[GHC] #13130: atomicModifyMutVar# has incorrect type

[GHC]

#13130: atomicModifyMutVar# has incorrect type
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        Reporter:  carter            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       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 rwbarton):

 There's a comment about this in `compiler/prelude/primops.txt.pp`. But it
 should be a Haddock comment so it shows up in the documentation. A note on
 why we don't use an unboxed tuple would also be nice.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13130#comment:1>
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