[GHC] #14776: Add equality rule for `eqString`

[GHC]

#14776: Add equality rule for `eqString`
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        Reporter:  mpickering        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      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 carter):

 That’s no sound for floating point when nan is treated as a valid value
 rather than an error/ exception trigger.

 In the former case this optimization will break ALL isNan code in Haskell.
 If we also have a mode where calculations that would produce a nan throw
 an exception, this rule would be safe.

 Is there a way to provide this rule but prevent it from running on floats
 and doubles ?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14776#comment:2>
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