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