[GHC] #13646: strict patterns with no bindings (e.g. `let !() = ...`) trigger -Wunused-pattern-binds

[GHC]

#13646: strict patterns with no bindings (e.g. `let !() = ...`) trigger -Wunused-
pattern-binds
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        Reporter:  exphp             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      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 exphp):

 I am conflicted on dfeuer's suggestion, since functions too can be bottom:

 {{{#!hs
 bottomsUp :: a -> a
 bottomsUp = error "oops"

 ...

 let !_ = bottomsUp  -- error: Lib: oops
 }}}
 So generating warnings on this would be a somewhat opinionated change. Yet
 at the same time, it is an opinion I perhaps agree with; such code is
 playing with fire!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13646#comment:3>
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