[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 simonpj):

 Harump. Maybe `let _ = rhs` ''should'' trigger a warning!  It's dead code
 after all.

 But the `let !_ = e in b` means just {{{e `seq` b}}} so I suppose you
 could argue that it non-warnable.

 Does anyone else care?

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9127#comment:1>
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