[GHC] #8646: Distinguish between update frames in rts/Printer.c

[GHC]

#8646: Distinguish between update frames in rts/Printer.c
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        Reporter:  Tarrasch          |            Owner:  simonmar
            Type:  feature request   |           Status:  patch
        Priority:  lowest            |        Milestone:  _|_
       Component:  Runtime System    |          Version:  7.6.3
      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 ezyang):

 It might be more robust if, in the failure check, it re-checked the
 closure type, and reported "unknown update frame" or "not an update frame"
 instead.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8646#comment:2>
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