[GHC] #9127: Don't warn about pattern-bindings of the form `let !_ = rhs`

[GHC]

#9127: Don't warn about pattern-bindings of the form `let !_ = rhs`
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        Reporter:  refold            |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      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 igloo):

 Ah, well, it'll ''probably'' do what you want. But if, for example, at a
 usage site GHC can see the expression that has been put in `rqPostParams`
 and thinks that it is cheap, then it may evaluate a copy of it rather than
 sharing the value. #2273 is an example of something similar.

 To be sure, I think you'd need something like:
 {{{
 foo :: RequestBuilder
 foo = do rq <- rGet
          let !rqPostParams' = rqPostParams rq
          ...
          rPut $! (rq { rqPostParams = rqPostParams' })
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9127#comment:4>
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