[GHC] #13032: Redundant forcing of Given dictionaries

[GHC]

#13032: Redundant forcing of Given dictionaries
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        Reporter:  simonpj           |                Owner:
            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:                    |
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Comment (by goldfire):

 How can we know, though, that the equality isn't bottom? The "simple
 optimizer" used to indeed throw such `case`s away, but then #11230 was
 reported, and I wrote 1722fa106e10e63160bb2322e2ccb830fd5b9ab3. So we
 should proceed carefully here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13032#comment:1>
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