Re: [GHC] #10698: Forall'd variable ‘$rcobox’ is not bound in RULE lhs

[GHC]

#10698: Forall'd variable ‘$rcobox’ is not bound in RULE lhs
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        Reporter:  nomeata           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by nomeata):

 Hmm, guess I was misguided here. Of course, `Map`’s first argument is set
 by the user to have role Nominal. For some reason I assumed that, as long
 as the constructors are in scope, I can still coerce it. But that’s not
 the way we went. So the whole rule is unfortunately not possible.

 Anyways, ghc-7.8 has a proper error message, instead of a weird warning:
 {{{
 T10698.hs:19:53:
     Could not coerce from ‘Map k1 a’ to ‘Map k2 a’
       because the first type argument of ‘Map’ has role Nominal,
       but the arguments ‘k1’ and ‘k2’ differ
       arising from a use of ‘coerce’
     from the context (Coercible k1 k2)
       bound by the RULE "mapKeysMonotonic/coerce" at T10698.hs:19:1-58
     In the expression: coerce
     When checking the transformation rule "mapKeysMonotonic/coerce"
 }}}
 so the bug is about the regression of the error message.

 Will commit a test case right away.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10698#comment:1>
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